Human Physiology An Integrated Approach 6/E

33 Pages·2017·4.62 MB·English

by  Dee Unglaub Silverthorn

17
Mechanics of Breathing
The Respiratory System
Bones and Muscles of the Thorax Surround the Lungs
Pleural Sacs Enclose the Lungs
Airways Connect Lungs to the External Environment
The Airways Warm, Humidify, and Filter Inspired Air
Alveoli Are the Site of Gas Exchange
Pulmonary Circulation Is High-Flow, Low-Pressure
Gas Laws
Air Is a Mixture of Gases
Gases Move Down Pressure Gradients
Boyle’s Law Describes Pressure-Volume Relationships
Ventilation
Lung Volumes Change During Ventilation
During Ventilation, Air Flows Because of Pressure Gradients
Inspiration Occurs When Alveolar Pressure Decreases
Expiration Occurs When Alveolar Pressure Increases
Intrapleural Pressure Changes During Ventilation
Lung Compliance and Elastance May Change in Disease States
Surfactant Decreases the Work of Breathing
Airway Diameter Determines Airway Resistance
Rate and Depth of Breathing Determine the Effi ciency of Breathing
Gas Composition in the Alveoli Varies Little During Normal Breathing
Ventilation and Alveolar Blood Flow Are Matched
Auscultation and Spirometry Assess Pulmonary Function
This being of mine,
whatever it really
is, consists of a little
fl esh, a little breath,
and the part which
governs.
— Marcus Aurelius Antoninus
( C . E . 121–180)
B ackground Basics
Ciliated and exchange
epithelia
Pressure, volume, fl ow,
and resistance
Pulmonary circulation
Colored x-ray
Surface tension
of the lung
Autonomic and somatic showing the
motor neurons branching
Velocity of fl ow airways.
600
Mechanics of Breathing
I
magine covering the playing surface of a racquetball court cavity to control their contact with the outside air. Internalization
(about 75 m2) with thin plastic wrap, then crumpling up the creates a humid environment for the exchange of gases with the
wrap and stuffi ng it into a 3-liter soft drink bottle. Impossible? blood and protects the delicate exchange surface from damage.
Maybe so, if you use plastic wrap and a drink bottle. But the lungs Internalized lungs create another challenge, however: how to
of a 70-kg man have a gas exchange surface the size of that plastic move air between the atmosphere and an exchange surface deep
wrap, compressed into a volume that is less than that of the bottle. within the body. Air flow requires a muscular pump to create
Th is tremendous surface area for gas exchange is needed to supply pressure gradients. More complex respiratory systems therefore
the trillions of cells in the body with adequate amounts of oxygen. consist of two separate components: a muscle-driven pump and a
A erobic metabolism in cells depends on a steady supply thin, moist exchange surface. In humans, the pump is the muscu-
of oxygen and nutrients from the environment, coupled with loskeletal structure of the thorax. Th e lungs themselves consist of
the removal of carbon dioxide. In very small aquatic animals, the exchange epithelium and associated blood vessels.
simple diff usion across the body surface meets these needs. Dis- Th e four primary functions of the respiratory system are:
tance limits diff usion rate, however, so most multicelled animals
1 E xchange of gases between the atmosphere and the blood.
require specialized respiratory organs associated with a circula-
Th e body brings in O for distribution to the tissues and
tory system. Respiratory organs take a variety of forms, but all 2
eliminates CO waste produced by metabolism.
possess a large surface area compressed into a small space. 2
2 H omeostatic regulation of body pH. The lungs can alter 17
Besides needing a large exchange surface, humans and
body pH by selectively retaining or excreting CO .
other terrestrial animals face an additional physiological chal- 2
3 P rotection from inhaled pathogens and irritating substances.
lenge: dehydration. The exchange surface must be thin and
Like all other epithelia that contact the external environ-
moist to allow gases to pass from air into solution, and yet at
ment, the respiratory epithelium is well supplied with de-
the same time it must be protected from drying out as a result
fense mechanisms to trap and destroy potentially harmful
of exposure to air. Some terrestrial animals, such as the slug
substances before they can enter the body.
(a shell-less snail), meet the challenge of dehydration with be-
4 V ocalization. Air moving across the vocal cords creates
havioral adaptations that restrict them to humid environments
vibrations used for speech, singing, and other forms of
and nighttime activities.
communication.
