CHAPTER OUTLINE I t is a clear, crisp fall morning. The leaves are turning, and the air carries a hint of the chill to come. Many of the leaves have already fallen, and the last flowers of the season are in bloom. For Marie’s friends, the heat of the summer is past, and this is a time of energy and exuberance. But for Marie, it is a season fraught with threats because her very survival is in danger. The air is filled with mold spores, leaf dust, and tiny fragments of dead plants. For her, those piles of leaves are simply huge colonies of mold and heaps of dust that make breathing, the essential act of life, difficult. Most people don’t give a second thought to breathing; but not Marie, not in mold season. She has so little energy that she must stop to rest on a flight of stairs. Oxygen is so scarce in her blood that her lips and even her fingernails sometimes turn blue. Marie is an asthmatic. Her lungs re- spond to mold and airborne dust with an inflammatory reaction that threatens her very survival during allergy season. Every minute, humans must transport oxygen to their cells and remove the carbon dioxide produced by cellular respiration. Human life depends on the integrated functioning of the cardiovascular and respiratory sys- tems because neither system, by itself, can supply what we need to survive. And for Marie and millions of other asthmatics, this gas exchange process cannot be taken for granted. But to understand how asthma and other diseases affect the res- piratory system, we must first get a grasp of the intricate anatomy and physiology of the airways and lungs. 366 The Respiratory System: Movement of Air ■ The Respiratory System Provides Us with Essential Gas Exchange as well as Vocalization p. 000 ■ External Respiration Brings Supplies for Internal Respiration p. 000 ■ Transport of Oxygen and Carbon Dioxide Requires Hemoglobin and Plasma p. 000 ■ Respiratory Health Is Critical to Survival p. 000 ■ In Order to Respire, Air Must Be Moved into and out of the Respiratory System p. 000 12 human_ch12_366-403v2.qxd 23-01-2007 16:05 Page 366
Transcript
CHAPTER OUTLINE
It is a clear, crisp fall morning. The leaves areturning, and the air carries a hint of the chill
to come. Many of the leaves have already fallen,and the last flowers of the season are in bloom.For Marie’s friends, the heat of the summer ispast, and this is a time of energy andexuberance. But for Marie, it is a season fraughtwith threats because her very survival is indanger. The air is filled with mold spores, leafdust, and tiny fragments of dead plants. For her,those piles of leaves are simply huge colonies ofmold and heaps of dust that make breathing,the essential act of life, difficult. Most peopledon’t give a second thought to breathing; butnot Marie, not in mold season. She has solittle energy that she must stop to rest ona flight of stairs. Oxygen is so scarce in herblood that her lips and even herfingernails sometimes turn blue.
Marie is an asthmatic. Her lungs re-spond to mold and airborne dust with aninflammatory reaction that threatens hervery survival during allergy season. Everyminute, humans must transport oxygen totheir cells and remove the carbon dioxideproduced by cellular respiration. Humanlife depends on the integrated functioningof the cardiovascular and respiratory sys-tems because neither system, by itself,can supply what we need to survive. Andfor Marie and millions of other asthmatics,this gas exchange process cannot betaken for granted. But to understand howasthma and other diseases affect the res-piratory system, we must first get a graspof the intricate anatomy and physiology ofthe airways and lungs.
366
The Respiratory System: Movement of Air
■ The Respiratory System ProvidesUs with Essential Gas Exchange aswell as Vocalization p. 000
■ External Respiration Brings Suppliesfor Internal Respiration p. 000
■ Transport of Oxygen and CarbonDioxide Requires Hemoglobin andPlasma p. 000
■ Respiratory Health Is Critical toSurvival p. 000
■ In Order to Respire, Air Must BeMoved into and out of theRespiratory System p. 000
The Respiratory System Provides Us With Essential Gas Exchange as Well as Vocalization 369368 CHAPTER 12 The Respiratory System: Movement of Air
Explore the overall function of the respiratory system.
Identify the structures of the upper and lower respiratory tracts.
LEARNING OBJECTIVES
The Respiratory System Provides Us WithEssential Gas Exchange as Well as Vocalization
hus far in our treatment of survival, wehave talked about protecting ourselvesfrom the environment, moving throughthe environment, and sensing and re-
acting to external and internal changes. We have ex-plored how the cardiovascular system (Chapter 11)moves nutrients, gases, and waste though the body. Butthe cardiovascular system must cooperate with the res-piratory system, which brings oxygen into the bodyand expels carbon dioxide. The respiratory system alsofilters incoming air, maintains blood pH, helpscontrol fluid and thermal homeostasis, and
produces sound. Other-wise, speech (and there-fore biology lectures!)would be impossible.
The respiratory systemhas two anatomical divisions,the upper respiratory tractand the lower respiratorytract, with separate but re-lated functions (Figure
12.1). The upper tract con-ditions air as it enters thebody, and the lower respira-tory tract allows oxygen to en-ter the blood, and waste gasesto leave it.
THE UPPER RESPIRATORY TRACT HAS AN INSPIRING ROLE
The structures of the upper respiratory tract—the nose,pharynx, and larynx—warm, moisten, and filter the in-coming air (Figure 12.2). The nose is one of the
T
first body parts that small chil-dren can identify. We are fa-miliar with the external por-tion of the nose, consisting ofthe nasal bone and hyalinecartilage, covered by skin and muscle. The division be-tween the two nostrils, or external nares, is a plate ofhyaline cartilage called the septum. The septum is at-tached to the vomer bone at its base. Both the septumand the cartilages that make up the sides of the nose
Upper respiratory tractRespiratory organs
in the face and neck.
Lower respiratory tractRespiratory organs
found within the
thoracic cavity,
including the
bronchial tree and
the lungs.
Larynx Voice box
or Adam’s apple.
Pharynx Throat.
Frontal sinus
Frontal bone
Olfactory epithelium
SuperiorMiddle Nasal
conchae Inferior
External naris
Maxilla
Oral cavity
Palatine bone
Soft palate
Hyoid bone
Uvula
Palatine tonsil
Oropharynx
Epiglottis
Esophagus
Trachea
Larynx
Thyroid cartilage
Cricoid cartilage
Thyroid gland
Sphenoid bone
Superior
Middle
Inferior
Nasopharynx
Orifice of auditory (eustachian) tube
Sagittal section of the left side of the head and neck showing the location of respiratory structures
Nasal meatuses
Sagittalplane
Cartilaginous framework:
Anterolateral view of external portion of nose showing cartilaginous and bony framework
Lateral nasal cartilages
Septal cartilage
Bony framework:
Frontal bone
Nasal bones
Maxilla
A
B
External nose anatomy including septum Figure 12.2
Differentiate the conducting zone from the respiratory zone.
Discuss the anatomy and physiology of the alveolar sac.
The Respiratory System Provides Us With Essential Gas Exchange as Well as Vocalization 371370 CHAPTER 12 The Respiratory System: Movement of Air
serve to support the nasal openings. If a blow to thenose moves these cartilages to the side of the vomer,airflow is blocked. To treat this “deviated septum,” sur-geons restore the septum into position and open bothnasal passageways. Surgery on the nose (called rhino-plasty) can also be done for cosmetic reasons, usuallyby breaking the nasal bone and reshaping the nasalcartilages.
As mentioned, the nasal cavity warms, filters,and moistens incoming air, and does so far better thanthe mucus membranes of the mouth. Swirls and ridgesin the nasal cavity slow the air. As inhaled air movesthrough this convoluted space, it contacts the nasal ep-ithelium. The epithelium in the upper respiratory tractis pseudostratified ciliated columnar epithelium. In thenasal region, this tissue is covered in mucus and con-stantly washed by tears draining from the eyes.
A large blood supply warms the nasal epithe-lium, and both the warmth and moisture are trans-ferred to the inhaled air (Figure 12.3). If you haveever bumped your nose, you know of this large bloodsupply. Most of us have had a bloody nose at least once,and have been surprised by the remarkable quantity ofblood that seems to leak out.
Filtering is a vital function of the nose becauseinhaled particles would seriously inhibit airflow in thelower respiratory tract. Coarse hairs in the nostrils filterout larger particles, and the mucus of the nasal passagesfurther filters incoming air by trapping small particles.
A final function of the nasal epithelium is thesense of smell (as described in Chapter 8). To smellsomething more clearly, we often take deep breaths toensure that airborne compounds reach the patch ofnasal epithelium that is studded with chemosensoryneurons.
The internal nares, the twin openings at theback of the nasal passageway, lead to the nasopharynx,or upper throat (Figure 12.4). The passageway be-tween the nose and throat is normally open for breath-ing, but it must close when we swallow. The uvula, afleshy tab of tissue that hangs down in the back of thethroat, contracts when touched by solids, moving up-ward and closing the internal nares. When your doctorasks you to say “Ahh” during a throat examination, youcontract the uvula and move it up to allow the doctor tosee the nasopharynx and the tonsils on the posterior
wall of the pharynx. If you laugh or cough while drink-ing, the uvula may spasm, and liquids may leak past it.These liquids may be forced out the external nares,causing a burning sensation as they travel the nasalpassages—and some slight embarrassment.
