QuestionsToDB4

CHAPTER 20 Respiratory System
 
STUDENT LEARNING OBJECTIVES
At the completion of this chapter, you should be able to do the following:
1.Outline the general flow of air through the respiratory system.
2.Describe the functions of the following: nose, pharynx, larynx.
3.Compare the organs of the upper respiratory tract with those of the lower respiratory tract.
4.Describe the structure of the bronchi and alveoli, and give their functions.
5.Describe the structure and function of the lungs.
6.Describe the process of pulmonary ventilation.
7.Outline the various measures of pulmonary volumes and capacities.
8.Discuss what is meant by partial pressure of gases and its significance in gas exchange.
9.Describe the process of the transport of blood gases.
10.Discuss how pulmonary function is regulated.
LANGUAGE OF SCIENCE AND MEDICINE
Before reading the chapter, say each of these terms out loud. This will help you avoid stumbling over them as you read.
 
alveolus (al-VEE-oh-lus)
[alve- hollow, -olus little] pl., alveoli (al-VEE-oh-lye)
apex (AY-peks)
[apex tip] pl., apices (AY-pih-seez)
base (BAYS)
[bas- foundation]
bicarbonate (bye-KAR-boh-nayt)
[bi- two, -carbon- coal (carbon), -ate oxygen compound]
bronchiole (BRONG-kee-ohl)
[bronch- windpipe, -ol- little]
bronchopulmonary segment (brong-koh-PUL-moh-nair-ee)
[bronch- windpipe, -pulmon- lung, -ary relating to]
carbaminohemoglobin (kahr-bam-ih-no-hee-mohGLOH-bin)
[carb- coal (carbon), -amino- ammonia compound (amino acid), -hemo- blood, -glob- ball, -in substance]
carbon monoxide (CO) (KAR-bon mon-OKS-ide)
[mono- single, -ox- sharp (oxygen), -ide chemical]
chronic obstructive pulmonary disease (COPD) (KRON-ik ob-STRUK-tiv PUL-moh-nair-ee)
[chron- time, -ic relating to, pulmon- lung, -ary relating to]
cleft palate (kleft PAL-ett)
concha (KONG-kah)
[concha sea shell] pl., conchae (KONG-kee)
cough reflex (REE-fleks)
[re- back or again, -flex bend]
cribriform plate (KRIB-rih-form)
[cribr- sieve, -form shape]
diving reflex (REE-fleks)
[re- back or again, -flex bend]
elastic recoil (eh-LAS-tik REE-koyl)
[elast- drive or propel, -ic relating to]
endotracheal intubation (en-doh-TRAY-kee-al in-tooBAY-shun)
[endo- within, -trache- rough duct, -al relating to, in- within, -tub- tube, -ation process]
epiglottis (ep-ih-GLOT-is)
[epi- upon, -glottis tongue] pl., epiglottides or epiglottises (ep-ih-GLOT-ih-deez, ep-ih-GLOT-ih-seez)
expiration (eks-pih-RAY-shun)
[ex- out, -[s]pir- breathe, -ation process]
expiratory reserve volume (ERV) (eks-PYE-rah-tor-ee)
[ex- out of, -[s]pir- breathe, -tory relating to]
functional residual capacity (FRC) (FUNK-shun-al ree-ZID-yoo-al kah-PASS-ih-tee)
glottis (GLOT-is)
[glottis tongue] pl., glottides or glottises (GLOT-ih-deez, GLOT-ih-seez)
hemoglobin (Hb) (hee-moh-GLOH-bin)
[hemo- blood, -glob- ball, -in substance]
hiccup (HIK-up)
[imitation of hiccup sound]
hilum (HYE-lum)
[hilum least bit] pl., hila (HYE-lah)
inspiration (in-spih-RAY-shun)
[in- in, -spir- breathe, -ation process]
inspiratory capacity (IC) (in-SPY-rah-tor-ee kah-PASS-ih-tee)
[in- in, -spir- breathe, -tory relating to]
inspiratory reserve volume (IRV) (in-SPY-rah-tor-ee)
[in- in, -spir- breathe, -tory relating to]
intrapleural space (in-trah-PLOO-ral)
[intra- within, -pleura- rib, -al relating to]
laryngopharynx (lah-rin-goh-FAIR-inks)
[laryng- voicebox (larynx), -pharynx throat] pl., laryngopharynges or laryngopharynxes (lah-rin-goh-FAIR-in-jeez, lah-rin-goh-FAIR-inks-ehz)
larynx (LAIR-inks)
[larynx voicebox] pl., larynges or larynxes (lah-RIN-jeez, LAIR-inks-ehz)
lingual tonsil (LING-gwal TAHN-sil)
[ling- tongue, -al relating to]
lower respiratory tract (RES-pih-rah-tor-ee TRAKT)
[re- again, -spir- breathe, -tory relating to, tractus trail]
medullary rhythmicity area (MED-oo-lair-ee rith-MIH-sih-tee)
[medulla- middle, -ary relating to, rhythm- rhythm, -ic- relating to, -ity condition]
nasal septum (NAY-zal SEP-tum)
[nas- nose, -al relating to, septum partition]
nasopharynx (nay-zoh-FAIR-inks)
[naso- nose, -pharynx throat] pl., nasopharynges or nasopharynxes (nay-zoh-FAIR-in-jeez, nay-zoh-FAIR-inks-ehz)
olfactory epithelium (ohl-FAK-tor-ee ep-ih-THEE-lee-um)
[olfact- smell, -ory relating to, epi- upon, -theli- nipple, -um thing] pl., epithelia (ep-ih-THEE-lee-ah)
oropharynx (or-oh-FAIR-inks)
[oro- mouth, -pharynx throat] pl., oropharynges or oropharynxes (or-oh-FAIR-in-jeez, or-oh-FAIR-inks-ehz)
palatine tonsil (PAL-ah-tyne TAHN-sil)
[palat- palate, -ine relating to]
paranasal sinus (pair-ah-NAY-zal SYE-nus)
[para- beside, -nas- nose, -al relating to, sinus hollow]
parietal pleura (pah-RYE-ih-tal PLOO-rah)
[parie- wall, -al relating to, pleura rib] pl., pleurae (PLOOR-ee)
partial pressure (PAR-shal PRESH-ur)
pharyngeal tonsil (fah-RIN-jee-al TAHN-sil)
[pharyng- throat, -al relating to]
pharynx (FAIR-inks)
[pharynx throat] pl., pharynges or pharynxes (FAH-rin-jeez, FAIR-inks-ehz)
pleura (PLOO-rah)
[pleura rib] pl., pleurae (PLOOR-ee)
pneumothorax (noo-moh-THOH-raks)
[pneumo- air or wind, -thorax chest]
primary bronchus (BRONG-kus)
[prim- first, -ary relating to, bronchus windpipe] pl., bronchi (BRONG-kye)
primary principle of ventilation (PRY-mair-ee PRIN-sip-al of ven-tih-LAY-shun)
[prim- first, -ary relating to, princip- foundation, vent- fan or create wind, -tion process]
residual volume (RV) (ree-ZID-yoo-al)
respiratory center (RES-pih-rah-tor-ee SEN-ter)
[re- again, -spir- breathe, -tory relating to]
respiratory mucosa (RES-pih-rah-tor-ee myoo-KOH-sah)
[re- again, -spir- breathe, -tory relating to, mucus slime]
respiratory portion (RES-pih-rah-tor-ee POR-shun)
[re- again, -spir- breathe, -tory relating to]
secondary bronchus (SEK-on-dair-ee BRONG-kus)
[second- second, -ary relating to, bronchus windpipe] pl., bronchi (BRONG-kye)
sneeze reflex (sneez REE-fleks)
[re- back or again, -flex bend]
spirogram (SPY-roh-gram)
[spir- breathe, -gram drawing]
spirometer (spih-ROM-eh-ter)
[spir- breathe, -meter measurement]
surfactant (sur-FAK-tant)
[combination of surf(ace) act(ive) a(ge)nt]
thyroid cartilage (THY-royd KAR-tih-lij)
[thyr- shield, -oid like]
tidal volume (TV) (TYE-dal)
[tid- time, -al relating to]
total lung capacity (TLC)
 
trachea (TRAY-kee-ah)
[trachea rough duct] pl., tracheae or tracheas (TRAY-kee-ee, TRAY-kee-ahz)
tracheostomy (tray-kee-OS-toh-mee)
[trache- rough duct, -os- mouth or opening, -tom- cut, -y action]
turbinate (TUR-bih-nayt)
[turbin- top (spinning toy), -ate of or like]
upper respiratory tract (RES-pih-rah-tor-ee TRAKT)
[re- again, -spir- breathe, -tory relating to, tract- trail]
vestibular fold (ves-TIB-yoo-lar)
[vestibul- entrance hall, -al relating to]
vestibule (VES-tih-byool)
[vestibul- entrance hall]
vibrissa (vye-BRISS-ah)
[vibrissa nostril hair] pl., vibrissae (vye-BRISS-ee)
visceral pleura (VISS-er-al PLOO-rah)
[viscer- internal organ, -al relating to, pleura rib] pl., pleurae (PLOOR-ee)
vital capacity (VC) (VYE-tal kah-PASS-ih-tee)
[vita- life, -al relating to]
vocal fold
[voca- voice, -al relating to]
yawn
[yawn gape]
AFTER having surgery to remove a stomach tumor, Derrick woke up in the recovery room in extreme pain. It hurt to move; it hurt to blink; it hurt to take even a little breath. And here was this nurse demanding that he take a deep breath and cough. Was she crazy?
What Derrick didn’t realize was that his shallow respirations were not getting rid of as much carbon dioxide as usual. As a result, concentration of carbon dioxide in his bloodstream was building to a level that would negatively affect the homeostasis of his entire body.
Before reading this chapter, write down what you think was going on in Derrick’s respiratory system. Then compare your answer to that of the physician’s as we continue Derrick’s story at the end of this chapter.
 
Remember Derrick from the Introductory Story? See if you can answer the following questions about him now that you have read this chapter.

