761
CHAPTER NINETEEN
Respiratory System
the P
CO
2
in alveolar air is 40 mm Hg. Because of the differ-
ence in these partial pressures, carbon dioxide diffuses from
blood, where its partial pressure is higher, across the respi-
ratory membrane and into alveolar air. When blood leaves
the lungs, its P
CO
2
is 40 mm Hg, the same as the P
CO
2
of alve-
olar air. Similarly, the P
O
2
of blood entering the pulmonary
capillaries is 40 mm Hg, but reaches 104 mm Hg as oxygen
diffuses from alveolar air into the blood. Thus, because
equilibrium is reached, blood leaves the alveolar capillaries
with a P
O
2
of 104 mm Hg. (Some venous blood draining the
bronchi and bronchioles mixes with this blood before return-
ing to the heart, so the P
O
2
of left atrial, left ventricular, and
systemic arterial blood is 95 mm Hg.) Because of the large
volume of air always in the lungs as long as breathing con-
Cell of
capillary wall
Capillary lumen
Alveolus
Macrophage
Type II
(surfactant-
secreting) cell
Fluid with
surfactant
Respiratory
membrane
Type I
(squamous
epithelial) cell
of alveolar wall
Capillary
Alveolus
Red blood cell
Diffusion of CO
2
Diffusion of O
2
Capillary endothelium
Basement membrane of
capillary endothelium
Interstitial space
Respiratory
membrane
Basement membrane of
alveolar epithelium
Alveolar epithelium
Alveolar fluid
(with surfactant)
FIGURE 19.33
The respiratory membrane consists of the walls of the alveolus and the capillary.
tinues, alveolar P
O
2
stays relatively constant at 104 mm Hg.
Clinical Application 19.5 looks at a respiratory effect that
occurs under speciF
c conditions—high altitude.
The respiratory membrane is normally thin (about 1
micrometer thick), and gas exchange is rapid. However,
a number of factors may affect diffusion across the respi-
ratory membrane. More surface area, shorter distance,
greater solubility of gases, and a steeper partial pressure
gradient all favor increased diffusion. Diseases that harm
the respiratory membrane, such as pneumonia, or reduce
the surface area for diffusion, such as emphysema, impair
gas exchange. These conditions may require increased P
O
2
for treatment. Clinical Application 19.6 examines illnesses
that result from impaired gas exchange.
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