The eustachian, or auditory, tubes link the na-sopharynx and the middle ear. When your ears “pop,”these tubes are opening to equalize air pressure be-tween the middle and outer ear.
The oropharynx, the area directly behind thetongue, is covered by the uvula when it hangs down.This portion of the throat is devoted to activities of themouth. Food and drink pass through the oropharynxwith each swallow, so the mucous membrane and ep-ithelium lining this region are usually thicker and moredurable than elsewhere in the pharynx. The palatineand lingual tonsils are found in the oropharynx as well.
Internal nasal passages Figure 12.3
Note the convoluted nasal conchae that swirl the incoming air.
These are lined with a mucous membrane that adds moisture
and heat to the air before sending it on to the lower
respiratory tract.
The lowest level of the pharynx, called thelaryngopharynx, is the last part of the respiratory tractshared by the digestive and respiratory systems. Theend of the laryngopharynx has two openings. The ante-rior opening leads to the larynx and the rest of the res-piratory system. The posterior opening leads to theesophagus and the digestive system.
The larynx divides the upper and lower respi-ratory tracts. This structure, composed entirely of car-tilage, holds the respiratory tract open, guards thelower tract against particulate matter, and producesthe sounds of speech. The larynx is composed of ninepieces of hyaline cartilage: threesingle structures and three pairedstructures. The single pieces arethe thyroid cartilage, the epiglot-tis, and the cricoid cartilage (Fig-
ure 12.5).
The thyroid cartilage lies in the front of the lar-ynx. It is shield-shaped and often protrudes from thethroat. Because males produce more testosterone thanfemales, and testosterone stimulates cartilage growth,the thyroid cartilage in men is usually larger than in fe-males. One common name for the larynx, “Adam’s ap-ple,” refers to the larger larynx in men.
The epiglottis covers the opening to the lowerrespiratory tract to prevent food from entering (epimeans “on top of” and glottis means “hole”). Theepiglottis is a leaf-like flap of cartilage on the superior(upper) aspect of the larynx, covering the hole leadingto the lungs. A pair of small corniculate cartilages holdthe epiglottis in position above the glottis. The larynx isattached to the tongue muscles. When the tonguepushes against the roof of the mouth in preparation forswallowing, the larynx moves up toward the epiglottis.Food particles hitting the top of the epiglottis complete
Hyoid bone
Trachea
Pharyngeal tonsil
Opening of auditory(eustachian) tube
NASOPHARYNX
Palatine tonsil
OROPHARYNX
Epiglottis
LARYNGOPHARYNX(hypopharynx)
Lingual tonsil
Esophagus
Sagittal section showing the regions of the pharynx
mucus should be mucous (as in Fig. 12.3 caption) (EA)
helen walden
Note
Lowercase w in with
The Respiratory System Provides Us With Essential Gas Exchange as Well as Vocalization 373372 CHAPTER 12 The Respiratory System: Movement of Air
Anterior view Posterior view
Epiglottis
Hyoid bone
Corniculate cartilage
Thyroid cartilage(Adam’s apple)
Arytenoid cartilage
Cricoid cartilage
Thyroid gland
Tracheal cartilage
Larynx Thyroidgland
Hyoid bone
Ventricular fold (false vocal cord)
Thyroid cartilage
Vocal fold (true vocal cord)
Epiglottis
Cuneiform cartilage
Corniculate cartilage
Arytenoid cartilage
Cricoid cartilage
Tracheal cartilage
Sagittal section
Sagittalplane
A B
C
Larynx Figure 12.5
Thyroid cartilage
Cricoid cartilage
Vocal fold
Arytenoid cartilage
Superior view of cartilagesand muscles
Posteriorcricoarytenoidmuscle
Movement of vocal folds apart (abduction) View through a laryngoscope
Tongue
Epiglottis
Cuneiform cartilageCorniculate cartilage
Vocal folds(true vocal cords)
A B
Vocal folds Figure 12.6
the closure by causing the epiglottis to rest against thetop of the larynx. You can feel this movement by touch-ing your “Adam’s apple” and swallowing. You will feelthe entire larynx move up with your tongue.
The cricoid cartilage is the only complete ringof cartilage in the respiratory system. It is narrow infront but thick in the back of the larynx. The cricoidcartilage holds the respiratory system open. If it iscrushed, airflow is impeded and breathing becomesnearly impossible. In an emergency, it may be neces-sary to surgically open the airway below a crushedcricoid cartilage.
The larynx is called the “voice box” because itis the location of the vocal cords (Figure 12.6).
These are folds, covered bymucus membrane, and heldin place by elastic ligamentsstretched across the glottis.These folds vibrate as airmoves past them, producingsound. High-pitched soundsoccur when tension on the vo-cal folds increases, and low-pitched sounds occur whenthe tension is reduced. We un-consciously adjust tension on
the vocal folds by moving the paired laryngeal carti-lages. The arytenoid and cuneiform cartilages both pull
on the vocal folds to alter pitch. The amplitude, oramount the cords are vibrating, determines soundvolume.
As boys reach puberty and their testes producemore testosterone, their voices change. Testosteronestimulates the growth of cartilage in the larynx, alteringthe thickness of the vocal folds. Boys train their voicesthrough daily use to adjust their vocal fold tensionbased on the size of the larynx. As the larynx grows, thetension needed to produce the same sounds changes.In effect, the male must retrain his voice to maintainvocal tone. When the larynx is growing quickly, themale voice will often “crack” or “squeak” due to his in-ability to adjust the tension on his changing vocal folds.
The lower respiratorytract routes air to thelungs The main functionof the lower tract is to move in-haled air to the respiratorymembrane. Physiologically,the upper tract and the firstportion of the lower tractmake up the conducting zoneof the respiratory system,which conducts air from the atmosphere to therespiratory zone deeper in the body, where the actualexchange of gases takes place (Figure 12.7).
the lungs, so this would also move the lungs upwardin the thoracic cavity—a structural nightmare! The C-shaped cartilage allows the back of the trachea tocompress, so the lungs can remain in the thoracic cav-ity and the trachea can get the support it needs to re-main open.
In advanced first-aid classes, you learn to locatethese rings and identify a position between two rings.You can save someone with a crushed larynx from suf-focating by opening the trachea between “C” rings andinserting a temporary breathing tube so air can flow tothe lungs, bypassing the crushed larynx. This is called atracheotomy. Another way to restore breathing is calledintubation—the insertion of a tube through the mouthor nose, through the larynx and into the trachea. Thetube pushes obstructions aside and/or helps suctionthem out.
At the level of the fifth thoracic vertebra, thetrachea splits into two tubes called the primary bronchi,which lead to each lung. Despite their common func-tion, the two bronchi are slightly different. The rightprimary bronchus is shorter, wider, and more verticalthan the left. For this reason, inhaled objects often getlodged in the right primary bronchus. These twobronchi are constructed very much like the trachea andare held open with incomplete rings of cartilage intheir walls.
At the lower base of the trachea is an extremelysensitive area called the carina. The mucus membraneof the carina is more sensitive to touch than any otherarea of the larynx or trachea, so this spot triggers a dra-matic cough reflex when any solid object touches it.
Once inside the lungs, the primary bronchi di-vide into the secondary bronchi (see Figure 12.7).The right bronchus divides into three secondarybronchi, whereas the left splits into two. This branchingpattern continues like the branches of a tree, gettingsmaller and smaller as the tubes extend farther fromthe primary bronchus. The sequentially smaller tubesare called tertiary bronchi, bronchioles, terminal bron-chioles, and respiratory bronchioles. The respiratorysystem looks like an upside-down tree, with the base atthe nasal passages and the tiniest branches leading tothe “leaves” deep within the lungs.
The bronchial tree undergoes two majorchanges as it reaches deeper into the body:
1. The cells of the mucus membrane get smaller.The epithelium of the upper and beginningportion of the lower respiratory tract is pseu-dostratified ciliated columnar epithelium; thesefairly large cells secrete mucus, while the ciliasweep the mucus upward and outward to re-move dust and inhaled particles. The epithe-lium changes to the slightly thinner, ciliatedcolumnar epithelium in the larger bronchioles.The smaller bronchioles are lined with smallerciliated cuboidal epithelium. Terminal bronchi-oles have no cilia and are lined with simplecolumnar epithelium.If dust reaches all theway to the terminalbronchioles, it can beremoved only bymacrophages of theimmune system.