1. Which of these muscles would not contract when Derrick complied with his nurse’s instructions?
2. Diaphragm
3. Serratus anterior
4. Rectus abdominis
5. External intercostals
6. Which statement best describes the “mechanics” of Derrick’s inhalations?
7. The thoracic cavity decreases in size, lowering the alveolar pressure, and air flows from high (atmosphere) pressure to low (alveolar) pressure.
8. The thoracic cavity increases in size, lowering the alveolar pressure, and air flows from high (atmospheric) pressure to low (alveolar) pressure.
9. Air flows from high (atmospheric) pressure to low (alveolar) pressure and expands the thoracic cavity.
10. Air flows from high (intrapleural) pressure to low (alveolar) pressure and expands the thoracic cavity.
11. The increased carbon dioxide will make Derrick’s blood _____.
12. More acidic
13. More basic
14. More neutral
15. None of the above
16. How is carbon dioxide transported in Derrick’s blood?
17. Dissolved in the plasma
18. Bound to hemoglobin
19. In the form of bicarbonate
20. All of the above

To solve a case study, you may have to refer to the glossary or index, other chapters in this textbook, A&P Connect, Mechanisms of Disease, and other resources.
ANATOMY AND PHYSIOLOGY OF THE RESPIRATORY SYSTEM
Your respiratory system has a number of vital roles. First, it functions as a gas distributor and exchanger. It does this by supplying oxygen to your cells and removing carbon dioxide from them. Because the majority of cells are too far from the lungs to absorb oxygen directly, the lungs first use tiny sacs, called alveoli, to exchange gases from the respiratory system with those from the blood of the circulatory system.
Second, the respiratory system filters, warms, and humidifies the air we breathe, in addition to providing us with vocal communication and olfaction (the sense of smell).
Third, the respiratory system plays a vital physiological role in our bodies by regulating homeostasis of metabolism, circulation, electrolyte and water balance, and acidity of the blood. These processes are critical to proper body functioning.
In this chapter, we will explore both the structural and physiological aspects of the respiratory system.
STRUCTURAL PLAN OF THE RESPIRATORY SYSTEM
The respiratory system can be divided into upper and lower tracts. The organs of the upper tract are located outside of the thorax (in the head and neck), while the lower tract is located almost entirely within the thorax (Figure 20-1).
The upper respiratory tract is composed of the nose, nasopharynx, oropharynx, laryngopharynx, and larynx. The lower respiratory tract consists of the trachea, the bronchial tree, and the lungs. There are also accessory structures such as the oral cavity, the rib cage, as well as respiratory musculature such as the intercostals and the diaphragm.
 
FIGURE 20-1 Structural plan of the respiratory system.
UPPER RESPIRATORY TRACT
Nose
Structure of the Nose
The external portion of the nose protrudes from the face. It consists of a bony and cartilaginous framework overlaid by skin with many sebaceous glands (Figure 20-2). The two nasal bones meet the frontal bone of the skull at their superior end to form the root of the nose. They are surrounded laterally and inferiorly by the maxilla. The flaring cartilaginous expansion that forms and supports the outer side of each nostril opening is called the ala.
The palatine bones, which comprise the roof of the mouth, also form the base of the nasal cavity, separating it from the mouth. Sometimes the palatine bones fail to unite at their center, producing a condition called cleft palate. When this abnormality arises, the mouth is only partially separated from the nasal cavity. This can cause serious difficulty in swallowing and speaking unless surgically repaired.
 
FIGURE 20-2 Nasal septum. The nasal septum consists of the perpendicular plate of the ethmoid bone, the vomer, and the septal and vomeronasal cartilages.
 
FIGURE 20-3 Upper respiratory tract. In this midsagittal section through the upper respiratory tract, the nasal septum has been removed to reveal the turbinates (nasal conchae) of the lateral wall of the nasal cavity. The three divisions of the pharynx (nasopharynx, oropharynx, and laryngopharynx) are also visible.
The roof of the nose is separated from the cranial cavity by a portion of the ethmoid bone called the cribriform plate (see Figures 20-2 and 20-3). The cribriform plate is perforated by many small openings that permit branches of the olfactory nerve to enter, allowing for our sense of smell. The cribriform plate is fragile and can be damaged from trauma to the nose. Such damage makes it possible for infectious material to pass directly from the nasal cavity into the cranial fossa. Here it can infect the brain and its membranous coverings.
The hollow nasal cavity is separated by a midline partition, the nasal septum, to form left and right cavities (see Figure 20-2). The nasal septum is composed of four main structures: the perpendicular plate of the ethmoid bone above, along with the vomer bone and the septal nasal and vomeronasal cartilages below. The nasal septum of adults is frequently deviated to one side or the other, interfering with respiration and drainage of the sinuses. The nasal septum also has a rich blood supply. Nosebleeds, or epistaxis, often occur as a result of septal contusions from a blow to the nose or weak blood vessels combined with high blood pressure.
Each nasal cavity is divided into three passageways called meatuses. These passageways are created by the projection of the conchae, or turbinates, from the lateral walls of the internal portion of the nose (see Figure 20-3). The superior and middle conchae are processes of the ethmoid bones. The inferior conchae are separate bones.
The nostrils, called anterior nares (singular, naris), are enclosed by skin reflected from the ala of the nose. The nostrils open inside to an area called the vestibule, located just below the inferior meatus. Sebaceous glands, numerous sweat glands, and coarse hairs called vibrissae are found in the skin of the vestibule. Air enters the nose, passes over the vestibular skin, and then enters the respiratory portion of each nasal passage. Each passage then extends from the inferior meatus to the small funnel-shaped orifices of the posterior nares. The posterior nares are openings that pass air from the nasal cavity to the pharynx.
To briefly summarize for you, the passage of air to the pharynx begins with the anterior nares (nostrils), passes through the vestibule, through the inferior, middle, and superior meatuses (simultaneously), and then through the posterior nares to enter the pharynx.
Nasal Mucosa
After passing over the skin of the vestibule, air enters the respiratory portion of the nasal passage and passes over the respiratory mucosa. This mucous membrane has a pseudostratified ciliated columnar epithelium dense with goblet cells and is richly supplied with blood. For this reason, it is bright pink or red. Near the roof of the nasal cavity and over the superior turbinate and opposing portion of the septum is the olfactory epithelium. In contrast to the respiratory mucosa, this epithelium is paler and has a yellowish tint due to fewer blood vessels. This membrane contains many olfactory nerve cells and has a rich lymphatic plexus. Ciliated mucous membrane lines the rest of the respiratory tract down as far as the smaller bronchioles.
Paranasal Sinuses
There are four pairs of air-containing spaces called paranasal sinuses that open, or drain into the nasal cavity. They are named after the bones within which they are located: the frontal, maxillary, ethmoid, and sphenoid sinuses (see Figure 9-7, page 159). The sphenoid sinuses lie in the body of the sphenoid bone on either side of the midline close to the optic nerves and pituitary gland. Each sinus is lined by ciliated respiratory mucosa, which sweep their mucous secretions into the nose. The ethmoid sinus is actually a group of many small air cells, whereas the remaining sinuses are larger and interconnected.
The paired and often asymmetrical sinuses are small or poorly formed at birth. However, they increase in size with growth of the skull. They do not reach maximal size until after puberty. In adults, the actual sinus shapes vary considerably.
A number of health-related conditions are associated with the improper drainage of the paranasal sinuses.
Function of the Nose
The nose serves as a passageway for air to the lungs, but as you know, air can also be inhaled and exhaled through the mouth. However, air that enters through the nasal passageways is filtered of impurities, warmed, moistened, and chemically examined (by olfaction) to detect potentially irritating or toxic substances. The vibrissae, or nasal hairs, in the vestibule serve as an initial filter, screening large particulate matter from the air. Many of the remaining smaller particulates are filtered by mucous secretions from the respiratory membrane. The conchae, or turbinates, serve to slow and stir the air, which allows the air to more efficiently pass over the mucosae, as well as provide time for olfaction. Fluid from the lacrimal glands (see Figure 14-15, page 311) and additional mucus produced in the paranasal sinuses also help trap particulate matter and moisten air passing through the nose. These sinuses also serve to lighten bones of the skull and allow resonation of sounds during speech.
 

1. List the functions of the respiratory system.
2. Name the principal organs of the upper respiratory tract and the lower respiratory tract.
3. What are the paranasal sinuses? What is their relationship to the nose?

Pharynx
Structure of the Pharynx
Another name for the pharynx is the throat. It is a tubelike structure about 12.5 cm (5 inches) long that extends from the base of the skull to the esophagus and lies just anterior to the cervical vertebrae. It is made of muscle and is lined with mucous membrane.
The pharynx is divided into three anatomical divisions. The nasopharynx is located behind the nose and extends from the posterior nares to the level of the soft palate. The oropharynx is located behind the mouth and stretches from the soft palate above to the hyoid bone below. Finally, the laryngopharynx extends from the hyoid bone to the esophagus.
Seven openings are found in the pharynx (see Figure 20-3):
▪Right and left auditory (eustachian) tubes that open into the nasopharynx
▪Two posterior nares that open into the nasopharynx
▪The opening from the mouth into the oropharynx
▪The opening into the larynx from the laryngopharynx
▪The opening into the esophagus from the laryngopharynx
 
FIGURE 20-4 Laryngeal cartilages. Some softer tissues of the larynx and surrounding structures have been removed to make it possible to see the cartilages of the larynx. Note the position of the nearby thyroid gland. A, Anterior view. B, Posterior view.
The pharyngeal tonsils are located in the nasopharynx on its posterior wall opposite the posterior nares and are referred to as adenoids when they are enlarged. The oral and laryngeal divisions are able to collapse while the nasopharynx does not. However, the nasopharynx may become obstructed by enlarged adenoids, making it difficult or even impossible for air to travel from the nose into the throat.
Two pairs of tonsils are found in the oropharynx: the palatine tonsils, located back in the oropharynx, and the lingual tonsils, located at the base of the tongue. Palatine tonsils are generally the most commonly removed tonsils (see Figure 19-7, page 433).
Function of the Pharynx
The pharynx serves as a common pathway for the respiratory and digestive tracts, because both air and food must pass through this structure before reaching the appropriate tubes: the trachea for air and the esophagus for food. It also modifies speech production by changing shape, allowing humans to make different sounds, especially the variable sounds of vowels.
Larynx
Location of the Larynx
The larynx, or voice box, lies between the root of the tongue and the upper end of the trachea just below and in front of the lowest part of the pharynx (see Figure 20-1). It is like a vestibule opening into the trachea from the pharynx and normally extends between the third and sixth cervical vertebrae. However, the larynx is often higher in females and during childhood of both sexes. The lateral lobes of the thyroid gland and the carotid artery (within its sheath) touch the sides of the larynx.
Structure of the Larynx
The larynx is triangular and lined with ciliated mucous membrane. It consists predominantly of cartilages that are attached to one another and surrounding structures by muscles or fibrous and elastic tissue components (Figure 20-4). The cavity of the larynx extends from its triangular inlet at the epiglottis to the circular outlet at the lower border of the cricoid cartilage, where it is continuous with the lumen of the trachea (see Figure 20-3, B).
The mucous membrane is composed of two pairs of lateral folds called vestibular folds (“vocal cords”). The upper thick vestibular folds (“false vocal folds) have a minimal role in vocalization (Figure 20-5). In some cultures, they are used in a fascinating form of throat singing that produces many musical overtones. The false vocal folds protect the lower folds, the vocal folds, which do contribute to sound
 
