HUMAN BIOLOGY

184 ChapteR 10

pore that

allows air flow

to adjacent

alveoli

red blood

cell inside

pulmonary

capillary

fused

basement

membranes

of both

epithelial

tissues

capillary

endothelium

alveolar

(^) epithelium
pulmonary
capillary
CO 2
O 2
how Gases are Exchanged and transported
hemoglobin in red blood cells solves this problem. Hemo-
globin binds and transports both O 2 and CO 2. It enables
blood to carry seventy times more oxygen than it otherwise
would and to carry seventeen times more carbon dioxide
away from tissues.
Air inhaled into alveoli contains plenty of oxygen and
relatively little carbon dioxide. Just the opposite is true of
blood arriving from tissues—which, remember, enters lung
capillaries at the “end” of the pulmonary circuit (Sec-
tion 7.3). Thus, in the lungs, oxygen diffuses down its pres-
sure gradient into the blood plasma and then into red blood
cells, where up to four oxygen molecules rapidly form a
weak, reversible bond with the four heme groups in each
molecule of hemoglobin (Figure 10.13). Hemoglobin with
oxygen bound to it is called oxyhemoglobin, or HbO 2.
The amount of HbO 2 that forms depends on several
factors. One is the partial pressure of oxygen—that is, the
relative amount of oxygen in blood plasma. In general, the
higher its partial pressure, the more oxygen hemoglobin
will pick up, until oxygen is attached to all four hemes in
the hemoglobin molecule. HbO 2 will give up its oxygen in
tissues where the partial pressure of oxy gen is lower than
in the blood. Figure 10.14 gives an idea of the pressure gra-
dients in different body regions.
In highly active tissues—which have a greater demand
for oxygen—the chemical conditions loosen hemoglobin’s
grip on oxygen. For example, the binding of oxygen weak-
ens as temperature rises or as increasing acidity lowers the
pH. Several events contribute to a falling pH. The reaction
that forms HbO 2 releases hydrogen ions (H^1 ), making the
blood more acidic. Blood pH also falls as the level of CO 2
given off by active cells increases.
When tissues are chronically short of oxygen, red blood
cells increase their production of a compound called
2,3-diphosphoglycerate, or DPG for short. DPG reversibly
binds hemoglobin. As more of it binds to hemoglobin, the
more easily hemoglobin binds oxygen. This makes more
oxygen available to tissues.
n Gas exchange during respiration provides body cells with
oxygen for cellular respiration and picks up the carbon
dioxide cells produce as a waste product.
n Links to Acid and base balance 2.7, Diffusion 3.10, How
blood transports oxygen 8.2
Physiologists divide respiration into external and internal
phases. External respiration moves oxygen from alveoli into
the blood and carbon dioxide in the opposite direction.
During internal respiration, oxygen moves from the blood
into tissues, and carbon dioxide moves from tissues into
the blood.
alveoli are built for gas exchange
The alveoli in your lungs have an ideal structure for their
function of gas exchange. The wall of each alveolus is one
layer of epithelial cells, supported by a gossamer-thin base-
ment membrane. Hugging the alveoli are lung capillaries
(Figure 10.12A). They, too, have an extremely thin basement
membrane around their wall. In between the two basement
membranes is a film of fluid. It may seem like a lot of layers,
but the respiratory membrane they form is far narrower
than even a fine baby hair (Figure 10.12B and 10.12C). This
is why oxygen and carbon dioxide can diffuse easily across
it—oxygen moving in and carbon dioxide, out.
Some cells in the epithelium of alveoli release pulmo nary
surfactant. This substance reduces the surface tension of
the watery film between alveoli, which would otherwise
collapse the delicate alveoli. This can happen to premature
babies whose underdeveloped lungs do not yet have work-
ing surfactant-secreting cells. The result is a dangerous
disorder called infant respiratory distress syndrome.
hemoglobin is the oxygen carrier
Blood plasma cannot carry enough dissolved oxygen
and carbon dioxide to meet the body’s requirements. The
Figure 10.12 Animated! Gases are exchanged between blood in pulmonary capillaries and air in alveoli. (© Cengage Learning)
10.5
B Cutaway view of an alveolus and associated
pulmonary capillaries. Pores connect adjacent alveoli.
C Components of the respiratory
membrane.
a Each alveolus is surrounded by
a mesh of pulmonary capillaries.
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