2. The composition ofthe walls of thebronchi and bronchi-oles changes. Smaller tubes need less cartilageto hold them open, so the incomplete rings ofcartilage supporting the bronchi are graduallyreplaced by plates of cartilage in the bronchi-oles. These plates diminish in the smaller bron-chioles, until the walls of the terminal bronchi-oles have virtually no cartilage. As cartilagedecreases, the percentage of smooth muscle in-creases. Without cartilage, these small tubes can
The Respiratory System Provides Us With Essential Gas Exchange as Well as Vocalization 375
The conducting zone includes all the structures of theupper respiratory tract, as well as the trachea, bronchi,bronchioles, and terminal bronchioles. The respiratoryzone lies deep within the lungs, and includes only therespiratory bronchioles and the alveoli.
The trachea connects the larynx to thebronchi Beyond the larynx, air enters the trachea, a12-centimeter-long tube extending from the base of thelarynx to the fifth thoracic vertebra (Figure 12.8).The trachea is approximately 2.5 centimeters in diame-ter, and is composed of muscular walls embedded with16 to 20 “C”-shaped rings of hyaline cartilage. (Remem-
ber that the cricoid cartilage of the larynx is the onlycomplete ring of cartilage in the respiratory system.)The opening of each “C” is oriented toward the back.You can easily feel the tracheal rings through the skinof your throat, immediately below your larynx.
These cartilage “C” rings support the tracheaso that it does not collapse during breathing, whilealso allowing the esophagus to expand during swal-lowing. When you swallow a large mouthful of food,the esophagus pushes into the lumen of the tracheaas the mouthful passes. If the tracheal cartilages werecircular, the food would push the entire trachea for-ward. But the trachea is attached to the bronchi of
374 CHAPTER 12 The Respiratory System: Movement of Air
Lateral view of right lung Lateral view of left lung
ANTERIOR
Inferior lobe
POSTERIOR
View (c)View (b)
A
B C
The Respiratory System Provides Us With Essential Gas Exchange as Well as Vocalization 377
the mediastinal surface. Onthis surface lies the hilum ofthe lung. Entering and exitingthe lung at the hilum are thebronchi, along with the majorblood vessels, lymphatics, andnerve supply for the organ.
The pleura wraps the lungs The lungs are cov-ered in a serous membrane called the pleura that al-lows the lungs to expand and contract without tear-ing the delicate respiratory tissues (Figure
12.10). The pleura is anatomically similar to thepericardium around the heart in that they are com-
posed of two membranous layers separated by serousfluid. The visceral pleura is snug against the lung tis-sue, and the parietal pleura lines the thoracic cavity.The pleural cavity between the two pleural mem-branes contains serous fluid. The surface tension ofthe fluid between these two membranes creates aslight outward pull on the lung tissue. Have you no-ticed that a thin layer of water on a glass table holdsother glass objects to it? In the lungs, this same phe-nomenon causes adhesion between the visceral andparietal pleura. There is also a slight vacuum in thepleural space, created during development of thelungs and thoracic cavity. This vacuum is essential toproper lung functioning.
External anatomy of the lungs Figure 12.9
376 CHAPTER 12 The Respiratory System: Movement of Air
be completely shut by contraction of thissmooth muscle. In asthma and other constric-tive respiratory disorders, this smooth musclebecomes irritated and tightens, reducing thetube diameter, sometimes even effectively clos-ing it.
Epinephrine, a hormone that is releasedinto the bloodstream when we exercise or feelfright, relaxes smooth muscle. In the lungs, epi-nephrine relaxes the smooth muscle of the ter-minal bronchioles, increasing the diameter ofthe lumen and allowing greater airflow. This inturn increases the oxygen content of the bloodand allows the muscles to work more efficiently.Someone you know who has asthma probablycarries an “inhaler” filled with “rescue med-ication.” If you get a look at the label, theactive ingredient is probably epineph-rine, norepinephrine, or a derivative.Spraying these drugs on the walls of thebronchioles immediately relaxes thesmooth muscle, dramatically increasingtubule diameter.
The thoracic cavity houses the two organs ofrespiration, the lungs (Figure 12.9). These light-weight organs extend from just above the clavicle to thetwelfth thoracic vertebra and fill the rib cage. The baseof the lungs is the broad portion sitting on the di-aphragm. The apex is the small point extending abovethe clavicles.
Although the lungs are paired, they are notidentical. The right lung is shorter and fatter, and it hasthree lobes, whereas the left lung has only two lobes.The left lung is thinner and has a depression for theheart, called the cardiac notch, on the medial side. Thecentral portion of the thoracic cavity is called the medi-astinum; therefore, the medial portion of the lungs is
HilumSite of entry and
exit for the nerves,
blood vessels, and
lymphatic vessels
on most organs.
Parietal pericardium
Sternum
Parietal pleura
Left pleural cavity
Skin
Visceral pleura
Right lung
Rib
LATERAL
Body of fifth thoracic vertebra
Spinal cord
MEDIAL
ANTERIOR
Inferior view of a transverse section through the thoracic cavityshowing the pleural cavity and pleural membranes
The Respiratory System Provides Us With Essential Gas Exchange as Well as Vocalization 379
If the partial vacuum within the pleural space islost, inhalation becomes difficult. This can happen ifthe thoracic cavity is punctured through injury or acci-dent, causing either a pneumothorax (air in the pleuralspace) or a hemothorax (blood in the pleural space). Ifenough air or blood enters the pleural space, lung tis-sue in that area can collapse (Figure 12.11). Theair or blood must be evacuated, and pleural integrity
restored, to reinflate the lungand reestablish normalbreathing. Pleurisy is less dev-astating and more commonthan a collapsed lung. Inpleurisy, the pleural mem-branes swell after being in-flamed or irritated, and they
rub against each another. Every breath is painful, anddeep breathing, coughing, or laughing may be excruci-ating. Anti-inflammatory drugs can reduce these symp-toms.
The lobes of each lung are separate sections ofthe organ that can be lifted away from the other lobes,just as a butcher might separate lobes of beef liver. Airenters each lobe through one secondary bronchus. De-spite having different numbers of secondary bronchi,each lung has ten terminal bronchioles, each supplyingone bronchopulmonary segment.
Gases are exchanged in the respiratory zoneA bronchopulmonary segment looks somewhat like agrape on a grapevine (Figure 12.12). A single ter-minal bronchiole feeds all the respiratory membranesof each bronchopulmonary segment. One pulmonaryarteriole runs to each segment, and one pulmonaryvenule returns from it. Small groups of respiratorymembranes, called lobules, extend off the terminalbronchiole. These lobules are wrapped in elastic tissueand covered in pulmonary capillaries. Lobules are at-tached to the terminal bronchiole by a respiratorybronchiole.
The respiratory bronchiole leads to alveolarducts, which finally conduct air to the alveoli, the respi-ratory membranes for the entire system. Only after trav-eling through the entire set of tubes in the conductingzone can gases diffuse in the alveoli. Diffusion takesplace nowhere else in the respiratory system. It is here,and here alone, that oxygen enters the bloodstreamand carbon dioxide exits.
The alveolus is a cup-shaped membrane at theend of the terminal bronchiole. Alveoli are clusteredinto an alveolar sac at the end of terminal bronchiole.The key to respiration is diffusion of gases, and diffu-sion requires extremely thin membranes. The walls ofthe alveolar sac are a mere two squamous epithelialcells thick—one cell from the alveolar wall and onefrom the capillary wall. As mentioned in the beginningof the chapter, asthma impedes air flow to these respi-ratory membranes. (See also the I wonder . . . feature,“Why are asthma rates going up?”)
Diffusion of gases across the cell membrane re-quires a moist membrane, but moist membranes have atendency to stick together much like plastic food wrap(Figure 12.13). Septal cells, scattered through thelung, produce surfactant, a detergent-like fluid thatmoistens the alveoli and prevents the walls from stick-ing together during exhalation. (Imagine how a smalllayer of watery detergent would release the bonding ofa ball of plastic wrap.) The surfactant also serves as a bi-ological detergent, solubilizingoxygen gas to promote uptake.
Because septal cells beginsecreting surfactant only duringthe last few weeks of pregnancy,premature babies often have dif-
378 CHAPTER 12 The Respiratory System: Movement of Air
Pleurisy Inflamma-
tion of the covering
surrounding the
lungs, causing
painful breathing.
Collapsed lung Figure 12.11
Terminal bronchiole
Pulmonary arteriole
Lymphatic vessel
Respiratory bronchiole
Pulmonary venule
Elastic connective tissue
Pulmonary capillary
Visceral pleura
Alveoli
Diagram of a portion of a lobule of the lung Lung lobule
Terminal bronchiole
Blood vessel
Respiratory bronchiole
Alveolar ducts
Alveoli
Visceral pleura
Alveolar sacs
Alveolar ducts
Alveolar sac
about 30xLM
A B
Lobule anatomy Figure 12.12
Details of respiratory membrane
Alveolus
Diffusion of CO2
Diffusion of O2
Alveolar fluid with surfactant
Interstitial space
Type I alveolar cell
Epithelial basement membrane
Capillary basement membrane
Capillary endothelium
Red blood cell
Basic gas movement across the respiratorymembrane Figure 12.13
The arrows in this diagram demonstrate the movement of
gases across the respiratory membrane. Oxygen diffuses
from the alveoli to the blood in the capillary, while carbon
dioxide diffuses in the opposite direction.