FIGURE 20-5 Vocal folds. Vocal folds (“cords”) viewed from above.
production during speech. The vocal folds of most females are only about 70% as large as those of males, explaining much of the gender differences in human vocal pitch.
The vocal folds and the space between them are together designated as the glottis (see Figure 20-5). The laryngeal cavity above the vestibular folds is called the vestibule (see Figure 20-3, B).
Cartilages of the Larynx
There are nine cartilages that form the framework of the larynx. The three largest—the thyroid cartilage, the epiglottis, and the cricoid cartilage—are single (or fused) structures, while the other six are three pairs of smaller accessory cartilages named the arytenoid, corniculate, and cuneiform cartilages.
▪The thyroid cartilage (Adam’s apple) is the largest in the larynx and usually is larger, with less fat lying over it, in men than in women.
▪The epiglottis is a small leaf-shaped cartilage that projects upward behind the tongue and hyoid bone. It is attached to the thyroid cartilage. The epiglottis is mobile and blocks the trachea when swallowing food or liquids.
▪The smaller cartilages, along with the cricoid cartilage, articulate and form attachments for the vocal folds (see Figure 20-4).
Muscles of the Larynx
The larynx and its components are moved by intrinsic muscles (with origins and insertions on the larynx), as well as by extrinsic muscles (which attach to the larynx but have their origins from another structure, such as the hyoid bone). The intrinsic muscles are important for controlling vocal fold length and tension and in regulating the shape of the laryngeal inlet. Contractions of the extrinsic muscles physically move the larynx and its parts. However, both intrinsic and extrinsic muscles are important during speech, respiration, and swallowing. During swallowing, for example, the muscles that connect the arytenoid cartilage with the epiglottis raise the larynx and help prevent entry of food or fluid into the trachea. You can feel this for yourself: swallow and feel how the larynx rises in position, toward your epiglottis. Specific intrinsic muscles of the larynx also function to influence the pitch of the voice by either lengthening and tensing or shortening and relaxing the vocal folds.
Function of the Larynx
The larynx functions in respiration as part of the pathway to the lungs. It is ciliated, and its mucous membranes help filter, moisten, and warm the air entering the lungs. In addition, it protects the airway from the entrance of solids or liquids during swallowing, as we have just seen. Finally, the larynx also serves as the organ of voice production. When exhaled air passes upward toward the glottis from the trachea, it causes the vocal folds to vibrate. This vibration can be modulated by the controlled passage of air from below the larynx and by a number of muscles that increase or decrease the tension of the vocal cords. The sounds produced by the vocal folds can be further altered by the shape of the mouth, nose, and sinuses during vocal production. The size and shape of the nose, mouth, pharynx, and bony sinuses help determine the quality of the voice.
A&P CONNECT
Edema (swelling) of the mucosa covering the vocal folds and other laryngeal tissues can be a potentially lethal condition. Even a moderate amount of swelling can obstruct the glottis so much that air cannot get through and asphyxiation results. To find out more, check out Swollen Larynx online at A&P Connect.
 
 

4. Describe the three main divisions of the pharynx.
5. Where are the tonsils located?
6. Distinguish between the true and false vocal folds and their functions.

LOWER RESPIRATORY TRACT
Trachea
Structure of the Trachea
The trachea, or windpipe, is a tube about 11 cm (4.5 inches) long and 2.5 cm (1 inch) wide that extends from the larynx in the neck to the primary bronchi in the thoracic cavity (see Figure 20-1). The wall of the trachea is composed of strong C-shaped cartilaginous rings (Figure 20-6, A) that are embedded in smooth muscle. The epithelium of the trachea, like most of the rest of the respiratory tract, is pseudostratified ciliated columnar epithelium (Figure 20-6, B).
Function of the Trachea
The trachea provides a sturdy open passageway from the upper respiratory tract into the lungs so that air can pass through unobstructed. The cilia on its epithelium also continue to filter particulates from the air as it passes to the lungs. Endotracheal intubation, a procedure that keeps the trachea open, is described for you in Box 20-1.
Bronchi and Alveoli
Structure of the Bronchi
The trachea divides into two primary bronchi, the right bronchus being slightly larger and more vertical than the left. The walls of the bronchi have incomplete cartilaginous rings for support outside of the lungs, and complete rings within the lungs. Ciliated mucosae line the bronchi. Upon
 
FIGURE 20-6 Cross section of the trachea. A, The inset at the top shows from where the section was cut. B, The micrograph shows a transverse section of the trachea. Note the mucosa of ciliated epithelium. Hyaline cartilage occurs below the glandular submucosa and is not visible in this section (×70).
 
FIGURE 20-7 Alveoli. A, Respiratory bronchioles subdivide to form tiny tubes called alveolar ducts, which end in clusters of alveoli called alveolar sacs. B, Scanning electron micrograph of a bronchiole, alveolar ducts, and surrounding alveoli. The arrowhead indicates the opening of alveoli into the alveolar duct.
entering the lungs, each primary bronchus divides into smaller branches called secondary bronchi. These then branch to form tertiary bronchi, and these subdivide into bronchioles (see Figure 20-1). Bronchioles continue to branch into smaller and smaller tubes until they end in microscopic terminal bronchioles, which pass air into respiratory bronchioles and then to one or more alveolar sacs (Figure 20-7). In our two lungs, alveolar sacs contain numerous (300 million!) smaller sacs called alveoli.
BOX 20-1 Health Matters
Keeping the Trachea Open
Often, a tube is placed through the mouth, pharynx, and larynx into the trachea before surgery or before patients leave the operating room, especially if they have been given a muscle relaxant. This procedure is called endotracheal intubation. The purpose of the tube is to ensure an open airway (see parts A and B of the figure). To ensure that the tube enters the trachea rather than the nearby esophagus (which leads to the stomach), anatomical landmarks such as the vocal folds are visualized. Likewise, the distinct feel of the V-shaped groove called the interarytenoid notch (see Figure 20-5) can help guide the proper insertion of the tube.
Another procedure done frequently in today’s modern hospitals is a tracheostomy, that is, the cutting of an opening into the trachea (part C of the figure). A surgeon may perform this procedure so that a suction device can be inserted to remove secretions from the bronchial tree or so that mechanical ventilation can be used to improve ventilation of the lungs.
 
Structure of the Alveoli
The alveoli are made up of a single layer of simple squamous epithelial tissue. This single layer of cells allows oxygen and carbon dioxide gas to pass quickly down their concentration gradients between the air in the alveoli and the dissolved gases in the capillaries surrounding the alveoli. The inner surface of the alveoli is coated with a fluid containing surfactant, which helps reduce surface tension—the force of attraction between water molecules—of the alveolar fluid. This helps keep the alveoli from collapsing so they are always available for gas exchange.
Function of the Bronchi and Alveoli
The bronchi continue to filter, warm, and humidify air that is inhaled while the alveoli perform the main function of the respiratory system: gas exchange with capillaries. One of the most important portions of the filtration system of the bronchi is the mucous membrane, which produces more than 125 ml of mucus daily (the equivalent of about half a can of soda!). This mucus forms a continuous sheet that is moved upward by cilia toward the pharynx from the lower respiratory tract (see Figure 20-6, B). These cilia move in only one direction and can quickly remove pollutants from the airway. However, cilia can be paralyzed by prolonged exposure to cigarette smoke. This paralysis results in accumulation of mucus and particulates and, of course, produces the familiar “smoker’s cough.”
 

7. Why don’t the trachea and primary bronchi collapse during inspiration?
8. What is meant by the term bronchial tree?
9. Describe the characteristics of the alveoli that enable them to exchange gases.

Lungs
Structure of the Lungs
The lungs are cone-shaped organs that fill the pleural portion of the thoracic cavity completely (Figure 20-8). The medial walls of the lungs are concave and provide room for the heart. The primary bronchi and pulmonary blood vessels are bound together by connective tissue and enter each lung through a slit on the medial surface called the hilum. The inferior surfaces of the lungs rest on the muscular diaphragm, which generates the “suction” of air by making the lungs larger for a time, thus creating a vacuum. The broad inferior surface of the lung (which rests on the diaphragm) constitutes the base, whereas the pointed upper margin is the apex (Figure 20-9).
Each lung is divided into lobes by fissures: the left into two lobes and the right into three (see Figure 20-9, B). The lobes of the lungs can be further divided into functional units called bronchopulmonary segments. Visceral pleura covers the outer surfaces of the lungs and adheres to it like the skin of an apple, thus providing protection from abrasion within the pleural cavity.
Function of the Lungs
The lungs perform both air distribution and gas exchange. The capillaries of the circulatory system and the alveoli of the lungs perform the essential function of exchanging oxygen and carbon dioxide so that these gases can be carried in the blood to and from the cells in the body. These structures are extremely efficient, predominantly due to the fact that both have an enormous
 
FIGURE 20-8 Bony structures of the chest, anterior view. These structures form a protective and expandable cage around the lungs and heart.
surface area relative to their volume. In fact, it has been estimated that if the lungs’ 300 million alveoli were opened up and flattened out, they would cover the surface of a tennis court! It is this surface area that allows for rapid diffusion of gases between alveolar and capillary membranes.
Box 20-2 describes how portions of the lung can be surgically removed in extreme cases of lung damage.
A&P CONNECT
Some types of lung cancer may be curable if detected early. Learn how light can be used to treat lung cancer! Check out Photodynamic Therapy online at A&P Connect.
 