A more common solution is anatomical: an internalized
respiratory epithelium. Human lungs are enclosed in the chest I n addition to serving these functions, the respiratory sys-
tem is also a signifi cant source of water loss and heat loss from
the body. These losses must be balanced using homeostatic
compensations.
RUNNING PROBLEM
I n this chapter you will learn how the respiratory system
Emphysema carries out these functions by exchanging air between the envi-
ronment and the interior air spaces of the lungs. Th is exchange
You could hear her whistling, wheezing breathing preceding
is the b ulk fl ow of air, and it follows many of the same principles
her down the hall. “Diagnosis: COPD,” reads Edna Wilson’s
that govern the bulk fl ow of blood through the cardiovascular
patient chart. COPD—chronic obstructive pulmonary
disease—is the name given to diseases in which air system:
exchange is impaired by narrowing of the lower airways.
1 F low takes place from regions of higher pressure to regions
Most people with COPD have emphysema or chronic
of lower pressure.
bronchitis or a combination of the two. Individuals in whom
2 A muscular pump creates pressure gradients.
chronic bronchitis predominates are sometimes called “blue
3 R esistance to air fl ow is infl uenced primarily by the diam-
bloaters,” owing to the bluish tinge of their skin (from low
eter of the tubes through which the air is fl owing.
blood oxygen levels) and a tendency to be overweight. In
contrast, patients with emphysema have been nicknamed A ir and blood are both fl uids. Th e primary diff erence be-
“pink puff ers.” They tend to be thin, have normal (pink) skin
tween air fl ow in the respiratory system and blood fl ow in the
coloration, and often breathe out through pursed lips, which
circulatory system is that air is a less viscous, compressible mix-
helps open their airways. More than 12 million people in
ture of gases while blood is a noncompressible liquid.
the United States have COPD. Its most common cause is
smoking, and most people can avoid the disease simply by
not smoking. Unfortunately, Edna has been a heavy smoker
for 35 of her 47 years. The Respiratory System
T he word r espiration has several meanings in physiology
( Fig. 17.1 ). Cellular respiration refers to the intracellular
reaction of oxygen with organic molecules to produce carbon
601
Mechanics of Breathing
consists of structures involved in ventilation and gas exchange
EXTERNAL RESPIRATION
( F ig. 17.2 ):
The respiratory and circulatory systems coordinate to move oxygen 1 Th e c onducting system of passages, or airways , that lead
and CO between the atmosphere and the cells.
2 from the external environment to the exchange surface of
Exchange I: CO2 O2 the lungs.
atmosphere 2 Th e alveoli (singular alveolus) {a lveus, a concave vessel},
to lung
a series of interconnected sacs and their associated p ulmo-
(ventilation) Airways
nary capillaries. Th ese structures form the exchange sur-
face, where oxygen moves from inhaled air to the blood,
Alveoli
of lungs and carbon dioxide moves from the blood to air that is
CO2 O2 about to be exhaled.
Exchange II:
3 Th e bones and muscles of the thorax (chest cavity) and ab-
lung to blood
CO2 O2 domen that assist in ventilation.
Transport of
Pulmonary Th e respiratory system can be divided into two parts. Th e
gases in
circulation
the blood upper respiratory tract consists of the mouth, nasal cavity,
pharynx, and larynx. Th e lower respiratory tract consists of the
trachea, two primary bronchi { bronchos, windpipe; singular—
bronchus}, their branches, and the lungs. Th e lower tract is also
known as the thoracic portion of the respiratory system because
Systemic it is enclosed in the thorax.
circulation
Bones and Muscles of the Thorax
Exchange III: CO2 O2 Surround the Lungs
blood to cells
CO2 O2 Th e thorax is bounded by the bones of the spine and rib cage
Cellular and their associated muscles. Together the bones and muscles
Cells respiration are called the thoracic cage . Th e ribs and spine (the chest wall )
Nutrients
form the sides and top of the cage. A dome-shaped sheet of skel-
ATP
etal muscle, the diaphragm , forms the fl oor ( Fig. 17.2 b).