Premature babiesInfants born prior
to the normal
gestational period
of 40 weeks.
ficulty breathing. Every inhalation requires a gasp to re-inflate the collapsed alveoli because their walls stick to-gether. In the late 1980s, artificial surfactant was givenas an inhalant during the first week of life to premature
infants. Now this drug is routinely given to prematureinfants to enhance respiration before septal cells beginproducing surfactant.
Details of respiratory membraneSection through an alveolus showing its cellular components
Monocyte
Alveolar macrophage
Alveolus
Red blood cellin pulmonarycapillary
Type I alveolar cell
Respiratory membrane
Type II alveolar (septal) cell
Elastic fiber
Reticular fiber
Alveolus
Diffusion of CO2
Diffusion of O2
Alveolar fluid with surfactant
Interstitial space
Type I alveolar cell
Epithelial basement membrane
Capillary basement membrane
Capillary endothelium
Red blood cell
A B
The Respiratory System Provides Us With Essential Gas Exchange as Well as Vocalization 381
Alveolar macrophages, or dust cells, also patrolthe alveoli (Figure 12.14). These immune cells re-
move any inhaled particles that escaped the mucus andcilia of the conducting zone.
380 CHAPTER 12 The Respiratory System: Movement of Air
I WO
ND
ER...
Why are asthma rates going up?
Asthma, a constriction of the bronchi that causes wheezingand shortness of breath, has become an epidemic. An esti-mated 14 to 30 million Americans have asthma, including atleast 6 million children, many in inner cities. The diseasesends about 500,000 people to the hospital each year. In-credibly, the rate of diagnosed asthma patients has dou-bled since the 1980s.
Part of the increase may be due to better diagnosis,but is something else making this disease more common?The answer must lie in the environmental causes of asthmaand/or in the human response to those causes. Asthma canresult from exposure to irritants and allergens, includingpollen, cockroaches, mold, cigarette smoke, air pollutants,respiratory infections, exercise, cold air, and some medi-cines. Researchers have examined these exposures andfound some important clues to the asthma epidemic:
• Asthma hospitalizations peak just after school starts inthe fall. In a Canadian study, schoolchildren aged 5 to 7were going to emergency rooms a few days beforepreschoolers and adults. Of the wheezing schoolchild-ren, 80 to 85 percent had active rhinovirus (commoncold) infections, as did 50 percent of adults. The re-search suggests that the common cold is spread by chil-dren (partly because immune systems are still develop-ing) and that rhinovirus infections may trigger manyasthma attacks.
• Poverty and environmental pollution both help to ex-plain why inner-city African Americans have such highrates of asthma. One potent asthma allergen is the cuti-cle (shell) of a cockroach, an insect often found incrowded inner cities. Compounding this problem are thechaotic home lives characteristic of inner-city families,
which can also interfere with timely administration ofmedicines to prevent asthma symptoms.
• Children who lived on a farm before age 5 have signifi-cantly lower rates of asthma, wheezing, and use ofasthma medicine, compared to children who live intown. Although allergies play a key role in asthma, thefarm children did not have lower rates of hay fever, anallergic reaction to pollen.
None of these studies exactly explains the surge inasthma diagnoses, but the last one does offer a clue. Somescientists suspect that early exposure to dirt and/or infec-tious disease somehow “tunes” the immune system to re-duce the hyperactive reaction that contributes to the in-flammation of asthma. Early exposure to endotoxin, acomponent of the cell wall of gram-negative bacteria, hasbeen associated with low rates of asthma. But the picture iscomplicated: Endotoxin also inflames lung tissue in healthypeople, and some studies have linked it to more wheezing,not less.
With the causes of the asthma epidemic still uncertain,the best take-home message is this: Most cases of asthmaare controllable. If you suffer from it, know what triggersyour disease and take action to reduce your exposure toyour personal asthma triggers. Take your preventative med-ications as prescribed and get the suggested immuniza-tions to prevent viral infections from triggering attacks.
Anatomy of an alveolar sac Figure 12.14
The respiratory membrane consists of a layer of alveolar cells, an epithelial basement membrane, the capillary basement membrane
and the endothelium of the capillary. These membranes are found at the end of the respiratory tree, as seen in figure 12.13a.
CONCEPT CHECK
List the structures of theupper and lower respiratorytract in the order that airpasses through them duringinhalation.
What is the function ofthe larynx?
Where are oxygen andcarbon dioxide actuallyexchanged in therespiratory system?
he anatomy of the respiratory systemeases the exchange of gases between theair and the body. But how is external airbrought into the depths of the respira-
tory system during inhalation (or inspiration)? Inhala-tion (and the opposite movement, called exhalation orexpiration) are governed by muscular movements ofthe thoracic cavity. Inhalation is an active process, re-quiring muscular contractions, but exhalation requiresonly that those muscles relax. The combined inflow andoutflow of air between atmosphere and alveoli is calledpulmonary ventilation. Pulmonary ventilation is gov-erned by Boyle’s law, which states that the volume of agas varies inversely with its pressure (Figure
12.15). In other words, if you increase the size of acontainer of gas without adding gas molecules, the pres-sure must decrease.
When you inhale, your muscles expand yourthoracic cavity (Figure 12.16). Your diaphragmcontracts, dropping the bottom from the thoracic cav-ity. This dropping of the diaphragm alone causes mostof the size increase in the thoracic cavity during an in-
382 CHAPTER 12 The Respiratory System: Movement of Air In Order to Respire, Air Must be Moved Into and Out of the Respiratory System 383
In Order to Respire, Air Must be Moved Into and Out of the Respiratory System
LEARNING OBJECTIVES
Explain inhalation and exhalation in terms of muscle use. Describe the various lung volumes, and explain their relationship.
Thalation. The intercostal muscles also contract, raisingthe ribs slightly. (You can feel this by holding your sidesas you breathe and feeling your ribs expand and con-tract.) The lungs are connected to the walls of the tho-racic cavity through the pleura, so the lungs aredragged along with the moving walls of the thoraciccavity. The volume of the lungs thus increases duringinhalation. The pressure inside them must thereforedrop, causing gas molecules to rush in from the envi-ronment immediately outside your nostrils. Because airmoves from high-pressure zones to low-pressure zones,air will move from the environment into your lungs toequilibrate this pressure gradient. This is how inhala-tion occurs.
Drowning occurs when water, which is tooheavy to remove from the lungs, is pulled into thelungs. Our respiratory muscles cannot push out the wa-ter, and water carries too little oxygen to diffuse intoour blood. In fact, oxygen will diffuse in the oppositedirection, from the blood to the water!
When the muscles that expanded the thoraciccavity relax, the thoracic cavity returns to its original
Inhalation: diaphragm dropping and volume increasing Figure 12.16
Pro
cess
Dia
gra
m
Inhalation
ExhalationDiaphragm:
Rectus abdominis
Transversus abdominis
Internal oblique
Scalenes
Sternocleidomastoid
MUSCLES OF INHALATION
Diaphragm
External intercostals
External oblique
Internal intercostals
MUSCLES OF EXHALATION
Muscles of inhalation and their actions (left); muscles of exhalation and their actions (right)
During inhalation, the ribs move upward and outward like the handle on a bucket
Changes in size of thoracic cavity during inhalation and exhalation
Run in to previous line and break at sternocleido- Note that word is misspelled; delete the i after sterno (EA)
the pleura. This vacuum forms during fetal develop-ment, when the walls of the thoracic cavity enlargefaster than the lungs. The parietal pleura is pulled out-ward with the expanding walls while the visceralpleura remains attached to the lungs. The resultingvacuum is essential to respiration because it preventscollapse of the thin alveoli during exhalation. Thewalls of the alveoli spring inward as the air leaves therespiratory tract, but the alveolar walls do not collapseand stick together, partly because of the outward pullof the vacuum between the pleura. In addition, theslight vacuum helps the lungs enlarge and fill with airon the next inhalation.
YOUR BRAIN STEM IS SETTING YOURRESPIRATORY RATE
As you read this text, you are breathing at a steady rate.These constant, day-in, day-out breaths are called yourresting rate. Respiratory rate is governed by themedulla oblongata and the pons. The respiratory cen-ter in the medulla oblongata causes rhythmic contrac-tions of the diaphragm, stimulating contraction for twoseconds and allowing three seconds of rest. This cyclerepeats continuously unless overridden by higher brainfunction (Figure 12.18). You can override themedullary signal by holding your breath or by forciblyexhaling, but you cannot hold your breath until youdie. Many small children use this threat to blackmailadults, but let them try! The pons in the brain stem willnot let anybody “forget” to breathe. Once the carbondioxide level builds to a critical point, the child will passout, and the pons will regain control of breathing. Youcan bet that child will resume breathing.