Thorax
Structure of the Thoracic Cavity
The thoracic cavity is divided by pleura to form three divisions. The mediastinum occupies the middle of the cavity, and the lateral spaces occupied by the lungs are called pleural divisions.
The parietal pleura lines the entire thoracic cavity by attaching to the inside of the ribs and superior surface of the diaphragm. Lying within the parietal pleura and continuous with it is the visceral pleura, which covers the outer surface of the lungs. An intrapleural space lies between the parietal and visceral pleura. It contains fluid for lubrication so that
 
FIGURE 20-9 Anterior view of trachea, bronchi, and lungs. A, The lower respiratory tract has been dissected from a cadaver and its organs separated to show them clearly. B, Lobes of the lungs.
the lungs can move with ease in the thoracic cavity. You should note that each lung is encased in its own pleura and is separate from the other lung (Figure 20-10).
BOX 20-2 Health Matters
Lung Volume Reduction Surgery
More than 2 million Americans, most of whom are older than age 50 and are current or former smokers, have emphysema—a major cause of disability and death in the United States. Emphysema is one of a number of conditions classified as a chronic obstructive pulmonary disease, or COPD.
Lung volume reduction surgery (LVRS) is a “treatment of last resort” for severe cases of emphysema. It involves the removal of 20% to 30% of each lung. Diseased tissue is generally removed from the upper or apical areas of the superior lobes. Evidence from a number of large clinical trials has now shown that the LVRS procedure may benefit or at least help stabilize emphysema patients whose lung function continues to decline despite aggressive pulmonary rehabilitation efforts and other more conservative forms of treatment.
 
Emphysema. A, Scanning electron micrograph (SEM) of normal lung with many small alveoli. B, SEM of lung tissue affected by emphysema. Alveoli have merged into large air spaces, thereby reducing the surface area available for gas exchange.
Although lung damage caused by emphysema is irreversible, in some cases the disease may be halted or its progression slowed by LVRS. In the end stages of this chronic disease, breathing becomes labored as the lungs fill with large irregular spaces resulting from the enlargement and rupture of many alveoli (see illustration). The LVRS procedure removes part of the diseased lung tissue and increases available space in the pleural cavities. As a result, the diaphragm and other respiratory muscles can more effectively move air into and out of the remaining lung tissue, thereby improving pulmonary function and making breathing easier.
LVRS may reduce the need for lung transplantation procedures and enhance the effectiveness of supporting medical treatments such as nutritional supplementation and exercise training in the treatment of selected late-stage emphysema patients. Newer and less invasive techniques involving smaller incisions and tiny video equipment inserted into the thoracic cavity (video-assisted thoracic surgery) are now being used for many LVRS procedures. As a result, the relatively long hospital stays and home recovery periods previously required after more traditional open-chest surgery have been shortened.
Function of the Thoracic Cavity
The thoracic cavity must enlarge greatly to create space for incoming air during respiration. This is accomplished as the ribs lift upward during inhalation so that both the depth and the width of the thorax are enlarged. The diaphragm also contracts during this action, which flattens it and makes it move downward. This creates an even larger change in the volume of the thoracic cavity. When the diaphragm relaxes, it moves upward, compressing the lungs and pushing waste gases out.
 

10. What is a “lobe” of a lung? How many lobes are there in the left and right lungs?
11. How does the diaphragm enable ventilation?

RESPIRATORY PHYSIOLOGY
Functionally, the respiratory system is composed of an integrated set of processes that include the following:
▪External respiration: pulmonary ventilation (breathing) and gas exchange in the pulmonary capillaries of the lungs
▪Transport of gases by the blood
▪Internal respiration: gas exchange in the systemic blood capillaries and cellular respiration
▪Overall regulation of respiration
Figure 20-11 summarizes the essential processes of pulmonary function. This set of processes will serve as a general framework for understanding respiratory physiology. If you wish to review the biochemistry of cell respiration, go to Chapter 4. Chapter 22 will review metabolism in even greater detail.
 
FIGURE 20-10 Lungs and pleura (transverse section). Note the parietal pleura lining the right and left pleural divisions of the thoracic cavity before folding inward near the bronchi to cover the lungs as the visceral pleura. The intrapleural space separates the parietal and visceral pleura. The heart, esophagus, and aorta are shown in the central mediastinum.
 
FIGURE 20-11 Overview of respiratory physiology. Respiratory function includes external respiration (ventilation and pulmonary gas exchange), transport of gases by blood, and internal respiration (systemic tissue gas exchange and cellular respiration). Cellular respiration is discussed separately in Chapter 4. Regulatory mechanisms centered in the brainstem use feedback from blood gas sensors to regulate ventilation.
PULMONARY VENTILATION
Mechanism of Pulmonary Ventilation
Air moves into the lungs because the volume within the lungs increases (and the air pressure within them is lowered) as the chest cavity expands and the diaphragm contracts. This expansion of the pulmonary cavity creates a pressure difference, or pressure gradient: The air outside of the body is at a higher pressure than the air within the lungs. This pressure gradient in turn causes air to rush into the lungs. Fluids and gases always move down their pressure gradients. When applied to the flow of air in the pulmonary airways, we call this concept the primary principle of ventilation. Thus, when air moves from the atmosphere of high pressure into the lower pressure of the lungs, inspiration occurs. In contrast, when the diaphragm pushes against the lungs, air pressure in the lungs is increased so that it is higher than that of the air pressure in the atmosphere. As a result, air again rushes down the pressure gradient and outside of the body, and expiration occurs.
Under standard conditions, air in the atmosphere exerts a pressure of 760 mm Hg. Air in the alveoli at the end of one expiration and before the beginning of another inspiration also exerts a pressure of 760 mm Hg. This explains why, at that moment, air is neither entering nor leaving the lungs. So, in effect, the mechanism that produces pulmonary ventilation is one that creates a gas pressure gradient between the atmosphere and the alveolar air. The pulmonary ventilation mechanism, therefore, modifies the alveolar pressure (PA, pressure within the alveoli of the lungs) to be either lower or higher than the atmospheric pressure (or barometric pressure, PB) to produce inspiration or exhalation, respectively.
These pressure gradients are established by enlarging or reducing the size of the thoracic cavity, which, as we have seen, is caused by the contraction and relaxation of respiratory muscles. These muscles produce changes in pressure that can be roughly modeled by using a balloon in a jar (Figure 20-12). The bell-shaped jar represents the rib cage (thoracic cavity), a rubber sheet across the open bottom of the bell jar represents the diaphragm, and a balloon represents the lungs. The space between the balloon and the jar represents the intrapleural space. Expanding the thorax by pulling the diaphragm downward increases the thoracic volume—thus decreasing intrapleural pressure (PIP). Because the balloon is stretchy, the decrease in PIP causes a similar decrease in the balloon pressure (analogous to alveolar pressure, PA). This creates a pressure gradient that results in airflow into the balloon (inspiration). If we let go of the elastic diaphragm, it moves back to its original position, decreasing the volume of the chest cavity, thus increasing the air pressure in the lung so that it is higher than that of the atmosphere. This causes air to rush out of the balloon (expiration).
Inspiration and Expiration
Contraction of the diaphragm alone, or contraction of both the diaphragm and the external intercostal muscles together, produces quiet inspiration. Contraction of the diaphragm (thus lowering its position) increases the volume of the thoracic cavity, and at the same time, the contraction of the external intercostals pulls the anterior end of each rib up and out (Figure 20-13). This action enlarges the
 
FIGURE 20-12 Balloon model of ventilation. The cartoons show a classic model in which a jar represents the rib cage (thoracic cavity), a rubber sheet represents the diaphragm, and a balloon represents the alveoli of the lungs. The space between the jar and balloon represents the intrapleural space. A, Inspiration, caused by downward movement of the diaphragm. B, Expiration, caused by elastic recoil of the diaphragm upward (Pressure values are expressed in mm Hg.)
 
FIGURE 20-13 Movement of the rib cage during breathing. Inspiratory muscles pull the ribs upward and thus outward, as in a bucket handle.
thorax from front to back as well as from side to side. As the thorax enlarges, the lungs expand to fill this space, causing the pressure in the bronchioles and alveoli to lower. This creates the pressure gradient necessary to have high-pressure air in the atmosphere move into the lungs in order to equalize the pressures inside and outside the lungs. Quiet expiration is a passive process that begins when the pressure gradients that resulted from inspiration are reversed by the relaxation of the inspiratory muscles. During forced expiration, abdominal and intercostal muscles can shrink the thoracic cavity such that pressure of air in the alveoli (alveolar pressure) is much larger than atmospheric pressure, thus causing a larger volume of air to move more quickly out of the lungs.
Regardless of the force exerted on the lungs and the thoracic cavity during breathing, a phenomenon called elastic recoil causes the lungs to return to their typical volume before the next inspiration.
Figure 20-14 gives you an overview of the mechanical aspects of inspiration and expiration. Please review this figure before continuing.
 

12. What is meant by pulmonary ventilation?
13. How does expansion of the thoracic cavity affect air pressure inside the lungs?
14. What is meant by the “primary principle of ventilation”?

 
FIGURE 20-14 Inspiration and expiration. A, Mechanism of inspiration. Note the role of the diaphragm and the chest-elevating muscles (pectoralis minor and external intercostals) in increasing thoracic volume, which decreases pressure in the lungs and thus draws air inward. B, Mechanism of expiration. Note that relaxation of the diaphragm plus contraction of chest-depressing muscles (internal intercostals) reduces thoracic volume, which increases pressure in the lungs and thus pushes air outward.
Pulmonary Volumes and Capacities
Pulmonary Volumes
Differing volumes of air move into and out of the lungs depending on the force with which one breathes. These volumes can be measured by a spirometer and recorded as graphics called spirograms (Figure 20-15). There is also always a residual volume of air that remains in the lungs between expiration and inspiration. All of these volumes are important to efficiency of gas exchange in the alveoli.
BOX 20-3 Health Matters
Pneumothorax
Air in the pleural space may accumulate when the visceral pleura ruptures and air from the lung rushes out, or when atmospheric air rushes in through a wound in the chest wall and parietal pleura. In either case, the lung collapses and normal respiration is impaired. Air in the thoracic cavity is a condition known as pneumothorax (see figure). To apply some of the information you have learned about the respiratory mechanism, let us suppose that a surgeon makes an incision through the chest wall into the pleural space, as is done in one of the dramatic, modern open-chest operations. What change, if any, can you deduce takes place in respirations? Compare your deductions with those in the next paragraph.
 