Two sets of intercostal muscles , internal and external,
Fig. 17.1
connect the 12 pairs of ribs (F ig. 1 7.2a ). Additional muscles, the
sternocleidomastoids and the s calenes , run from the head and
neck to the sternum and fi rst two ribs.
d ioxide, water, and energy in the form of ATP. External respi- F unctionally, the thorax is a sealed container fi lled with three
ration, the topic of this chapter and the next, is the movement membranous bags, or sacs. One, the p ericardial sac, contains the
of gases between the environment and the body’s cells. External heart. Th e other two bags, the p leural sacs, each surround a lung
respiration can be subdivided into four integrated processes, il- {p leura, rib or side}. The esophagus and thoracic blood vessels
lustrated in Figure 17.1 : and nerves pass between the pleural sacs ( Fig. 17.2 c).
1 Th e exchange of air between the atmosphere and the lungs.
Pleural Sacs Enclose the Lungs
Th is process is known as v entilation, or breathing. I nspi-
ration (inhalation) is the movement of air into the lungs.
Th e lungs ( Fig. 17.2 b, d) consist of light, spongy tissue whose
Expiration (exhalation) is the movement of air out of the
volume is occupied mostly by air-fi lled spaces. Th ese irregular
lungs. Th e mechanisms by which ventilation takes place
cone-shaped organs nearly fill the thoracic cavity, with their
are collectively called the mechanics of breathing .
bases resting on the curved diaphragm. Semi-rigid conducting
2 T he exchange of O and CO between the lungs and the
2 2 airways—the bronchi—connect the lungs to the main airway,
blood.
the trachea.
3 Th e transport of O and CO by the blood.
2 2 E ach lung is surrounded by a double-walled pleu-
4 Th e exchange of gases between blood and the cells.
ral sac whose membranes line the inside of the thorax and
External respiration requires coordination between the re- cover the outer surface of the lungs ( Fig. 17.3 ). Each pleu-
spiratory and cardiovascular systems. Th e respiratory system ral membrane, or p leura, contains several layers of elastic
602
Mechanics of Breathing
c onnective tissue and numerous capillaries. Th e opposing layers
Concept Check Answers: End of Chapter
of pleural membrane are held together by a thin fi lm of pleu-
ral fl uid whose total volume is only about 25–30 mL in a 70-kg 1. What is the diff erence between cellular respiration and external
man. Th e result is similar to an air-fi lled balloon (the lung) sur- respiration?
rounded by a water-fi lled balloon (the pleural sac). Most illus-
2. Name the components of the upper respiratory tract and those of the
trations exaggerate the volume of the pleural fl uid, but you can lower respiratory tract.
appreciate its thinness if you imagine spreading 25 mL of water
3. Based on the total cross-sectional area of diff erent airways, where is
evenly over the surface of a 3-liter soft drink bottle.
the velocity of air fl ow highest and lowest?
Pleural fluid serves several purposes. First, it creates a
moist, slippery surface so that the opposing membranes can 4. Give two functions of pleural fl uid.
slide across one another as the lungs move within the thorax.
5. N ame the components (including muscles) of the thoracic cage. List the
Second, it holds the lungs tight against the thoracic wall. To vi- contents of the thorax.
sualize this arrangement, think of two panes of glass stuck to-
6. Which air passages of the respiratory system are collapsible?
gether by a thin fi lm of water. You can slide the panes back and
forth across each other, but you cannot pull them apart because
of the cohesiveness of the water. A similar fl uid bond between
17
the two pleural membranes makes the lungs “stick” to the tho- The Airways Warm, Humidify,
racic cage and holds them stretched in a partially infl ated state,
and Filter Inspired Air
even at rest.
During breathing, the upper airways and the bronchi do more
than simply serve as passageways for air. Th ey play an important
A irways Connect Lungs to the role in conditioning air before it reaches the alveoli. Condition-
External Environment ing has three components:
1 W arming air to body temperature (37 (cid:2)C ), so that core
Air enters the upper respiratory tract through the mouth and
body temperature does not change and alveoli are not
nose and passes into the p harynx, a common passageway
damaged by cold air;
for food, liquids, and air {p harynx, throat}. From the pharynx,
2 A dding water vapor until the air reaches 100% humidity, so
air flows through the larynx into the trachea , or windpipe
that the moist exchange epithelium does not dry out; and
( Fig. 1 7.2 b). The larynx contains the v ocal cords , connective
3 F iltering out foreign material, so that viruses, bacteria, and
tissue bands that vibrate and tighten to create sound when air
inorganic particles do not reach the alveoli.
moves past them.
