Chapter 22
The Respiratory System
825
22
Hyperbaric oxygen chambers
provide clinical applications of
Henry’s law. Tese chambers contain O
2
gas at pressures higher
than 1 atm and are used to force greater-than-normal amounts
of O
2
into the blood of patients suffering from carbon mon-
oxide poisoning (see p. 829) or tissue damage following radia-
tion therapy. Hyperbaric therapy is also used to treat individuals
with gas gangrene, because the anaerobic bacteria causing this
infection cannot live in the presence of high O
2
levels.
Scuba diving provides another illustration of Henry’s law. If
divers rise rapidly from the depths, dissolved nitrogen forms
bubbles in their blood, causing “the bends.”
Homeostatic Imbalance
22.11
Although breathing O
2
gas at 2 atm is not a problem for short
periods,
oxygen toxicity
develops rapidly when P
O
2
is greater
than 2.5–3 atm. Excessively high O
2
concentrations generate
huge amounts of harmful free radicals, resulting in profound
CNS disturbances, coma, and death.
Composition of Alveolar Gas
As shown in ±able 22.4, the gaseous makeup of the atmosphere is
quite different from that in the alveoli. Te atmosphere is almost
entirely O
2
and N
2
; the alveoli contain more CO
2
and water vapor
and much less O
2
. Tese differences reflect the effects of:
Gas exchanges occurring in the lungs (O
2
diffuses from the
alveoli into the pulmonary blood and CO
2
diffuses in the op-
posite direction)
Humidification of air by conducting passages
Te mixing of alveolar gas that occurs with each breath. Be-
cause only 500 ml of air enter with each tidal inspiration, gas
in the alveoli is actually a mixture of newly inspired gases
and gases remaining in the respiratory passageways between
breaths.
Te alveolar partial pressures of O
2
and CO
2
are easily
changed by increasing breathing depth and rate. A high AVR
brings more O
2
into the alveoli, increasing alveolar P
O
2
and rap-
idly eliminating CO
2
from the lungs.
External Respiration
During external respiration (pulmonary gas exchange), dark
red blood flowing through the pulmonary circuit is trans-
formed into the scarlet river that is returned to the heart for
distribution by systemic arteries to all body tissues. Tis color
change is due to O
2
uptake and binding to hemoglobin in red
blood cells (RBCs), but CO
2
exchange (unloading) is occurring
equally fast.
Te following three factors influence external respiration:
Tickness and surface area of the respiratory membrane
Partial pressure gradients and gas solubilities
Ventilation-perfusion coupling that matches alveolar venti-
lation with pulmonary blood perfusion
Let’s look at these factors one by one.
Thickness and Surface Area
of the Respiratory Membrane
In healthy lungs, the respiratory membrane is only 0.5 to 1 μm
thick, and gas exchange is usually very efficient.
Homeostatic Imbalance
22.12
Te effective thickness of the respiratory membrane increases dra-
matically if the lungs become waterlogged and edematous, as in
pneumonia or leF heart failure (see p. 685). Under such condi-
tions, even the 0.75 s that red blood cells spend in transit through
the pulmonary capillaries may not be enough for adequate gas
exchange, and body tissues suffer from oxygen deprivation.
Te greater the surface area of the respiratory membrane, the
more gas can diffuse across it in a given time period. In healthy
lungs, the alveolar surface area is enormous. Spread flat, the to-
tal gas exchange surface of these tiny sacs in an adult male’s
lungs is about 90 m
2
—approximately 40 times greater than the
surface area of his skin!
Homeostatic Imbalance
22.13
Certain pulmonary diseases drastically reduce the alveolar sur-
face area. ²or instance, in emphysema the walls of adjacent al-
veoli break down and the alveolar chambers enlarge. ±umors,
Table 22.4
Comparison of Gas Partial Pressures and Approximate Percentages in the Atmosphere
and in the Alveoli
 
ATMOSPHERE (SEA LEVEL)
ALVEOLI
GAS
APPROXIMATE
PERCENTAGE
PARTIAL PRESSURE
(mm Hg)
APPROXIMATE
PERCENTAGE
PARTIAL PRESSURE
(mm Hg)
N
2
78.6
597
74.9
569
O
2
20.9
159
13.7
104
CO
2
0.04
0.3
5.2
40
H
2
O
0.46
3.7
6.2
47
 
100.0%
760
100.0%
760
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