Pneumothorax. Diagram showing air entering the thoracic cavity, causing lung collapse.
Intrapleural pressure, of course, immediately increases from its normal subatmospheric level to the atmospheric level. More pressure than normal is therefore exerted on the outer surface of the lung and causes it to collapse. It could even collapse the other lung. Why? Because the mediastinum is a mobile rather than a rigid partition between the two pleural sacs. This anatomical fact allows the increased pressure in the side of the chest that is open to push the heart and other mediastinal structures over toward the intact side, where they would exert pressure on the other lung. Pneumothorax can also result from disruption of the visceral pleura and the resulting flow of pulmonary air into the pleural space.
Pneumothorax results in many respiratory and circulatory changes. They are of great importance in determining medical and nursing care but lie beyond the scope of this book.
The volume of air exhaled after a normal breath is termed tidal volume (TV) and is about 500 ml (or 0.5 L) in an average resting adult. After releasing tidal air, an individual can force still more air out of the lungs. This is called the expiratory reserve volume (ERV)—typically about 1,000 ml for an average adult. Inspiratory reserve volume (IRV) is the amount of air that can be forcibly inspired over and above a normal inspiration, usually about 3,300 ml. It is measured by having the individual inhale forcefully after a normal inspiration. Regardless of the forcefulness of an exhalation, some air remains in the lungs, trapped in the alveoli. This air is called the residual volume (RV) and amounts to about 1,200 ml. Between breaths, an exchange of oxygen and carbon dioxide occurs between the trapped residual air in the alveoli and the blood. This process helps keep amounts of oxygen and carbon dioxide constant in the blood.
In a condition called pneumothorax (Box 20-3), the RV is eliminated when the lung collapses. Even after the RV is forced out, the collapsed lung has a porous, spongy texture and floats in water because of trapped air (equal to about 40% of the RV).
 
FIGURE 20-15 Pulmonary ventilation volumes and capacities. A spirogram.
Pulmonary Capacities
A pulmonary capacity is the sum of two or more pulmonary “volumes.” The vital capacity (VC), for example, is the sum of the IRV, TV, and ERV (see Figure 20-15). The vital capacity represents the largest volume of air an individual can move in and out of the lungs. In general, a larger person has a larger vital capacity than a smaller person. VC can depend on the size of a person’s thoracic cavity, his or her posture, and a number of other factors. For example, an individual has a larger VC standing up than lying down. The volume of blood in the lungs can also affect VC. The vital capacity may be lowered if the lungs contain more blood than normal and the alveolar air space is diminished. Excess fluid in the pleural or abdominal cavities also decreases vital capacity, as does the disease emphysema. In emphysema, alveoli become stretched and lose their elasticity and are unable to recoil during expiration, leaving an increased RV in the lungs, thus making each inspiration and expiration require more effort.
In diagnosing lung disorders, a physician may need to know the inspiratory capacity (IC), which is the maximal amount of air an individual can inspire after a normal expiration. As you can determine from Figure 20-15, IC is the sum of the TV and the IRV. Functional residual capacity (FRC) is the amount of air left in the lungs at the end of a normal expiration and is thus the sum of the ERV and the RV. The total lung capacity (TLC) is the sum of all four lung volumes (see Figure 20-15).
Table 20-1 summarizes typical pulmonary volumes and capacities for you.
Remember that only the volume of air that reaches the alveoli can be involved in gas exchange. This means that the rest of the air filling the pharynx, larynx, trachea, and bronchi is effectively “dead space.” When alveoli cannot perform their function due to disorders such as chronic obstructive pulmonary disease (COPD), this dead space is increased, making gas exchange less efficient and breathing a difficulty.
 

15. What is the difference between a pulmonary volume and a pulmonary capacity?
16. What is meant by the term vital capacity? What is it equal to?
17. What is a spirogram?

PULMONARY GAS EXCHANGE
Partial Pressure
Partial pressure means the pressure exerted by any one gas in a mixture of gases or in a liquid. According to the law of partial pressures, the partial pressure of a gas in a mixture of gases is directly related to the concentration of that gas in the mixture and to the total pressure of the mixture. About 78% of the atmosphere is made up of nitrogen, which means the partial pressure of nitrogen (PN2) is 78% of the total atmospheric pressure. Thus, PN2 is 592.8 mm (that is, 78% of 760 mm).
TABLE 20-1 Pulmonary Volumes and Capacities
NAME
DESCRIPTION
TYPICAL VALUE
Volumes
Tidal volume (TV)
Volume moved into or out of the respiratory tract during a normal respiratory cycle
500 ml (0.5 L)
Inspiratory reserve volume (IRV)
Maximum volume that can be moved into the respiratory tract after a normal Inspiration
3,000-3,300 ml (3.0-3.3 L)
Expiratory reserve volume (ERV)
Maximum volume that can be moved out of the respiratory tract after a normal expiration
1,000-1,200 ml (1.0-1.2 L)
Residual volume (RV)
Volume remaining in the respiratory tract after maximum expiration
1,200 ml (1.2 L)
Capacities
Vital capacity (VC)
Largest volume of air that can be moved in and out of the lungs: TV + IRV + ERV
4,500-5,000 ml (4.5-5.0 L)
Inspiratory capacity (IC)
Maximal amount of air that can be inspired after a normal expiration: TV + IRV
3,500-3,800 ml (3.5-3.8 L)
Functional residual capacity (FRC)
Amount of air left in the lungs at the end of a normal expiration: ERV + RV
2,200-2,400 ml (2.2-2.4 L)
Total lung capacity (TLC)
Total volume of air that the lungs can hold: TV + IRV + ERV + RV
5,700-6,200 ml (5.7-6.2 L)
The partial pressure of a gas such as oxygen (O2) in a liquid such as blood is directly proportional to the amount of that gas dissolved in the liquid, which in turn is determined by the partial pressure of the gas in the environment of the liquid. The partial pressure of oxygen (PO2) in both the alveoli and arterial blood is about 100 mm Hg, while in venous blood, it is about 37 mm Hg. Gas molecules will diffuse into alveolar blood from its gaseous environment in the alveoli and dissolve until the partial pressure of the gas in the blood becomes equal to its partial pressure in the alveoli. Therefore, arterial blood PO2 and partial pressure of carbon dioxide (PCO2) are usually very nearly equal to alveolar PO2 and PCO2.
Exchange of Gases in the Lungs
As we’ve seen, the exchange of gases in the lungs takes place between alveolar air and blood flowing through lung capillaries. It is important to note that the alveoli present not only a membrane that exchanges these gases efficiently but also a barrier between the internal world of the body and the outside environment. With this interpretation, the airways can be viewed simply as inward extensions of the external
 
FIGURE 20-16 External-internal barrier. The respiratory membranes of the lung represent an interface or barrier that gases must cross to enter or exit the body’s internal environment. The pulmonary airway is merely an extension of the external environment.
environment (Figure 20-16). This means that inspired air is not part of the internal environment.
Gases in the alveoli are exchanged in both directions across the respiratory membrane. This is because the blood returning from the body is low in oxygen but high in carbon dioxide, so its PO2 is much lower than that of alveolar air while its PCO2 is much higher (Figure 20-17). This causes oxygen to diffuse rapidly into the blood while carbon dioxide diffuses out of the blood. Gas exchange in the alveoli is facilitated by their very thin walls (only 0.004 mm thick) and the fact that alveolar and capillary surface areas are extremely large relative to their volume.
A&P CONNECT
Many conditions leave us feeling like we need more oxygen—whether it is strenuous exercise or an abnormal respiratory or cardiovascular condition. Getting extra oxygen for therapeutic, sports, and even recreational use is explored in Oxygen Supplements online at A&P Connect.
 
 

18. How does the partial pressure of a gas relate to its concentration?
19. What determines the pressure gradient for oxygen and carbon dioxide?

HOW BLOOD TRANSPORTS GASES
Blood transports oxygen and carbon dioxide either as solutes or combined with other chemicals. Because fluids can dissolve only small amounts of gases, most of the oxygen and
 
FIGURE 20-17 Pulmonary gas exchange. A, As blood enters a pulmonary capillary, oxygen diffuses down its pressure gradient (into the blood). Oxygen continues diffusing into the blood until equilibration has occurred (or until the blood leaves the capillary). B, As blood enters a pulmonary capillary, carbon dioxide diffuses down its pressure gradient (out of the blood). As with oxygen, carbon dioxide continues diffusing as long as there is a pressure gradient. PO2 and PCO2 remain relatively constant in a continually ventilated alveolus.
carbon dioxide transported in the blood is chemically united (soon after being dissolved) with other compounds such as hemoglobin, plasma protein, or water. Once gas molecules are bound to another molecule, their plasma concentration decreases so that more gas can diffuse from the alveoli into the plasma. This mechanism greatly increases the overall amount of gas that can be transported by the blood. The primary gas transport molecule of the blood is hemoglobin.
Hemoglobin
Hemoglobin (Hb) is a conjugated protein made of four polypeptide chains (two alpha chains and two beta chains). In Figure 20-18, you can see that each chain is associated with an iron-containing heme group, and each iron atom can bind an oxygen molecule (O2). In a way, hemoglobin acts
 
FIGURE 20-18 Hemoglobin. Sketch showing that hemoglobin is a quaternary protein consisting of four different tertiary (folded) polypeptide chains—two alpha (α) chains and two beta (β) chains. Each chain has an associated iron-containing heme group. Oxygen (O2) can bind to the iron of the heme group, or carbon dioxide (CO2) can bind to amine groups of the amino acids in the polypeptide chains.
like an oxygen sponge. However, it can also act as a carbon dioxide sponge by absorbing carbon dioxide molecules from solution.
Gas Transport in the Blood
Oxygen
Oxygen travels in two forms: as dissolved oxygen in the plasma and as oxygen associated with hemoglobin (oxyhemoglobin).
 