T he trachea is a semiflexible tube held open by 15 to 20 I nhaled air is warmed by the body’s heat and moistened by
C-shaped cartilage rings. It extends down into the thorax, where water evaporating from the mucosal lining of the airways. Un-
it branches (division 1) into a pair of p rimary bronchi , one der normal circumstances, by the time air reaches the trachea,
bronchus to each lung (F ig. 1 7.2b ). Within the lungs, the bronchi it has been conditioned to 100% humidity and 37 (cid:2)C . Breath-
branch repeatedly (divisions 2–11) into progressively smaller ing through the mouth is not nearly as eff ective at warming and
bronchi (F ig. 1 7.2e ). Like the trachea, the bronchi are semi-rigid moistening air as breathing through the nose. If you exercise
tubes supported by cartilage. outdoors in very cold weather, you may be familiar with the
Within the lungs, the smallest bronchi branch to become ache in your chest that results from breathing cold air through
bronchioles, small collapsible passageways with walls of smooth your mouth.
muscle. Th e bronchioles continue branching (divisions 12–23) Air is filtered both in the trachea and in the bronchi.
until the respiratory bronchioles form a transition between the These airways are lined with ciliated epithelium whose cilia
airways and the exchange epithelium of the lung. are bathed in a watery saline layer ( F ig. 17.5 ). The saline is
T he diameter of the airways becomes progressively produced by epithelial cells when Cl- secreted into the lumen
smaller from the trachea to the bronchioles, but as the individ- by apical anion channels draws Na+ into the lumen through the
ual airways get narrower, their numbers increase geometrically paracellular pathway ( Fig. 17.5 c). Movement of solute from the
( F ig. 17.4 ). As a result, the total cross-sectional area in- ECF to the lumen creates an osmotic gradient, and water follows
creases with each division of the airways. Total cross- the ions into the airways. Th e CFTR channel, whose malfunc-
sectional area is lowest in the upper respiratory tract and tion causes cystic fi brosis, is one of the anion channels found on
greatest in the bronchioles, analogous to the increase in cross- the apical surface of this epithelium.
sectional area that occurs from the aorta to the capillaries in A sticky layer of mucus fl oats over the cilia to trap most
the circulatory system. inhaled particles larger than 2 mm . Th e mucus layer is secreted
603
Fig. 17.2 ANATOMY SUMMARY
The Lungs and Thoracic Cavity
(a) Muscles of the thorax, neck, and abdomen (b) The respiratory system is divided
create the force to move air during breathing. into upper and lower regions.
Pharynx
Nasal cavity
Upper Vocal cords
Sternocleido-
respiratory
mastoids Tongue
system
Scalenes
Esophagus Larynx
Trachea
Lower
respiratory
system
External Internal
intercostals intercostals
Diaphragm
Abdominal
muscles

Left lung
Right lung
Muscles Muscles
Diaphragm
of inspiration of expiration Right bronchus Left bronchus
(c) Sectional view of chest. Each lung is enclosed in (d) On external view, the right lung is divided
two pleural membranes. The esophagus and aorta into three lobes, and the left lung is
pass through the thorax between the pleural sacs. divided into two lobes.
Apex
Pleural
Esophagus Aorta membranes
Superior
lobe
Superior
lobe
Right Left
lung lung
Middle
lobe
Heart
Inferior Inferior
lobe lobe
Right pleural Pericardial Left pleural
cavity cavity cavity Base Cardiac notch
Superior view
604
The Bronchi and Alveoli
(e) Branching of airways creates (f) Structure of lung lobule. Each cluster of alveoli is
about 80 million bronchioles. surrounded by elastic fibers and a network of capillaries.
Larynx
Bronchiole
Branch of
pulmonary artery
Bronchial artery, Smooth muscle
nerve and vein
The trachea
branches into Trachea
two primary
bronchi.
Branch of Elastic
Left primary pulmonary fibers
Cartilage bronchus vein
ring
Capillary
Lymphatic
beds
vessel
The primary bronchus
divides 22 more times,
terminating in a cluster Secondary
of alveoli. bronchus
Alveoli
Bronchiole
Alveoli
(g) Alveolar structure
(h) Exchange surface of alveoli
Capillary Elastic fibers
Alveolar Nucleus of
epithelium endothelial cell
RBC
Type I alveolar
cell for gas
exchange
Capillary
Endothelium Plasma
Endothelial
cell of capillary 0.1-
1.5
Type II alveolar μm
cell (surfactant
cell) synthesizes Alveolar
surfactant. air space
Alveolus
Surfactant Fused
Limited basement
interstitial membranes
fluid
Alveolar Blue arrow represents gas exchange
macrophage between alveolar air space and the plasma.
ingests foreign
material.