The body can sense the levels of carbon dioxideand oxygen in the blood through chemoreceptors inthe carotid artery and aorta. High carbon dioxide lev-
els immediately trigger an in-crease in the depth and rate ofrespiration. These chemore-ceptors are quite sensitive tocarbon dioxide and respondto a 10 percent increase in car-bon dioxide levels by doublingthe respiratory rate. In con-
trast, a much larger decrease in oxygen level is neededbefore these receptors will cause the respiratory rate toincrease.
Different respiratory volumes describe dif-ferent types of breath During normal breath-ing, the volume of air inhaled per minute reflects therespiratory rate and the volume of each normal breath,called the tidal volume (TV). Tidal volume, approxi-mately 500 ml, is somewhat more than the amount ofair that is actually exchanged because the trachea, lar-ynx, bronchi, and bronchioles are “anatomic deadspaces” that do not participate in gas exchange. Thesedead spaces have a volume of about 150 ml. So eachtidal breath delivers about 350 ml of air to the respira-tory membranes, with 150 ml filling the conducting res-piratory tract.
Just as you can consciously control your breath-ing rate, you can increase the volume of breath by con-tracting more muscles during inhalation. During a“forced inhalation,” the average adult male can inhaleapproximately 3,300 ml of additional air, and the aver-age adult female can force in approximately 1,900 ml.This volume is called inspiratory reserve volume (IRV).
In Order to Respire, Air Must be Moved Into and Out of the Respiratory System 385
size, which raises pressure in the thoracic cavity abovethat outside the nostrils (Figure 12.17). Again, be-cause air moves toward areas of low pressure, therespired air exits the respiratory tract. During exhala-tion, the lungs act like a bicycle pump: The containerholding the air gets smaller, gas pressure rises, and assoon as it exceeds the pressure outside the pump, airleaves the container.
Exhalation is a passive process, mainly involvingmuscular relaxation. If we must forcibly exhale, as insighing or yelling, we contract muscles that directly and
indirectly shrink the thoracic cavity. The abdominalmuscles are primarily involved in forcible exhalation.When these muscles contract, they push on the abdom-inal organs, pushing the diaphragm up. You can provethis by placing a hand on your abdomen and forcefullyexhaling. You will feel these muscles contract as youforce out the air.
Recall that the alveoli are thin and moist. Sur-factant helps prevent these membranes from gum-ming up and sticking together during exhalation. Asecond factor is the vacuum between the two layers of
384 CHAPTER 12 The Respiratory System: Movement of Air
ic pressure = 760 mmHg Atmospheric pressure = 760 mmHg
During inhalation (diaphragm contracting)
eolarpressure =760 mmHg
Alveolarpressure =758 mmHg
Intrapleuralpressure =754 mmHg
Intrapleurpressure =756 mmHg
During e
Atmospher
Alveolarpressure =762 mmHg
Intrapleuralpressure =756 mmHg
1. 2.
3.
At rest (diaphragm relaxed)
Atmospher
al
Alv
xhalation (diaphr agm relaxing)
ic pressure = 760 mmHg
Pressure changes in pulmonary ventilation Figure 12.17
ChemoreceptorsSensory receptors
that detect small
changes in levels of
specific chemicals,
such as carbon
dioxide.
RESPIRATORY CENTER: Midbrain
Sagittalplane
Apneustic area
Pneumotaxic area
Medullary rhythmicity area:
Inspiratory area
Pons
Medullaoblongata
Spinal cord
Expiratory area
Sagittal section of brain stem
Respiratory centers in the brain and majorarteries Figure 12.18
Pressure changes within the thoracic cavity occur as the volume
of the cavity increases and decreases. During inhalation,
the diaphragm contracts, the chest expands and the lungs are
pulled outward. All of these decrease pressure within the lungs,
allowing external air to rush in. Relaxing the diaphragm and
the intercostals drops the volume of the lungs, forcing the air in
out of you.” In our terms, your problem was a loss ofresidual volume. The force of falling momentarilyshrank the thoracic cavity beyond what muscular con-tractions could achieve and forced out a portion of theresidual volume. Your first breath was painful and may
even have produced awkward noises as you reinflatedthe empty alveoli to refill your RV. This is just what in-fants do with their first few gasps after birth (which arecommonly mistaken for crying).
External Respiration Brings Supplies for Internal Respiration 387
Similarly, we can exhale much more than the500 ml tidal volume after a normal tidal inhalation, upto about 1,000 ml for males and 700 ml for females, inthe expiratory reserve volume (ERV). This volume is
lower that IRV because exhalation is a passive process;we do not have muscles that directly compress the tho-racic cavity beyond that used in a tidal breath. The bestwe can do is indirectly pressurize the thoracic cavity bycontracting the abdominal muscles, forcing the con-tents of the abdominal cavity up against the diaphragm(Figure 12.19).
Vital capacity (VC) measures the total volumeof air your lungs can inhale and exhale in one hugebreath, which is essentially the maximum amount of airyour lungs can move in one respiratory cycle (a bigbreath in and back out again) (Figure 12.20). VCis the sum of inspiratory reserve volume, tidal volume,and expiratory reserve volume. For most people, theVC is between 3,100 and 4,800 ml; males generally havea larger volume.
The amount of air that remains in the lungs af-ter forced expiration is called residual volume (RV).The residual volume holds the alveoli open and fills the“anatomical dead spaces.” The RV is usually between1,100 and 1,200 ml. You can add your RV to your VC tofind your total lung capacity.
Have you ever fallen from a tree or a swing andlanded on your back? Perhaps you could not breathefor a minute because you had “gotten the wind knocked
386 CHAPTER 12 The Respiratory System: Movement of Air
1,000 mL
2,000 mL
3,000 mL
4,000 mL
5,000 mL
6,000 mL
INSPIRATORYRESERVEVOLUME3,100 mL
(1,900 mL)
TIDALVOLUME 500 mL
EXPIRATORYRESERVEVOLUME1,200 mL(700 mL)
RESIDUALVOLUME1,200 mL
(1,100 mL)
INSPIRATORYCAPACITY3,600 mL
(2,400 mL)
FUNCTIONALRESIDUALCAPACITY2,400 mL
(1,800 mL)
VITALCAPACITY4,800 mL
(3,100 mL)
TOTALLUNG
CAPACITY6,000 mL
(4,200 mL)
LUNG VOLUMES LUNG CAPACITIES
Exhalation
Inhalation
Start ofrecord
End ofrecord
Respiratory volumes Figure 12.20
CONCEPT CHECK
How does Boyle’s lawexplain pulmonaryventilation?
Give the equation fortotal lung capacity.(Hint: See Figure 12.21.)
What is therelationship betweenERV and IRV?
Using expiratory reserve volume Figure 12.19
Normal tidal volume moves air into and out of the lungs without
taxing the respiratory muscles. When a larger volume of air must
be exchanged, the intercostals, the scalenes, and the abdominal
muscles are used as well. The volume of air exhaled can be
increased from approximately 500 ml to over 3,000 ml—plenty of
air with which to celebrate birthdays!
force lies between the partialpressures of the gases in thealveolar air and the capillaryblood. In internal respiration,it is the partial pressure differ-ences between the gases in thecapillary blood and the tissuefluid that cause diffusion ofthose gases.
The air we breathe is composed of many gases.Nitrogen is the most common, making up 78.1 percentof the atmosphere by volume. Oxygen is the secondmost common gas, occupying 20.9 percent of the totalvolume. Water vapor varies by location and weather,ranging from 0 to 4 percent of volume. And finally, car-bon dioxide makes up a measly 0.4 percent of air byvolume. The air pressure in any mass of air is a sum ofthe partial pressures of each constituent gas, and thepressure exerted by each gas is direclty related to itsproportion in the atmosphere. In air, 78.1 percent ofthe pressure is generated by nitrogen molecules; 20.9percent by oxygen, and 0.4 percent by carbon dioxide.Knowing that atmospheric pressure is usually close to760 mm Hg, we can calculate the partial pressures ofeach gas.
hus far we have discussed only pul-monary ventilation—the moving of airinto the respiratory system. Once gasesare in the alveoli, external respiration
occurs. External respiration is the exchange of gasesbetween the air in the alveoli and the blood in the res-piratory capillaries. A second respiratory process—in-ternal respiration—is the exchange of gases betweenthe blood in the systemic capillaries and the body’scells.
EXTERNAL RESPIRATION SECURES OXYGEN, DISPOSES OF CARBON DIOXIDE
The exchanges of both external and internal respira-tion are driven by the partial pressures of oxygen andcarbon dioxide. In external respiration, the driving
External Respiration Brings Supplies for Internal Respiration
T
LEARNING OBJECTIVES
Define internal and external respiration.