FIGURE 20-19 Carbon dioxide–hemoglobin reaction. Carbon dioxide can bind to an amine group (NH2) in an amino acid within a hemoglobin (Hb) molecule to form carbaminohemoglobin (HbNCOOH−) and a hydrogen ion. The highlighted areas show where the original carbon dioxide molecule is in each part of the equation.
Of these two forms of transport, oxyhemoglobin carries the vast majority of the total oxygen transported by the blood.
Carbon Dioxide
Only about 10% of the carbon dioxide in blood travels as dissolved gas; the rest travels as carbaminohemoglobin (Figure 20-19) and as bicarbonate, according to the equation below:
 
Carbaminohemoglobin is an important carrier of blood carbon dioxide and is formed when CO2 binds to an amine group on hemoglobin. This association is made more quickly and efficiently when there is more CO2 in the blood and is also slowed when there is a lowering of PCO2 in the blood.
Figure 20-20, which summarizes all three forms of carbon dioxide transport, shows that once bicarbonate ions are
 
FIGURE 20-20 Carbon dioxide transport in the blood. As the illustration shows, CO2 dissolves in the plasma. Some of the dissolved CO2 enters red blood cells (RBCs) and combines with hemoglobin (Hb) to form carbaminohemoglobin (HbCO2). Some of the CO2 entering RBCs combines with H2O to form carbonic acid (H2CO3), a process facilitated by the enzyme carbonic anhydrase (CA) present inside each cell. Carbonic acid then dissociates to form H+ and bicarbonate (HCO3−). The H+ combines with Hb, whereas the HCO3− diffuses down its concentration gradient into the plasma. As HCO3− leaves each RBC, Cl− enters and prevents an imbalance in charge—a phenomenon called the chloride shift.
formed, they diffuse down their concentration gradient into the plasma.
When carbon dioxide enters the blood and generates carbaminohemoglobin or bicarbonate, it also produces H+ ions as a result. Thus, when CO2 enters into the system, it increases the amount of hydrogen ions. This lowers the pH of the blood and makes it more acidic. In Chapter 23 we’ll explore, briefly, why this is significant.
Carbon Monoxide
Gases other than oxygen and carbon dioxide can also bind to hemoglobin. For example, carbon monoxide (CO) binds more than 200 times more strongly to hemoglobin than oxygen does. (Carbon monoxide is a molecule produced by incomplete combustion in furnaces and engines.) In effect, carbon monoxide can replace oxygen at its binding sites with hemoglobin, greatly reducing the amount of oxygen carried in your blood. In addition, CO binds so strongly that it is hard to remove. This can cause a deadly situation. One treatment is to administer 100% oxygen (with a tight mask) to an afflicted person. Sometimes, under critical circumstances, the patient
 
FIGURE 20-21 Systemic gas exchange. A, As blood enters a systemic capillary, O2 diffuses down its pressure gradient (out of the blood). O2 continues diffusing out of the blood until equilibration has occurred (or until the blood leaves the capillary). B, As blood enters a systemic capillary, CO2 diffuses down its pressure gradient (into the blood). As with O2, CO2 continues diffusing as long as there is a pressure gradient.
is placed in a pressure chamber where the PO2 can be driven so high it literally displaces the CO from the hemoglobin, allowing oxygen once again to bind to its carrier.
A & P CONNECT
Variations of hemoglobin exist in the body to temporarily store or carry oxygen. Find out why we need more than one type of oxygen carrier in the body in Oxygen-binding Proteins online at A&P Connect.
SYSTEMIC GAS EXCHANGE
Exchange of gases in tissues takes place between arterial blood flowing through tissue capillaries (Figure 20-21), according to their pressure gradients. Thus, oxygen diffuses out of arterial ends of the capillaries and into cells, while carbon dioxide diffuses out of cells and into venous ends of the capillaries. As cells use oxygen to metabolize sugars and other organic compounds, the PO2 of the cells lowers compared to the PO2 of arterial capillaries. This causes a movement of oxygen into the cells, to equalize the pressure. The arterial blood also has a low PCO2 in comparison to metabolizing cells, which are producing more and more carbon dioxide. As a result, carbon dioxide travels down its pressure gradient into the venous capillaries, which then transport blood back to the lungs to rid the body of its waste CO2.
As activity increases in any tissue, its cells necessarily use oxygen more rapidly. This decreases intracellular PO2, which means there is a larger pressure gradient between blood and tissues, which accelerates oxygen diffusion out of the tissue capillaries. Similarly, there is an increase in CO2 production in the cells, which increases the difference in pressures between the cells and the capillaries, so more CO2 moves out of the cells and into the venous capillaries. The increasing PCO2 and decreasing PO2 produce two effects—they favor oxygen dissociation from oxyhemoglobin and carbon dioxide associating with hemoglobin to generate carbaminohemoglobin. This means that, as activity increases, carbon dioxide is more efficiently removed from the system while oxygen is more efficiently unloaded from hemoglobin in order to supply the highly active cells.
REGULATION OF PULMONARY FUNCTION
Respiratory Control Centers
Various mechanisms maintain the relative constancy of the blood PO2 and PCO2. This homeostasis of blood gases is maintained primarily by means of changes in ventilation—the rate and depth of breathing. The main integrators that control the nerves that affect the inspiratory and expiratory muscles are located within the brainstem. Together these integrators are simply called the respiratory centers (Figure 20-22) and they serve together to regulate our breathing patterns.
The basic rhythm of the respiratory cycle of inspiration and expiration is generated by the medullary rhythmicity area. This area contains two regions of interconnected control centers: the dorsal respiratory group (DRG) and the ventral respiratory group (VRG). It is thought, based on research with animal models, that the VRG controls the basic rhythm of breathing while the DRG integrates information from chemoreceptors for PCO2 and signals the VRG to alter breathing rhythm to restore homeostasis.
 
FIGURE 20-22 Regulation of breathing. The dorsal respiratory group (DRG) and ventral respiratory group (VRG) of the medulla represent the medullary rhythmicity area. The pontine respiratory group (PRG) and apneustic center of the pons influence the basic respiratory rhythm by means of neural input to the medullary rhythmicity area. The brainstem also receives input from other parts of the body; information from chemoreceptors, baroreceptors, and stretch receptors can alter the basic breathing pattern, as can emotional (limbic) and sensory input. Despite these subconscious reflexes, the cerebral cortex can override the “automatic” control of breathing to some extent to do such activities as sing or blow up a balloon. Green arrows show flow of information to the respiratory control centers. The purple arrow shows the flow of information from the control centers to the respiratory muscles that drive breathing.
Additional regulatory centers in the pons, the pontine respiratory group (PRG) and the apneustic center can influence the activity of the medullary rhythaicity area.
Box 20-4 examines unusual breathing reflexes such as coughing and sneezing.
Factors that Influence Breathing
Feedback information to the medullary rhythmicity area comes from sensors throughout the nervous system, as well as from other control centers. For example, changes in pH and the partial pressures of carbon dioxide and oxygen within the systemic arterial blood all influence the medullary rhythmicity area.
The normal PCO2 (about 38 to 40 mm Hg) in the blood acts on chemoreceptors in the medulla, which also monitor the pH of blood. When CO2 increases or pH decreases (becoming more acidic) even slightly above the homeostatic values, it stimulates central chemoreceptors, which are present throughout the brainstem. Larger increases in arterial carbon dioxide also stimulate peripheral chemoreceptors in the carotid bodies and the aorta. Both of these will result in faster breathing relative to the increase of CO2 in the blood. Figure 20-23 summarizes this negative feedback response (which acts much like a thermostat in your house).
 
FIGURE 20-23 Negative feedback control of respiration. This diagram summarizes the feedback loop that operates to increase the respiratory rate in response to high plasma PCO2. Increased cellular respiration during exercise causes a rise in plasma PCO2, which is detected by central chemoreceptors in the brain and perhaps peripheral chemoreceptors in the carotid sinus and aorta. Feedback information is relayed to integrators in the brainstem that respond to the increase in PCO2 above the set-point value by sending nervous correction signals to the respiratory muscles, which act as effectors. The effector muscles increase their alternating contraction and relaxation, thus increasing the rate of respiration. As the respiration rate increases, the rate of CO2 loss from the body increases and PCO2 drops accordingly. This brings the plasma PCO2 back to its set-point value.
BOX 20-4 FYI
Unusual Breathing Reflexes
The cough reflex is stimulated by foreign matter in the trachea or bronchi. The epiglottis and glottis reflexively close, and contraction of the expiratory muscles causes air pressure in the lungs to increase. The epiglottis and glottis then open suddenly, resulting in an upward burst of air that removes the offending contaminants—a cough.
 
The sneeze reflex is similar to the cough reflex, except that it is stimulated by contaminants in the nasal cavity. A burst of air is directed through the nose and mouth, forcing the contaminants (and mucus) out of the respiratory tract. Droplets from a sneeze can travel more than 161 km/hr (100 miles/hr) and travel 3 m (9.8 ft). Research suggests that many pathogenic microbes produce symptoms that trigger sneezing in order to spread themselves to other people. Scientists call this altered host behavior and identify it as a mechanism of microbes to efficiently spread themselves to additional human hosts.
The term hiccup is used to describe an involuntary, spasmodic contraction of the diaphragm. When such a contraction occurs, generally at the beginning of an inspiration, the glottis suddenly closes, producing the characteristic sound. Hiccups lasting for extended periods can be disabling. They may be produced by irritation of the phrenic nerve or the sensory nerves in the stomach or by direct injury or pressure on certain areas of the brain. Fortunately, most cases of hiccups last only a few minutes and are harmless.
A yawn is slow, deep inspiration through an unusually widened mouth. Yawns were once thought to be reflexes that increase ventilation when blood oxygen content is low, but newer evidence suggests that this is unlikely. A current theory states that we yawn for the same reason we occasionally stretch—to prepare our muscles and our circulatory system for action. Recent alternate hypotheses suggest that yawning cools the brain or otherwise regulates body temperature—or that yawning is triggered by neurotransmitters related to mood. The variety of hypotheses show one thing for certain: we do not currently understand the physiology of yawning!
A protective physiological response called the diving reflex is responsible for the astonishing recovery of apparent drowning victims—including some who may have been submerged for more than 40 minutes! Survivors are most often preadolescent children who have been immersed in water below 20° C (68° F). Apparently, the colder the water, the better the chance of survival. Victims initially appear dead when pulled from the water. Breathing has stopped; they have fixed, dilated pupils; they are cyanotic; and their pulse has stopped.
Studies have shown that, when the head and face are immersed in ice-cold water, there is immediate shunting of blood to the core body areas with peripheral vasoconstriction and slowing of the heart (bradycardia). Metabolism is slowed, and tissue requirements for oxygen and nutrients decrease. The diving reflex is a protective response of the body to cold water immersion and is a function of such physiological and environmental parameters as water temperature, age, lung volume, and posture.
Control of Respirations During Exercise
Respirations increase abruptly at the beginning of exercise and decrease even more markedly as it ends. This much is known. The mechanism that accomplishes this increased ventilation rate, however, is not known. It is not identical to the one that produces moderate increases in breathing. Numerous studies have shown that arterial blood PCO2, PO2, and pH do not change enough during exercise to produce the degree of hyperpnea (faster, deeper respirations) observed. Presumably, many chemical and nervous factors and temperature changes operate as a complex, but still unknown, mechanism for regulating respirations during exercise (Figure 20-24).
 

