826
UNIT 4
Maintenance of the Body
22
mucus, or inflammatory material also reduce surface area by
blocking gas flow into the alveoli.
Partial Pressure Gradients and Gas Solubilities
Partial pressure gradients of O
2
and CO
2
drive the diffusion of
these gases across the respiratory membrane. A steep oxygen
partial pressure gradient exists across the respiratory membrane
because the P
O
2
of deoxygenated blood in the pulmonary arter-
ies is only 40 mm Hg, as opposed to a P
O
2
of approximately
104 mm Hg in the alveoli. As a result, O
2
diffuses rapidly from
the alveoli into the pulmonary capillary blood
(Figure 22.17)
.
Equilibrium—that is, a P
O
2
of 104 mm Hg on both sides
of the respiratory membrane—usually occurs in 0.25 second,
which is about one-third of the time a red blood cell spends in
a pulmonary capillary
(Figure 22.18)
. Te lesson here is that
blood can flow through the pulmonary capillaries three times
as quickly and still be adequately oxygenated.
Carbon dioxide diffuses in the opposite direction along
a much gentler partial pressure gradient of about 5 mm Hg
(45 mm Hg to 40 mm Hg) until equilibrium occurs at 40 mm Hg.
Expiration then gradually expels carbon dioxide from the alveoli.
Even though the O
2
pressure gradient for oxygen diffusion
is much steeper than the CO
2
gradient, equal amounts of these
gases are exchanged. Why? Te reason is because CO
2
is 20
times more soluble in plasma and alveolar fluid than O
2
.
Ventilation-Perfusion Coupling
For optimal gas exchange, there must be a close match, or cou-
pling, between
ventilation
(the amount of gas reaching the al-
veoli), and
perfusion
(the blood flow in pulmonary capillaries).
Both are controlled by local autoregulatory mechanisms that
continuously respond to local conditions:
P
O
2
controls perfusion by changing
arteriolar
diameter.
P
CO
2
controls ventilation by changing
bronchiolar
diameter.
Influence of Local
P
O
2
on Perfusion
We begin with perfusion
because we introduced its autoregulatory control in Chapter
19. If alveolar ventilation is inadequate, local P
O
2
is low because
Inspired air:
P
O
2
P
CO
2
160 mm Hg
0.3 mm Hg
External
respiration
Blood leaving
lungs and
entering tissue
capillaries:
P
O
2
P
CO
2
100 mm Hg
40 mm Hg
Pulmonary
veins (P
O
2
100 mm Hg)
O
2
CO
2
O
2
CO
2
Pulmonary
arteries
O
2
O
2
CO
2
CO
2
Alveoli of lungs:
P
O
2
P
CO
2
104 mm Hg
40 mm Hg
Heart
Alveoli
Blood leaving
tissues and
entering lungs:
P
O
2
P
CO
2
40 mm Hg
45 mm Hg
Systemic
veins
Systemic
arteries
O
2
CO
2
O
2
CO
2
O
2
CO
2
Tissues:
P
O
2
less than 40 mm Hg
P
CO
2
greater than 45 mm Hg
Internal
respiration
Figure 22.17
Partial pressure gradients promoting gas
movements in the body.
Top: Gradients promoting O
2
and CO
2
exchange across the respiratory membrane in the lungs. Bottom:
Gradients promoting gas movements across systemic capillary
membranes in body tissues. (Note that the small decrease in
P
O
2
in
blood leaving lungs is due to partial dilution of pulmonary capillary
blood with less oxygenated blood.)
P
O
2
(mm Hg)
50
40
100
150
0
0.25
0.50
Time in the
pulmonary capillary (s)
0.75
0
P
O
2
104 mm Hg
End of
capillary
Start of
capillary
Figure 22.18
Oxygenation of blood in the pulmonary
capillaries at rest.
Note that the time from blood entering the
pulmonary capillaries (indicated by 0) until the
P
O
2
is 104 mm Hg is
approximately 0.25 second.
previous page 860 Human Anatomy and Physiology (9th ed ) 2012 read online next page 862 Human Anatomy and Physiology (9th ed ) 2012 read online Home Toggle text on/off