Discuss the movement of gas from air to blood and from blood to
mm Hg. This gradient allows oxygen to leave theblood and enter the respiring cells without requiringenergy from the body.
Cellular respiration produces carbon dioxide,and the partial pressure of carbon dioxide in the tissues
388 CHAPTER 12 The Respiratory System: Movement of Air External Respiration Brings Supplies for Internal Respiration 389
is about 45 mm Hg. Blood in the capillary beds has acarbon dioxide partial pressure of 40 mm Hg. Thissmaller gradient is still enough to cause carbon dioxideto diffuse from the cells to the blood, which carries itoff to the lungs and releases it into the alveolar air.
Dalton’s law Figure 12.21
Each gas in the atmosphere exerts a different partial pressure. Together they all add up to atmospheric pressure. As these gases
float around in the environment, they independently diffuse from areas of high concentration to areas of low concentration.
Atmospheric pressure is the sum of the pressures of all these gases:
Atmospheric pressure (760 mmHg) � PN2� PO2
� PH2O � PCO2� Pother gases
We can determine the partial pressure exerted by each component in the mixture by multiplying the percentage of the gas in
the mixture by the total pressure of the mixture. Atmospheric air is 78.6% nitrogen, 20.9% oxygen, 0.04% carbon dioxide, and
0.06% other gases; a variable amount of water vapor is also present, about 0.4% on a cool, dry day. Thus, the partial pressures
Why discuss partial pressure? Because it ex-plains the movement of oxygen and carbon dioxide inrespiration. Dalton’s law states that gases move inde-pendently down their pressure gradients, from higherto lower pressure (Figure 12.21). So oxygen willdiffuse from the air in the alveoli into the blood,whereas carbon dioxide will diffuse from blood to thealveoli. Each gas independently moves toward an areaof lower pressure without affecting any other gas.
The partial pressure of oxygen in the air of thealveoli is approximately 100 mm Hg, whereas the par-tial pressure of oxygen in the tissues hovers near 40 mmHg. Through simple diffusion, oxygen moves from theair in the alveoli through the thin respiratory mem-brane and into the blood. By the time blood in the res-piratory capillaries completes its journey through thelungs, the partial pressure of oxygen in the blood hasequilibrated with that of the air. Blood returning to theheart’s left atrium carries oxygen with a partial pressureof 100 mm Hg, ready to be pumped to the tissues (Fig-
ure 12.22).While oxygen is diffusing into the blood, car-
bon dioxide is leaving it. The partial pressure of car-bon dioxide in the blood returning to the left side ofthe heart is about 40 mm Hg. Blood picks up carbondioxide as it courses through the tissues, and by the
time it reaches the alveoli, the partial pressure of car-bon dioxide is 45 mm Hg, higher than the 40 mm Hgin the alveolar air. This CO2 pressure gradient causescarbon dioxide to diffuse from the blood to the alveo-lar air. When the blood leaves the lungs and enters theleft atrium, its carbon dioxide partial pressure hasdropped to 40 mm Hg. The difference between 40 and45 mm Hg tells us how much of this waste gas was re-moved from the body.
INTERNAL RESPIRATION SUPPLIESOXYGEN TO THE CELLS AND REMOVESTHEIR GASEOUS WASTE
Internal respiration is the exchange of gases betweenthe blood and the cells (see Figure 12.22). Forsurvival, oxygen in the arteries must reach the tissues,and carbon dioxide generated in the cells must leavethe body. In the capillaries of the systemic circulation,the two gases again diffuse in opposite directions.Oxygen enters the tissues, and carbon dioxide dif-fuses out, again based on partial pressure. The partialpressure of oxygen in the capillary beds of the sys-temic circuit is approximately 95 mm Hg, whereas thepartial pressure of oxygen in most tissues is about 40
Summary of the events of externaland internal respirationFigure 12.22
So far, mmHg has been closed up in text and art. From here on, it's done with a space.
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Ok to have (a) and (b) rather than NGS style in this figure?
390 CHAPTER 12 The Respiratory System: Movement of Air Transport of Oxygen and Carbon Dioxide Requires Hemoglobin and Plasma 391
CONCEPT CHECK
What is the differencebetween external andinternal respiration?
Explain how Dalton’slaw governs themovement of oxygenand carbon dioxide inthe body.
How does the bloodtransport oxygen andcarbon dioxide?
LEARNING OBJECTIVES
Understand the role of hemoglobin in respiration.
Recognize the role of carbon dioxide in maintaining blood pH.
Transport of Oxygen and Carbon DioxideRequires Hemoglobin and Plasma
espiration involves not only the struc-tures of the respiratory system, but alsothe functioning of the cardiovascularsystem. The respiratory system moves
the gases in and out of the body, while the cardiovascu-lar system transports them within the body. The pul-monary capillaries exchange gases in the lungs, whilethe systemic capillaries exchange gases in the body. Thefinal piece to this puzzle is to determine how thesegases are carried through the cardiovascular system be-tween these two capillary beds.
HEMOGLOBIN TRANSPORTS OXYGEN
As we know, the hemoglobin molecule carries oxygenin the bloodstream (Figure
12.23). Hemoglobin picksup oxygen through a bond be-tween the oxygen moleculeand the iron atom of theheme molecule. Hemoglobinhas a high affinity for oxygen
R
Hemoglobin with oxygen binding siteindicated Figure 12.23
AffinityAn attraction be-
tween particles that
increases chances
of combining.
under some conditions but will release it under otherconditions. The oxygen–hemoglobin dissociationcurves discussed in Chapter 11 and reviewed belowdemonstrate hemoglobin’s unique characteristics.
The bond between oxygen and hemoglobin isreversible. Oxygen binds to the iron atom in the hemo-globin molecule when the partial pressure of oxygen ishigh, the pH is high, and the temperature is low. In ar-eas where these conditions do not exist, hemoglobin re-leases oxygen. Even minute changes in temperature orpH will cause hemoglobin to release oxygen (Figure
12.24). Such differences exist in active tissue—mus-cles generate lactic acid and heat while contracting.Contraction requires oxygen to fuel ATP production,which produces lactic acid. An increase in lactic acid, inturn, lowers the pH. Waste heat warms the muscle, andall of these factors increase oxygen delivery to the mus-cle cells.
Several mechanisms transport carbon diox-ide Hemoglobin is best known for carrying oxygen,but it also conveys about 23 percent of total carbondioxide through the bloodstream. This carbon dioxidebinds to the protein portion of hemoglobin, formingcarbaminohemoglobin (see Figure 12.23).
Another 7 percent of the blood-borne carbondioxide is carried as dissolved CO2 gas. The majorshare of blood-borne carbon dioxide (about 70 per-
cent of total carbon dioxide)is carried as bicarbonate ionin plasma. Bicarbonate ion isproduced in steps. First, car-bon dioxide and water com-bine to form carbonic acid in-
side red blood cells. The enzyme carbonic anhydrasespeeds this reaction, allowing red blood cells to re-move most of the carbon dioxide from the blood. Thiscarbonic acid then dissociates into a hydrogen ion anda bicarbonate ion. The hydrogen ion is picked up byhemoglobin, forming reduced hemoglobin. The bicar-bonate ion is transferred out of the RBC in exchangefor a chloride ion coming in to the RBC. The largetransport of chloride ions into the RBCs, called thechloride shift, is an exchange reaction that requiresno ATP because it merely switches the positions of theanions. The bicarbonate in the plasma then serves as a
Low bloodPCO2
Effect of PCO2 on affinity of hemoglobin for oxygen
Per
cent
sat
urat
ion
of h
emog
lobi
n
0 10 20 30
PO2 (mmHg)
40 50 60 70 80 90
10
20
30
40
50
60
70
80
90
100
Normal bloodPCO2
High blood PCO2
Per
cent
sat
urat
ion
of h
emog
lobi
n
0 10 20 30
PO2 (mmHg)
40 50 60 70 80 90
10
20
30
40
50
60
70
80
90
100
Normal blood pH(7.4)
Low blood pH(7.2)
High blood pH(7.6)
Effect of pH on affinity of hemoglobin for oxygenA
B
Effects of temperature and pH on hemoglobinbinding Figure 12.24
Note that mmHg is closed up here in art as it is in other art.
blood is due to a high concen-tration of oxyhemoglobin.But blood inside your body isnever as crimson as what isspilled when you cut yourself.The partial pressure of oxygenin the atmosphere is farhigher than anywhere in yourbody, so hemoglobin quickly picks up more oxygenwhen you bleed.
392 CHAPTER 12 The Respiratory System: Movement of Air Respiratory Health is Critical to Survival 393
buffer, helping to maintainblood pH (Figure 12.25).Without this buffering, wecould not control our internalpH, and we would perish.