20. In what form is most oxygen transported in the blood?
21. In what form is most carbon dioxide transported in the blood?
22. How does carbon dioxide affect the pH of blood?

 
FIGURE 20-24 Normal effects of maximum exercise in an athlete. This graph shows that the breathing rate (vertical axis) is much higher in an athlete exercising maximally than would be expected for any given blood carbon dioxide pressure (PCO2) (horizontal axis). As you can see at the normal points of a PCO2 of 40 mm Hg, the exercising athlete’s breathing (ventilation) rate is 120 L/min. However, at rest the athlete’s breathing rate is only about 5 or 6 L/min at the same PCO2—thus showing that PCO2 is not the major factor causing an increased rate of breathing during exercise.
MECHANISMS OF DISEASE
There are many common disorders and diseases of the respiratory system. These include disorders associated with the upper respiratory tract, such as rhinitis, laryngitis, and tonsillitis, as well as anatomical disorders that can cause sleep apnea and chronic nosebleeds. Disorders and diseases of the lower respiratory tract are also common, and include acute bronchitis, pneumonia, and less common diseases such as tuberculosis. Even more significant are various types of lung cancer, which are among the most common and deadly of all cancers.
Disorders associated with respiratory function include restrictive pulmonary disorders, obstructive pulmonary disorders, and chronic obstructive pulmonary diseases (COPD) such as bronchitis, emphysema, and asthma.
Find out more about these diseases and disorders of the respiratory system online at Mechanisms of Disease: Respiratory System.
Cycle of LIFE
Respiratory exchange of oxygen and carbon dioxide must occur between air in the lungs and blood and, following this, between blood and every body cell. Hemoglobin plays a vital role in the respiration process, in conjunction with the structural components of the body through which the respiratory gases must pass. Each component of the system may be affected by developmental defects, by age-related structural changes, or by loss of function during the life cycle.
Premature birth can cause potentially fatal respiratory problems. A very low–birth weight baby may have inadequate blood flow to the lungs, an inability to ventilate properly, and inadequate quantities of surfactant. Other diseases that cause serious respiratory problems include cystic fibrosis and asthma and certain types of obstructive pulmonary disease and emphysema in older adults. Pneumothorax occurs more frequently in young adult females as a complication of endometriosis in the thorax (see Chapter 25).
Numerous age-related changes affect vital capacity, make ventilation difficult, or reduce the oxygen or carbon dioxide carrying capacity of blood. For example, in older adulthood the ribs and sternum tend to become more fixed and less able to expand during inspiration, the respiratory muscles are less effective, and hemoglobin levels are often reduced. The result is a general reduction in respiratory efficiency in old age.
The BIG Picture
Understanding the relationship of structure to function is critical to an understanding of homeostasis in all of the body organ systems. The anatomy of the respiratory system components permits the distribution of air and the exchange of respiratory gases. This dual function ultimately allows for both exchange of gases between environmental air and blood in the lungs and, finally, gas exchange between blood and individual body cells. In addition to delivery of air to the tiny terminal air passageways and alveoli for gas exchange with blood, components of the upper respiratory tract effectively filter, warm, and humidify the air we breathe.
Respiratory functions are dependent on the structural organization of the system parts and on the interrelationship of those components with other body systems, including the nervous, cardiovascular, muscular, and immune systems. For example, nerves regulate the thoracic and abdominal muscles that drive breathing, as well as the smooth muscles that regulate airflow through the bronchial tree. The immune system guards against airborne pathogens and irritants.
The homeostatic balance of the entire body, and thus the survival of each and every cell, depends on the proper functioning of the respiratory system. This is because the mitochondria in each cell require oxygen for their energy conversions. In addition, each cell produces toxic carbon dioxide as a waste product from these energy conversions, and the internal environment must continually acquire new oxygen and discard carbon dioxide. If each cell were immediately adjacent to the external environment—that is, atmospheric air—this would require no special system. However, because almost every one of the 100 trillion cells that make up the body are far removed from the outside air, another method of satisfying this condition must be employed; this is where the respiratory system comes in.
By the physical process of ventilation, fresh external air continually flows less than a hair’s breadth away from the circulating fluid of the body—the blood. By means of diffusion, oxygen enters the internal environment and carbon dioxide leaves. The efficiency of this process is enhanced by hemoglobin, which immediately takes oxygen molecules out of solution in the plasma so that more oxygen can rapidly diffuse into the blood. The blood, the circulating fluid tissue of the cardiovascular system, carries the blood gases throughout the body—picking up gases where there is an excess and unloading them where there is a deficiency. In this manner, each cell of the body is continually bathed in a fluid environment that offers a constant supply of oxygen and an efficient system for removing carbon dioxide.
Specific mechanisms involved in respiratory function show the interdependence between body systems observed throughout our study of the human body. For example, without blood and the maintenance of blood flow by the cardiovascular system, blood gases could not be transported between the gas exchange tissues of the lungs and the various systemic tissues of the body. Without regulation by the nervous system, ventilation could not be adjusted to compensate for changes in the oxygen or carbon dioxide content of the internal environment. Without the skeletal muscles of the thorax, the airways could not maintain the flow of fresh air that is so vital to respiratory function. The skeleton itself provides a firm outer housing for the lungs and has an arrangement of bones that facilitates the expansion and recoil of the thorax, which is needed to accomplish inspiration and expiration. Without the immune system, pathogens from the external environment could easily colonize the respiratory tract and possibly cause a fatal infection.
Even more subtle interactions between the respiratory system and other body systems can be found. For example, the language function of the nervous system is limited without the speaking ability provided by the larynx and other structures of the respiratory tract. The homeostasis of pH, which is regulated by a variety of systems, is influenced by the respiratory system’s ability to adjust the body’s carbon dioxide levels (and thus the levels of carbonic acid).
CHAPTER SUMMARY
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ANATOMY AND PHYSIOLOGY OF THE RESPIRATORY SYSTEM

1. Respiratory system functions as a gas distributor and exchanger
2. Respiratory system filters, warms, and humidifies the air we breath, in addition to providing us with vocal communication and olfaction
3. Respiratory system also plays a vital physiological role in our bodies by regulating homeostasis of metabolism, circulation, electrolyte and water balance, and acidity of the blood

STRUCTURAL PLAN OF THE RESPIRATORY SYSTEM

1. Respiratory system can be divided into upper and lower tracts:
2. Upper respiratory tract—composed of the nose, nasopharynx, oropharynx, laryngopharynx, and larynx
3. Lower respiratory tract—consists of the trachea, the bronchial tree, and the lungs
4. Accessory structures include the oral cavity, rib cage, intercostals, and diaphragm

UPPER RESPIRATORY TRACT

1. Nose
2. Structure of the nose
3. External portion consists of a bony and cartilaginous framework overlaid by skin with many sebaceous glands
4. Two nasal bones meet the frontal bone of the skull at their superior end to form the root
5. Palatine bones form the base of the nasal cavity

(1) Cleft palate—condition when the palatine bones fail to unite at their center

1. Cribriform plate—separates the roof of the nose from the cranial cavity (Figures 20-2 and 20-3)
2. Nasal septum—separates the nasal cavity into right and left cavities; made up of four main structures (Figure 20-2):

(1) Perpendicular plate of the ethmoid bone
(2) Vomer bone
(3) Septal nasal cartilage
(4) Vomeronasal cartilage

1. Nasal cavity is divided into three passageways (meatuses): created by the projection of the conchae (Figure 20-3)
2. Nostrils (anterior nares)—enclosed by skin reflected from the ala of the nose; open inside to the vestibule
3. Passage of air to the pharynx begins with the anterior nares, passes through the vestibule, through the inferior, middle, and superior meatuses (simultaneously), and then through the posterior nares to enter the pharynx
4. Nasal mucosa—mucous membrane that air passes over; it contains a rich blood supply
5. Olfactory epithelium—contains many olfactory nerve cells and has a rich lymphatic plexus
6. Ciliated mucous membrane lines the rest of the respiratory tract down as far as the smaller bronchioles
7. Paranasal sinuses—four pairs of air-containing spaces that open or drain into the nasal cavity; each sinus is lined by ciliated respiratory mucosa
8. Functions of the nose
9. Serves as a passageway for air to the lungs
10. Air that enters through the nasal passageways is filtered of impurities, warmed, moistened, and chemically examined
11. Sinuses serve to lighten bones of the skull and allow resonation of sounds during speech
12. Pharynx (throat)—tubelike structure that extends from the base of the skull to the esophagus; made of muscle and is lined with mucous membrane
13. Pharynx is divided into the three anatomical divisions:
14. Nasopharynx—located behind the nose and extends from the posterior nares to the level of the soft palate
15. Oropharynx—located behind the mouth and stretches from the soft palate above to the hyoid bone below
16. Laryngopharynx—extends from the hyoid bone to the esophagus
17. Seven openings are found in the pharynx (Figure 20-3):
18. Right and left auditory (eustachian) tubes that open into the nasopharynx
19. Two posterior nares that open into the nasopharynx
20. The opening from the mouth into the oropharynx
21. The opening into the larynx from the laryngopharynx
22. The opening into the esophagus from the laryngopharynx
23. Pharyngeal tonsils—located in the nasopharynx; referred to as adenoids when they are enlarged
24. Palatine tonsils—located back in the oropharynx; most commonly removed
25. Lingual tonsils—located at the base of the tongue
26. Pharynx serves as a common pathway for the respiratory and digestive tracts
27. Larynx (voice box)—lies between the root of the tongue and the upper end of the trachea; below and in front of the lowest part of the pharynx (Figure 20-1)
28. Lined with ciliated mucous membrane
29. Consists predominantly of cartilages attached to one another; surrounded by muscles or fibrous and elastic tissue components (Figure 20-4)
30. Vestibular folds (false vocal cords)
31. Vocal folds—contribute to sound production during speech
32. Cartilages of the larynx—nine cartilages form the framework of the larynx (Figure 20-4):
33. Thyroid cartilage
34. Epiglottis
35. Cricoid cartilage
36. Arytenoid (two)
37. Corniculate (two)
38. Cuneiform (two)
39. Muscles of the larynx—intrinsic and extrinsic muscles
40. Intrinsic muscles—control vocal fold length and tension and are important in regulating the shape of the laryngeal inlet
41. Extrinsic muscles—physically move the larynx and its parts
42. Larynx forms part of the pathway to the lungs and produces voice