605
Mechanics of Breathing
THE PLEURAL SAC Concept Check A nswers: End of Chapter
The pleural sac forms a double membrane surrounding the lung, 7. Cigarette smoking paralyzes cilia in the airways and increases mucus
similar to a fluid-filled balloon surrounding an air-filled balloon. production. Why would these eff ects cause smokers to develop a
cough?
Pleural
membrane
Air-filled Air space
balloon of lung
Alveoli Are the Site of Gas Exchange
Fluid-filled
balloon The pleural fluid has a much Th e alveoli, clustered at the ends of terminal bronchioles, make
smaller volume than is up the bulk of lung tissue (F ig. 1 7.2f , g). Th eir primary function
suggested by this illustration.
is the exchange of gases between themselves and the blood.
Fig. 17.3 E ach tiny alveolus is composed of a single layer of
epithelium ( Fig. 17.2 g). Two types of epithelial cells are found
in the alveoli. The smaller but thicker type II alveolar cells
by goblet cells in the epithelium ( Fig. 17.5 b). Th e cilia beat with synthesize and secrete a chemical known as s urfactant.
an upward motion that moves the mucus continuously toward Surfactant mixes with the thin fl uid lining of the alveoli to aid
the pharynx, creating what is called the mucociliary escalator . lungs as they expand during breathing, as you will see later in
Mucus contains immunoglobulins that can disable many patho- this chapter. Type II cells also help minimize the amount of fl uid
gens. Once mucus reaches the pharynx, it can be spit out (e xpec- present in the alveoli by transporting solutes, followed by water,
torated ) or swallowed. For swallowed mucus, stomach acid and out of the alveolar air space.
enzymes destroy any remaining microorganisms. T he larger t ype I alveolar cells occupy about 95% of
Secretion of the watery saline layer beneath the mucus is the alveolar surface area and are very thin so that gases can dif-
essential for a functional mucociliary escalator. In the disease fuse rapidly through them (F ig. 1 7.2h ). In much of the exchange
cystic fi brosis, for example, inadequate ion secretion decreases area, a layer of basement membrane fuses the alveolar epithe-
fl uid movement in the airways. Without the saline layer, cilia be- lium to the capillary endothelium. In the remaining area only a
come trapped in thick, sticky mucus. Mucus cannot be cleared, small amount of interstitial fl uid is present.
and bacteria colonize the airways, resulting in recurrent lung The thin walls of the alveoli do not contain muscle be-
infections. cause muscle fi bers would block rapid gas exchange. As a result,
BRANCHING OF THE AIRWAYS
Cross-sectional
Name Division Diameter (mm) How many?
area (cm2)
Conducting system Trachea 0 15-22 1 2.5
Primary bronchi 1 10-15 2
Smaller 2 4
bronchi
3
4 1-10
5
6-11 1 x 104
2 x 104 100
Bronchioles 12-23 0.5-1
Exchange surface 8 x 107 5 x 103
Alveoli 24 0.3 3-6 x 108 >1 x 106
Fig. 17.4
606
Mechanics of Breathing
AIRWAY EPITHELIUM
(a) Epithelial cells lining the airways and submucosal (b) Cilia move the mucus layer toward the pharynx, removing trapped
glands secrete saline and mucus. pathogens and particulate matter.
Dust particle
Ciliated Mucus layer traps
epithelium inhaled particles.
Watery saline layer
allows cilia to
push mucus
toward pharynx.
Cilia
Goblet cell
secretes mucus.
Movement of mucus Nucleus of
columnar 17
Mucus layer Submucosal epithelial cell
Lumen of airway gland Basement
membrane
(c) One model of saline secretion by airway epithelial cells
Sina lluinmee lanyer Na+ H2O Cl–
2 1 NKCC brings Cl– into epithelial
cell from ECF.
Anion
Respiratory channel
epithelial 2 Apical anion channels,
cells including CFTR, allow Cl– to
enter the lumen.