Reduced hemoglobinhas a deep crimson, almostpurple color, which is why ve-
nous blood looks so blue when veiwed through our skincompared to arterial blood. The red color of arterial
CONCEPT CHECK
How is oxygen carried inthe blood?
What is the role ofhemoglobin in gastransport?
What is one positiverole of carbon dioxidein the blood?
BufferA compound that
absorbs hydrogen
ions or hydroxide
ions, stabilizing pH.
OxyhemoglobinHemoglobin
molecule with at
least one oxygen
molecule bound to
the iron center.
Anterior view Right lateral view
Frontal sinus
Sphenoidal sinus
Maxillary sinus
Ethmoidal cells
A B
Sinuses Figure 12.26
LEARNING OBJECTIVES
Discuss two common disorders of the upper respiratory tract.
Identify the symptoms of obstructive respiratory disorders.
Understand the main disorders of the lower respiratory tract.
Respiratory Health is Critical to Survival
he previous chapter introduced cardio-vascular disorders and outlined their ob-vious impact on respiration. If the blooddoes not circulate properly, or if it does
not carry enough oxygen, external and internal respi-ration are impaired.
The upper respiratory tract is susceptible to in-fection and inflammation of the nasal passages, sinuses,and larynx. One of the most common upper respira-tory diseases is sinusitis, an inflammation or swelling ofthe sinuses (“-itis” means inflammation). Sinuses arecavities in the skull, lined with the same type of mucusmembrane as the nasal passages (Figure 12.26).Sinuses exist in the frontal bone, ethmoid, sphenoid,and maxillary bones, but the largest are in the frontal
T
In A, the bicarbonate ion is being absorbed from the blood
into the RBC, where it is converted to carbon dioxide and
passed out to the alveolus. Oxygen is seen coming into the
RBC at the alveolus as well. In B, carbon dioxide is passing
from the tissues to the capillaries. Here is it picked up by the
RBC. Inside the RBC, the carbon dioxide is converted to
bicarbonate ions that are then pumped back out to the
blood where they serve as a buffer. Oxygen is seen leaving
the RBC and diffusing into the tissues, where it is used to
drive cellular activities.
Carbon dioxide transport in blood Figure 12.25Pro
cess Diag
ram
HCO3–
Inhaled
Exhaled
CO2
O2
CO2
O2 O2
Tissue cell
Interstitialfluid
Plasma
Exchange of O2 and CO2 in systemic capillaries (internal respiration)
Systemiccapillary wall
Red blood cell
HCO3–
CO2 + H2O
O2 + Hb–H
HCO3– + H+
Hb–O2
H2CO3
CO2
O2
CO2
O2 O2
CO2 + H2O
CO2 + Hb Hb–CO2
O2 + Hb–H
HCO3– + H+
Hb–O2+ H+
H2CO3
Exchange of O2 and CO2 in pulmonary capillaries (external respiration)
394 CHAPTER 12 The Respiratory System: Movement of Air Respiratory Health is Critical to Survival 395
Otitis media distended eardrumFigure 12.27
bone. When you succumb tothe common cold or flu,viruses swell the nasal mem-branes. Histamines are re-leased, and mucus productionincreases as the membranestry to rid the body of the virus.If the membrane lining a sinusswells, the opening can shut,preventing mucus producedin the sinus from draining andcausing it to build up pressurein the closed sinus. Residentpopulations of streptococcusor staphylococcus bacteria canalso grow unchecked in theclosed sinus. Acute sinusitisis usually caused by a commoncold and goes away on its ownwithin two to three weeks.Chronic sinusitis, in contrast,is more severe and its causesare less clear. Most people whosuffer from chronic sinusitisalso have allergies, asthma, or
a compromised immune system owing to a disease likeAIDS. Treating this type of sinusitis is also more diffi-
HistamineA compound in-
volved in allergic
reactions that
causes capillary
leakage and
increased fluid
movement to
affected tissues.
Acute sinusitisInflammation of the
sinuses with sudden
onset and usually of
short duration.
cult; antibiotics, inhalant steroids, or even oral steroidsmay be used depending on the case.
If you have a young child, you probably knowabout otitis media (Figure 12.27). This inflamma-tion of the middle ear fills the middle ear with fluid,distending the eardrum. A stretched eardrum cancause severe pain, and the eardrum can rupture as bac-teria within the trapped fluid multiply. Otitis media isusually caused by a bacterial infection that can betreated with antibiotics. The pathogens most often ar-rive through the eustachian tube, with its open connec-tion between the middle ear and the nasopharynx. Insmall children, the tube is almost horizontal, so fluidsin the mouth can easily travel to the middle ear, espe-cially since the bottom of the tube opens with each swal-low. As we age, our facial bones expand, tilting the eu-stachian tubes toward the vertical, so fluids do not flowso readily to the middle ear. For this reason, ear infec-tion rates drop with age.
Diseases of the lower respiratory tract are usu-ally either obstructive, meaning that something is ob-structing the normal flow of gases through the lungs,or constrictive, indicating that the airways have beennarrowed in some way.
CONSTRICTIVE DISEASES ARE SERIOUSBUT OFTEN SPORADIC
As the name implies, constrictive respiratory diseasesconstrict the airways. One common constrictive diseaseof the lower respiratory tract is bronchitis, an inflam-mation of the mucous membrane lining the bronchi.When this membrane swells, the lumen of the bronchi-ole constricts. Often these infected bronchioles alsoproduce more mucus, which can block air passages.The most common symptom of bronchitis is a deep, of-ten painful, cough. Acute bronchitis can be caused byviruses and occasionally bacteria. Chronic bronchitis ismost often caused by smoking and can last frommonths to years, depending on the severity of the reac-tion to smoke and the duration of the smoking habit.The main symptom of acute and chronic bronchitis is aproductive cough. In acute bronchitis, shortness ofbreath, tightness of the chest, and a general feeling ofillness often accompany the cough. Treatment for bron-
Hea
lth
, Wel
lnes
s, a
nd
Dis
ease
Chronic obstructive pulmonary disease: Why are chronic bronchitis and emphysema so deadly?
Chronic sinusitisInflammation of
the sinuses that
persists for long
periods of time.
Chronic obstructive pulmonary disease (COPD) is actually
two diseases—emphysema and chronic bronchitis—that
both obstruct airflow. (Doctors prefer the term COPD be-
cause individual patients often have both diseases.) In the
United States, the death rate from COPD has doubled in
the past 30 years, to an estimated 120,000 annually. Glob-
ally, scientists predict that COPD will be the third-largest
cause of death by 2020. The major cause is cigarette
smoking, but other airborne toxins and pollutants are
also to blame.
About 8.6 million Americans have been diagnosed
with chronic bronchitis, which starts when the bronchi
get inflamed. Patients develop scarring of the bronchi, ac-
companied by heavy mucus flow and chronic cough. Mu-
cus and thick bronchial walls obstruct airflow, and bacter-
ial infections can fester in the gathered mucus.
Emphysema begins when a pollutant or cigarette
smoke damages the alveoli, forming holes that cannot be
repaired. Delicate lung structures become fibrotic (filled
with fibers) and stiff, reducing their elasticity so exhaling
becomes difficult. The disease starts gradually, with a
shortness of breath, and gets worse with age. More than
80 percent of cases are caused by smoking. About 5 per-
cent of Americans suffer from genetic emphysema caused
by the lack of a protein necessary for lung function.
Both types of COPD reduce gas transfer in the lungs,
causing shortness of breath. Exercise, and even daily ac-
tivity, become difficult or impossible. COPD may be
treated with antibiotics, anti-inflammatories, and bron-
chodilators, which open the airways to ease breathing.
Advanced emphysema patients need supplemental oxy-
gen. An increasing number of COPD patients are receiv-
ing lung transplants. Although transplants can prolong
survival, the failure rate is much higher than with other
organ transplants.
Given the increasing death rate, and the fact that
emphysema is invariably fatal, new perspectives are
needed on COPD. An intensified battle against smoking is
an obvious first step that could bring many other benefits
(see the Ethics and Issues box, “Tobacco, the universal
poison,” on page 00). Researchers have found other clues
that could help explain and treat COPD. For example, a
20-year study found that asthmatics were 12 times as
likely to develop emphysema as other people. Asthma
and emphysema are considered separate diseases, but
this evidence suggests that the emphysema epidemic
may be part of the asthma epidemic. (For more on
asthma, see the I Wonder box, “Why are asthma rates go-
ing up?” on page 00.)
Other research examines what happens after a diag-
nosis of COPD. Stopping smoking can greatly extend one’s
life span. And exercise makes a difference. A study at Ohio
State University found that a 10-week exercise program in-
creased cognitive, psychological, and physical function in
COPD patients. After a COPD patient spends time in the
hospital to treat a life-threatening crisis, physical rehabilita-
tion can reduce symptoms and restore some quality of life.
