824
UNIT 4
Maintenance of the Body
22
Gas Exchanges Between
the Blood, Lungs, and Tissues
State Dalton’s law of partial pressures and Henry’s law.
Describe how atmospheric and alveolar air differ in
composition, and explain these differences.
Relate Dalton’s and Henry’s laws to events of external and
internal respiration.
As you’ve discovered, during
external respiration
oxygen enters
and carbon dioxide leaves the blood in the lungs by diffusion. At
the body tissues, where the process is called
internal respiration
,
the same gases move in opposite directions, also by diffusion. To
understand these processes, let’s examine the physical proper-
ties of gases and consider the composition of alveolar gas.
Basic Properties of Gases
Beyond Boyle’s law, two more gas laws provide most of the infor-
mation we need—
Dalton’s law of partial pressures
reveals how a
gas behaves when it is part of a mixture of gases, and
Henry’s law
helps us understand how gases move into and out of solution.
Dalton’s Law of Partial Pressures
Dalton’s law of partial pressures
states that the total pressure
exerted by a mixture of gases is the sum of the pressures exerted
independently by each gas in the mixture. Further, the pressure
exerted by each gas—its
partial pressure
—is directly propor-
tional to the percentage of that gas in the gas mixture.
As indicated in
Table 22.4
, nitrogen makes up about 79% of
air, and the partial pressure of nitrogen P
N
2
is 78.6%
3
760 mm
Hg, or 597 mm Hg. Oxygen, which accounts for nearly 21% of
air, has a partial pressure P
O
2
of 159 mm Hg (20.9%
3
760 mm
Hg). Together nitrogen and oxygen contribute about 99% of
the total atmospheric pressure. Air also contains 0.04% carbon
dioxide, up to 0.5% water vapor, and insignificant amounts of
inert gases (such as argon and helium).
At high altitudes, where the atmosphere is less influenced
by gravity, partial pressures decline in direct proportion to the
decrease in atmospheric pressure. For example, at 10,000 feet
above sea level where the atmospheric pressure is 523 mm Hg,
P
O
2
is 110 mm Hg.
Moving in the opposite direction, atmospheric pressure in-
creases by 1 atm (760 mm Hg) for each 33 feet of descent (in
water) below sea level. At 99 feet below sea level, the total pres-
sure exerted on the body is equivalent to 4 atm, or 3040 mm Hg,
and the partial pressure exerted by each component gas is also
quadrupled.
Henry’s Law
Henry’s law
states that when a gas is in contact with a liquid, the
gas will dissolve in the liquid in proportion to its partial pres-
sure. Accordingly, the greater the concentration of a particular
gas in the gas phase, the more and the faster that gas will go into
solution in the liquid.
At equilibrium, the partial pressures in the gas and liquid
phases are the same. If, however, the partial pressure of the
gas later becomes greater in the liquid than in the adjacent
gas phase, some of the dissolved gas molecules will reenter the
gaseous phase. So the direction and amount of movement of
a gas are determined by its partial pressure in the two phases.
±is flexible situation is exactly what occurs when gases are ex-
changed in the lungs and tissues. For example, when P
CO
2
in the
pulmonary capillaries is higher than in the lungs, CO
2
diffuses
out of the blood and enters the air in the alveoli.
How much of a gas will dissolve in a liquid at any given par-
tial pressure also depends on the
solubility
of the gas in the liq-
uid and the
temperature
of the liquid. ±e gases in air have very
different solubilities in water (and in blood plasma). Carbon
dioxide is most soluble. Oxygen is only 1/20 as soluble as CO
2
,
and N
2
is only half as soluble as O
2
. For this reason, at a given
partial pressure, much more CO
2
than O
2
dissolves in water,
and practically no N
2
goes into solution.
When a liquid’s temperature rises, gas solubility decreases.
±ink of club soda, which is produced by forcing CO
2
gas to
dissolve in water under high pressure. If you take the cap off a
bottle of club soda and leave it in the fridge, it will slowly go flat.
But if you leave it at room temperature, it will very quickly go
flat. In both cases, you end up with plain water—all the CO
2
gas
has escaped from solution.
Table 22.3
Nonrespiratory Air (Gas) Movements
MOVEMENT
MECHANISM AND RESULT
Cough
Taking a deep breath, closing glottis, and forcing air superiorly from lungs against glottis; glottis opens suddenly and a blast
of air rushes upward. Can dislodge foreign particles or mucus from lower respiratory tract and propel them superiorly.
Sneeze
Similar to a cough, except that expelled air is directed through nasal cavity as well as through oral cavity; depressed uvula
routes air upward through nasal cavity. Clears upper respiratory passages.
Crying
Inspiration followed by releasing air in a number of short expirations. Primarily an emotionally induced mechanism.
Laughing
Essentially same as crying in terms of air movements produced. Also an emotionally induced response.
Hiccups
Sudden inspirations resulting from spasms of diaphragm; believed to be initiated by irritation of diaphragm or phrenic
nerves, which serve diaphragm. Sound occurs when inspired air hits vocal folds of closing glottis.
Yawn
Very deep inspiration, taken with jaws wide open; not believed to be triggered by levels of oxygen or carbon dioxide in
blood. Ventilates all alveoli (not the case in normal quiet breathing).
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