3 Na+ goes from ECF to lumen
by the paracelllular pathway,
drawn by the electrochemical
gradient.
K+
ATP
4 NaCl movement from ECF to
3 Na+ Na+ Na+2Cl–K+ K+ lumen creates a concentration
H O gradient so water follows into
ECF 2 1 the lumen.
4
Fig. 17.5
lung tissue itself cannot contract. However, connective tis- proximity of capillary blood to alveolar air is essential for the
sue between the alveolar epithelial cells contains many elastin rapid exchange of gases.
and collagen fi bers that create elastic recoil when lung tissue is
stretched. Pulmonary Circulation
Th e close association of the alveoli with an extensive net- Is High-Flow, Low-Pressure
work of capillaries demonstrates the intimate link between the
respiratory and cardiovascular systems. Blood vessels fi ll 80– Th e pulmonary circulation begins with the pulmonary trunk,
90% of the space between alveoli, forming an almost continuous which receives low-oxygen blood from the right ventricle. Th e
“sheet” of blood in close contact with the air-fi lled alveoli. Th e pulmonary trunk divides into two pulmonary arteries, one to
607
Mechanics of Breathing
CLINICAL FOCUS Concept Check Answers: End of Chapter
8. Is blood fl ow through the pulmonary trunk greater than, less than, or
Congestive Heart Failure equal to blood fl ow through the aorta?
When is a lung problem not a lung problem? The 9. A person has left ventricular failure but normal right ventricular
answer: when it’s really a heart problem. Congestive heart function. As a result, blood pools in the pulmonary circulation,
failure (CHF) is an excellent example of the interrelationships doubling pulmonary capillary hydrostatic pressure. What happens to
among body systems and demonstrates how disruptions net fl uid fl ow across the walls of the pulmonary capillaries?
in one system can have a domino eff ect in the others. The
10. Calculate the mean pressure in a person whose pulmonary arterial
primary symptoms of heart failure are shortness of breath
pressure is 25 8 mm Hg.
( dyspnea ), wheezing during breathing, and sometimes a >
productive cough that may be pinkish from the presence
of blood. Congestive heart failure arises when the right
heart is a more eff ective pump than the left heart. When
Gas Laws
blood accumulates in the pulmonary circulation, increased
volume increases pulmonary blood pressure and capillary
Respiratory air fl ow is very similar in many respects to blood
hydrostatic pressure. Capillary fi ltration exceeds the ability
fl ow in the cardiovascular system because both air and blood
of the lymph system to drain interstitial fl uid, resulting in
pulmonary edema. Treatment of CHF includes increasing are fluids. Their primary difference is that blood is a non-
urinary output, which brings yet another organ system into compressible liquid but air is a compressible mixture of gases.
the picture. By current estimates, about 5 million Americans Figure 17.6 summarizes the laws that govern the behavior of
suff er from CHF. To learn more about this condition, visit the gases in air and provide the basis for the exchange of air between
American Heart Association web site ( www.americanheart. the external environment and the alveoli.
org ) or MedlinePlus, published by the National Institutes of In this course, blood pressure and environmental air pres-
Health ( www.nlm.nih.gov/medlineplus/heartfailure.html ).
sure (a tmospheric pressure ) are both reported in millimeters of
mercury (mm Hg). Respiratory physiologists sometimes report
gas pressures in centimeters of water instead, where 1 mm Hg =
1.36 cm H O , or in kiloPascals (kPa), where 760 mm Hg =
2
each lung. Oxygenated blood from the lungs returns to the left 101.325 kPa.
atrium via the pulmonary veins. A t sea level, normal atmospheric pressure is 760 mm Hg.
A t any given moment, the pulmonary circulation contains However, in this course we follow the convention of designating
about 0.5 liter of blood, or 10% of total blood volume. About 75 mL atmospheric pressure as 0 mm Hg. Because atmospheric pres-
of this amount is found in the capillaries, where gas exchange sure varies with altitude and because very few people live ex-
takes place, with the remainder in pulmonary arteries and veins. actly at sea level, this convention allows us to compare pressure
Th e rate of blood fl ow through the lungs is much higher than the diff erences that occur during ventilation without correcting for
rate in other tissues because the lungs receive the entire cardiac altitude.
output of the right ventricle: 5 L min. Th is means that as much
>
blood fl ows through the lungs in one minute as fl ows through
the entire rest of the body in the same amount of time! RUNNING PROBLEM
Despite the high fl ow rate, pulmonary blood pressure is low.