COPD is not a pretty picture. In most cases, the dis-
ease is much easier to prevent than to cure. For smokers,
prevention should start now.
AU: pls. providecaption to tie thesephotos to textabove
396 CHAPTER 12 The Respiratory System: Movement of Air Respiratory Health is Critical to Survival 397
Asthma attacks are usually triggered by an externalsource, such as exercise, viral infection, or inhalation ofcold air or an allergen, or by high levels of ozone in theair (see the I wonder . . . box on the increasing pre-vlance of asthma).
Asthma may result from an overactive immunesystem, and to many people, inhaling an allergen cancause an immediate and life-threatening constriction ofthe airways. Many asthma patients carry inhalers con-taining bronchodilator drugs to quickly open the air-ways during an acute attack. As a preventative measurebetween attacks, many chronic asthmatics inhale corti-costeriods to reduce the number and severity of asthmaattacks. Despite these medicines, however, asthma stillkills up to 5,000 people every year in the United States.
OBSTRUCTIVE DISEASES CAUSEPERMANENT LUNG DAMAGE
Although asthma is a serious disease, it does not perma-nently damage lung tissue. In contrast, the chronic ob-structive pulmonary diseases, including emphysemaand fibrosis, do damage or destroy the terminal andrespiratory bronchioles. The most common obstructivepulmonary diseases are pneumonia, tuberculosis, em-physema, and lung cancer (see Health, Wellness, andDisease box). After exhalation in all of these diseases,the tubes of the airway do not spring back open be-cause the elastic tissue is destroyed. Pressure builds inthe lungs as the patient tries to force air through thecollapsed tubes, damaging the delicate alveoli and re-ducing the respiratory surface area. The most commoncause of emphysema is smoking, but environmentalpollutants and even genetic factors can also be toblame. Pulmonary fibrosis, a destructive increase in col-lagen that also makes the lungs less elastic, often resultsfrom occupational exposure to silicon or otherirritants.
Lung tissue must remain warm and moist be-cause gases cannot diffuse across a dry membrane. Un-fortunately, these same conditions are perfect for bacte-rial growth. Bacteria living in the warm, moist, lungtissue cause two of the more common obstructive respi-ratory diseases: pneumonia and tuberculosis.
chitis includes rest, plenty of fluids, and perhaps over-the-counter cough medicine. If the cough persists, aninhalant bronchodilator may be prescribed to relax thesmooth muscle of the bronchi, open the constrictedtubes, and help clear the mucus (Figure 12.28).
Asthma is a constrictive pulmonary disease thatcan be life-threatening. During an asthma attack, thesmooth muscle of the bronchi contract, mucus produc-tion increases in these tubes, and the bronchi swell, in-terfering with the passage of air. Breathing grows labo-rious, and wheezing is common during exhalation.
Inhalers contain bronchodilator drugsFigure 12.28
Eth
ics
and
Issu
es
Tobacco, the universal poison
In 1964, the U.S. Surgeon General issued an influential Re-
port on Smoking and Health. The report looks tame given
how much we now know about the toxicity of tobacco
smoke, but it was an early acknowledgment that smoking
causes lung cancer. Today, smoking-related lung cancer kills
an estimated 174,000 people in the United States per year,
and the number is rising.
Some of the approximately 4,000 compounds in to-
bacco smoke attack the delicate epithelial cells lining the
respiratory tract and allow them to grow without control—
the hallmark of cancer. Because early tumors are invisible,
lung cancer is not usually detected until it has spread; there-
fore, the five-year survival rate is only 15 percent. Smoking
and tobacco smoke also:
• are the major cause of emphysema.
• increase the risk of acute myeloid leukemia, and cancerof the throat, mouth, bladder, kidney, stomach, cervix,and pancreas, according to the American Cancer Society.
• impair several functions of the uterine tube, which con-ducts both gametes and the embryo, and alters femalehormone effectiveness. Both effects could explain whysmoking women have high rates of reproductive prob-lems, including undersized and/or premature infants.
• kill nerve cells, interfering with smell and taste.
• elevate pulse and body temperature.
• increase the risk of heart disease by a factor of 2 to 4.
• raise the level of carbon monoxide and reduce the levelof oxygen in the blood, which in turn reduces the abilityto exercise or even comfortably move about.
• destroy cilia in the airways, reducing the ability to expelmucus.
• promote heartburn and peptic ulcers by increasing stom-ach acid production.
Nicotine causes its own set of problems. Nicotine is a
vasoconstrictor, which forces the heart to work harder.
Nicotine’s neurological effects include increased concentra-
tion, a reduction in hunger, and a subtle boost in mood.
Nicotine triggers the release of dopamine, a “feel-good”
neurotransmitter, making “coffin nails,” or cigarettes, highly
addictive.
The tobacco industry is expert at promoting the delu-
sion that smoking is “cool.” Many of their campaigns are tar-
geted, subtly or not, at young people. It’s only logical. With
so many customers dying each year, they need to replace
them with young, healthy smokers.
But many people are quitting, and fewer people are
starting. The number of cigarettes smoked per capita has
declined 59 percent from 1963 to 2004. Still, an estimated
45 million Americans smoke. Smoking causes about 30 per-
cent of all cancers and an estimated 438,000 premature
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ing reduces the risk, even for long-term smokers. Aswith other respiratory illnesses, the symptoms include achronic cough, possibly with bleeding, wheezing, andchest pain. Treatment may include surgical removal ofthe tumor, or destruction of the cancer with radiationor chemotherapy. Unlike other cancers, lung cancer isrelatively easy to prevent. Avoid smoking and exposureto environmental carcinogens such as like asbestos, sili-con, coal dust, and radon gas.
Cystic fibrosis (CF) results from a defectivegene that controls the consistency of mucus in thelungs. The CF version of this gene causes thick, sticky
Respiratory Health is Critical to Survival 399398 CHAPTER 12 The Respiratory System: Movement of Air
Pneumonia is a gen-eral term for a buildup of fluidin the lung, often as a re-sponse to bacterial or viral in-
fection. When the delicate membranes in the alveolibecome inflamed, they secrete fluid in an attempt toeradicate the pathogen, but this fluid inhibits gas ex-change across the membrane. Symptoms of pneumoniainclude a productive cough, lethargy, fever, chills, andshortness of breath. Treatment depends on the under-lying cause of the fluid buildup. Although pneumoniausually can be treated, it can be fatal, especially in pa-tients with weak immunity owing to other serious ill-nesses.
Tuberculosis (TB) is a disease caused by My-cobacterium tuberculosis infection. This tiny bacteriumcan pass from person to person in airborne dropletsgenerated by a sneeze and cough. The inhaled bacteriamultiply from one small region of the infected organ,called the “focus.” Because it is airborne, the focus inhumans is usually in the lung tissue. If the immune sys-tem can combat the disease, scar tissue may form at thefocus. In those rare instances where the body does noteliminate the infection, the bacteria can enter the lym-phatic system and infect just about any organ. The bac-terium can also remain dormant for years and thenreappear in the lungs without warning. Symptoms ofTB resemble those of pneumonia, including a produc-
tive (and often bloody) cough, fever, chills, and short-ness of breath. TB also causes weight loss and nightsweats. TB is usually diagnosed if a focus appears on achest X-ray. Previous exposure can be detected with asimple skin test, which is mandatory for children enter-ing U.S. public schools (Figure 12.29).
A century ago, TB was a major deadly healththreat, but antibiotics have reduced the incidence in in-dustrialized nations. Unfortunately, TB is on the riseagain because antibiotic-resistant strains have now ap-peared, and many patients must take multiple antibi-otics for many months to clear the infection. TB is oneof several cases where bacteria are starting to evade an-tibiotics that once controlled them. This shows howmisuse of antibiotics, combined with their widespreaduse in animal agriculture, may help breed antibiotic-re-sistant strains of bacteria.
Cancer can attack just about any organ system,but lung cancer causes one-third of all cancer deaths inthe United States. Lung cancer can affect the bronchior the alveoli. In either case, the cells proliferate, ob-struct airflow, and prevent gas exchange. Lung canceris primarily due to tobacco smoking; nearly 90 percentof all patients in the United States are current or for-mer smokers. The Ethics and Issues box on page 00 dis-cusses this in greater detail. Lung cancer takes years todevelop, but the risk of lung cancer increases with eachyear of smoking. The good news is that quitting smok-
mucus to be produced, rather than thin, fluid mucusthat is conducive to diffusion. This thick mucus trapsbacteria and slows airflow through the bronchial tree,and it may also block the pancreas and bile duct. Treat-ment for the lung obstruction includes physical therapyto dislodge the mucus (Figure 12.30), and newdrugs that may make the mucus more fluid. Approxi-mately 30,000 people in the United States are currentlyliving with cystic fibrosis. Another 1,000 are diagnosedyearly, usually before age 3. One promising line of re-search would use gene therapy to correct the defectthat causes CF.