LOWER RESPIRATORY TRACT

1. Trachea (windpipe)—tube that extends from the larynx in the neck to the primary bronchi in the thoracic cavity (Figure 20-8)
2. Wall of the trachea is composed of C-shaped cartilaginous rings (Figure 20-6)
3. Provides a sturdy open passageway from the upper respiratory tract into the lungs
4. Bronchi and Alveoli
5. Trachea divides into two primary bronchi; right bronchus slightly larger and more vertical than the left
6. Primary bronchi divide into smaller branches called secondary bronchi
7. Branch to form tertiary bronchi and then further on into bronchioles
8. Bronchioles continue to branch into microscopic terminal bronchioles; pass air into respiratory bronchioles and then alveolar sacs
9. Alveolar sacs contain numerous smaller sacs called alveoli
10. Alveoli are made up of a single layer of simple squamous epithelial tissue
11. Allows oxygen and carbon dioxide gas to pass quickly from alveoli to capillary
12. Surface of the alveoli is coated with a fluid containing surfactant; keeps the alveoli from collapsing
13. Functions of the bronchi and alveoli—distribute air to the lung’s interior
14. Lungs—cone-shaped organs that fill the pleural portion of the thoracic cavity completely (Figures 20-8 and 20-9)
15. Hilum—slit on the lung’s medial surface where the primary bronchi and pulmonary blood vessels enter
16. Base—broad inferior surface of the lung
17. Apex—pointed upper margin of the lung
18. Each lung is divided into lobes by fissures; the left into two lobes and the right into three
19. Lobes of the lungs can be further divided into functional units; bronchopulmonary segments
20. Visceral pleura—covers the outer surfaces of the lungs; provides protection from abrasion within the pleural cavity
21. Lungs perform both air distribution and gas exchange
22. Thorax—thoracic cavity is divided by pleura to form three divisions
23. Mediastinum—middle of thoracic cavity
24. Pleural divisions—the part occupied by the lungs
25. Parietal pleura—lines the entire thoracic cavity by attaching to the inside of the ribs and superior surface of the diaphragm
26. Visceral pleura—covers the lungs entirely
27. Functions of the thorax—brings about inspiration and expiration

RESPIRATORY PHYSIOLOGY

1. Respiratory system is composed of an integrated set of processes (Figure 20-11):
2. External respiration
3. Transport of gases by the blood
4. Internal respiration
5. Overall regulation of respiration

PULMONARY VENTILATION

1. Mechanism of pulmonary ventilation—air moves into the lungs because the volume within the lungs increases and the air pressure within them is lowered
2. Pressure gradient—air outside of the body is at a higher pressure than the air within the lungs
3. Pressure gradient causes air to rush into the lungs
4. Primary principle of ventilation
5. When air moves from the high pressure of the atmosphere into the lower pressure of the lungs, inspiration occurs
6. When the diaphragm pushes against the lungs, air pressure in the lungs is increased so that it is higher than that of the air pressure in the atmosphere; air rushes down the pressure gradient and outside of the body, and expiration occurs
7. Pressure gradients are established by enlarging or reducing the size of the thoracic cavity
8. Inspiration and expiration (Figure 20-14)
9. Inspiration—contraction of the diaphragm produces inspiration; as it contracts, it makes the thoracic cavity larger (Figure 20-13)

(1) Expansion of the thorax results in lungs expanding to fill space, causing pressure in bronchioles and alveoli to lower
(2) Air moves into the lungs in order to equalize pressure inside and outside

1. Expiration

(1) Quiet expiration—passive process that begins when the pressure gradients that resulted from inspiration are reversed by the relaxation of the inspiratory muscles
(2) Forced expiration—abdominal and intercostals muscles shrink thoracic cavity so pressure in alveoli is much large than atmospheric pressure, causing large volume of air to move quickly out of lungs
(3) Elastic recoil—lungs to return to their typical volume before the next inspiration

1. Pulmonary volumes—differing volumes of air move into and out of the lungs depending on the force with which one breathes
2. Volumes can be measured by a spirometer; recorded as graphics called spirograms (Figure 20-15)
3. Tidal volume (TV)—volume of air exhaled after a normal breath; about 500 ml
4. Expiratory reserve volume (ERV)—the volume of air an individual can force out of the lungs after releasing tidal air; about 1,000 ml
5. Inspiratory reserve volume (IRV)—amount of air that can be forcibly inspired over and above a normal inspiration; about 3,300 ml
6. Residual volume (RV)—amount of air that cannot be forcibly exhaled (1,200 ml)
7. Pulmonary capacity—sum of two or more pulmonary “volumes” (Figure 20-15 and Table 20-1)
8. Vital capacity (VC)—sum of the IRV, TV, and ERV
9. Inspiratory capacity (IC)—sum of the TV and the IRV
10. Functional residual capacity (FRC)—sum of the ERV and the RV
11. Total lung capacity (TLC)—sum of all four lung volumes

PULMONARY GAS EXCHANGE

1. Partial pressure—pressure exerted by any one gas in a mixture of gases or in a liquid
2. Law of partial pressure—the partial pressure of a gas in a mixture of gases is directly related to the concentration of that gas in the mixture and to the total pressure of the mixture
3. Exchange of gases in the lungs—exchange of gases in the lungs takes place between alveolar air and blood flowing through lung capillaries (Figure 20-16)

HOW BLOOD TRANSPORTS GASES

1. Blood transports oxygen and carbon dioxide either as solutes or combined with other chemicals
2. Hemoglobin (Hb)—quaternary protein made up of four polypeptide chains (two alpha chains and two beta chains)
3. Each chain is associated with an iron-containing heme group, and each iron atom can bind an oxygen molecule (O2) (Figure 20-18)
4. Gases other than oxygen and carbon dioxide can also bind to hemoglobin
5. Carbon monoxide
6. Gas transport in the blood
7. Oxygen—travels as dissolved oxygen in plasma and as oxygen associated with hemoglobin—(oxyhemoglobin)
8. Oxyhemoglobin carries the vast majority of the total oxygen transported by the blood
9. Carbon dioxide (Figure 20-20)
10. Only about 10% of the carbon dioxide in blood travels as dissolved gas
11. Carbaminohemoglobin is an important carrier of blood carbon dioxide (20% of total); formed when CO2 binds to an amine group on hemoglobin (Figure 20-19)
12. About 70% of total blood CO2 is transported as bicarbonate
13. When CO2 enters the blood, it also produces hydrogen ions
14. Carbon monoxide
15. CO binds more strongly to hemoglobin than oxygen does
16. Hard to remove CO from hemoglobin, which can be deadly since it replaces oxygen at the binding sites

SYSTEMIC GAS EXCHANGE

1. Exchange of gases in tissues takes place between arterial blood flowing through tissue capillaries (Figure 20-21)
2. Oxygen diffuses out of arterial capillaries and into cells according to their pressure gradients
3. Carbon dioxide diffuses out of cells and into venous capillaries according to their pressure gradients
4. Increasing PCO2 and decreasing PO2 produce two effects:
5. Oxygen dissociation from oxyhemoglobin
6. Carbon dioxide associating with hemoglobin to generate carbaminohemoglobin

REGULATION OF PULMONARY FUNCTION

1. Respiratory control centers—main integrators that control the nerves that affect the inspiratory and expiratory muscles; located within the brainstem (Figure 20-22)
2. Basic rhythm of the respiratory cycle of inspiration and expiration is generated by the medullary rhythmicity area
3. Contains two regions of interconnected control centers:
4. Dorsal respiratory group (DRG)—integrates information from chemoreceptors for PCO2; signals the VRG to alter breathing rhythm to restore homeostasis
5. Ventral respiratory group (VRG)—controls the basic rhythm of breathing
6. Factors that influence breathing—sensors from the nervous system and other control centers provide feedback to the medullary rhythmicity area
7. Changes in pH
8. Chemoreceptors—monitor pH in blood
9. Changes in partial pressures of carbon dioxide and oxygen
10. Peripheral chemoreceptors—stimulated upon increased arterial carbon dioxide
11. Control of respirations during exercise
12. Respirations increase abruptly at the beginning of exercise and decrease even more markedly as it ends
13. Mechanism that accomplishes this increased ventilation rate is not known

REVIEW QUESTIONS
Write out the answers to these questions after reading the chapter and reviewing the Chapter Summary. If you simply think through the answer without writing it down, you won’t retain much of your new learning.

1. Identify the major anatomical structures of the nose.
2. How are the conchae arranged in the nose? What are they?
3. The pharynx is common to what two systems?
4. What is the voice box? Of what is it composed? What is the Adam’s apple?
5. What is the epiglottis? What is its function?
6. Describe the structure and function of the trachea.
7. List the organs that are included in the upper respiratory tract. Do the same for the lower respiratory tract.
8. Identify the separate volumes that make up the total lung capacity.
9. Normally, about what percentage of the tidal volume fills the anatomical dead space?
10. What factors influence the amount of oxygen that diffuses into the blood from the alveoli?
11. Identify the major factors that influence breathing.
12. Describe the changes in respiration during a period of exercise.

CRITICAL THINKING QUESTIONS
After finishing the Review Questions, write out the answers to these items to help you apply your new knowledge. Go back to sections of the chapter that relate to items that you find difficult.

1. How would you describe the structure and function of the respiratory mucosa? Include the types of cells it contains and where these cells are located in the respiratory system.
2. Why do you think mucus production is especially important in the olfactory epithelium?
3. Make the distinction between air distribution and gas exchange in the respiratory system. Identify the organs that serve as air distributors and gas exchangers.
4. What is pulmonary ventilation? What evidence can you find to describe whether the lungs are active or passive during this process?
5. After strenuous exercise, inexperienced athletes will quite often attempt to recover and resume normal breathing by bending over or sitting down. Using the mechanics of ventilation, how would you modify the recovery practices of these athletes?