Edna has not been able to stop smoking, and her COPD
Pulmonary arterial pressure averages 25 8 mm Hg, much lower
> is a combination of emphysema and bronchitis. Patients
than the average systemic pressure of 120 80 mm Hg. Th e right
> with chronic bronchitis have excessive mucus production
ventricle does not have to pump as forcefully to create blood fl ow
and exhibit general infl ammation of the entire respiratory
through the lungs because resistance of the pulmonary circulation
tract. The mucus narrows the airways and makes breathing
is low. Th is low resistance can be attributed to the shorter total diffi cult.
length of pulmonary blood vessels and to the distensibility and
large total cross-sectional area of pulmonary arterioles. Q1: What does narrowing of the airways do to airway
resistance?
N ormally, the net hydrostatic pressure fi ltering fl uid out of
a pulmonary capillary into the interstitial space is low because
of low mean blood pressure. Th e lymphatic system effi ciently re-
moves fi ltered fl uid, and lung interstitial fl uid volume is usually
minimal. As a result, the distance between the alveolar air space
and the capillary endothelium is short, and gases diff use rapidly
between them.
608
Fig. 17.6 ESSENTIALS
Gas Laws
This figure summarizes the rules that govern the behavior of gases
in air. These rules provide the basis for the exchange of air between
the external environment and the alveoli.
(a) The ideal gas equation
Where P is pressure, V is volume, n is the moles of gas, T is absolute
PV = nRT
temperature, and R is the universal gas constant, 8.3145 j/mol × K
In the human body we can assume that the number of moles and temperature
are constant. Removing the constants leaves the following equation:
This relationship says that if the volume of gas
V = 1/P
increases, the pressure decreases, and vice versa.
(b) Boyle’s Law
Boyle’s law also expresses this inverse relationship between pressure and volume.
For example, the container on the left is 1 L (V )
1
P V = P V and has a pressure of 100 mm Hg (P ).
1 1 2 2 1
What happens to the pressure when the volume
decreases to 0.5 L?
100 mm Hg × 1 L = P × 0.5 L
2
200 mm Hg = P
2
The pressure has increased ×2.
The Ideal Gas law and Boyle’s law apply
V = 1.0 L V = 0.5 L
to all gases or mixtures of gases. 1 2
P = 100 mm Hg P = 200 mm Hg
1 2
(c) Dalton’s Law
Dalton’s law says that the total pressure of a mixture of gases is the sum of the pressures
of the individual gases. The pressure of an individual gas in a mixture is known as the
partial pressure of the gas (P ).
gas
For example, at sea level, atmospheric pressure (P ) is 760 mm Hg, In humid air, water vapor “dilutes” the contribution
atm
and oxygen is 21% of the atmosphere. What is the partial pressure of of other gases to the total pressure.
oxygen (P )?
O2
Partial Pressures (P ) of Atmospheric Gases at 760 mm Hg
To find the partial pressure of any one gas in a sample gas
of dry air, multiply the atmospheric pressure (P ) by the gas’s
atm Gas and its P in dry P in P in
relative contribution (%) to P : gas gas gas
atm percentage in air 25 ˚C air 25 ˚C air, 37 ˚C air,
100% humidity 100% humidity
Partial pressure of a gas = Patm × % of gas in atmosphere Oxygen (O2) 21% 160 mm Hg 155 mm Hg 150 mm Hg
Carbon dioxide
PO2 = 760 mm Hg x 21% PO2 (CO2) 0.03% 0.25 mm Hg 0.24 mm Hg 0.235 mm Hg
= 760 mm × 0.21 = 160 mm Hg
Water vapor 0 mm Hg 24 mm Hg 47 mm Hg
The partial pressure of oxygen (P ) in dry air at sea level
O2
is 160 mm Hg. To calculate the partial pressure of a gas in humid air, you must first subtract
the water vapor pressure from the total pressure. At 100% humidity and
The pressure exerted by an individual gas is determined only by its 25° C, water vapor pressure (PH2O) is 24 mm Hg.
relative abundance in the mixture and is independent of the
molecular size or mass of the gas.
P in humid air = (P – P ) × % of gas
gas atm H2O
P = (760 – 24) × 21% = 155 mm Hg
O2
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