Gases Are Exchanged at the Blood Capillaries
the steps in the process of external
the steps in the process of internal
ir is a mixture of gases—nitrogen, oxygen,
water vapor, carbon dioxide, and others. To
understand gas exchange in the body, you
must first become familiar with three physi-
cal principles of gases:
Gases travel by diffusion from areas of higher concen-
tration to areas of lower concentration.
In a mixture of gases such as air, each gas exerts pressure
independently of the other gases in the mixture. This
pressure is called a partial pressure (abbreviated by
is a particular gas). The sum of the partial
pressures of all the gases in a mixture equals the
total pressure. Partial pressure is a way to express
the concentration of a gas. A gas with a higher partial
pressure is more concentrated than one with a lower
partial pressure. Therefore, gases diffuse from areas of
higher partial pressures to areas of lower partial pressure.
While partial pressures force gases across membranes,
the solubility of the gases determines whether the
gases will stay where they have been moved. Solubility
indicates whether the material will remain dissolved
in a liquid. For example, neither oxygen nor carbon
dioxide is very soluble in the blood, so the body needs to
overcome this issue to transport these important gases.
Diffusion Moves Gases Across
the Capillary Membranes
By controlling the rate and depth of breathing, pulmonary
ventilation can control the concentrations of oxygen (PO
and carbon dioxide (PCO
) in the alveoli (Figure 13.5). In
contrast, the PO
of the blood are determined by
the rate of metabolism of the cells that consume oxygen
and produce carbon dioxide. As blood moves around the
body, it encounters regions of different PO
pulmonary and systemic capillaries.
the routes of oxygen and carbon diox-
ide transport in the blood.
In the pulmonary capillaries, where gaseous exchange
with the air of the alveoli (external respiration) occurs:
is lower in the blood than in the air when the blood
enters the pulmonary capillaries. Therefore, oxygen
diffuses from the alveoli into the pulmonary capillaries.
is higher in the blood than in the air when the blood
enters the pulmonary capillaries. Therefore, carbon dioxide
diffuses from the pulmonary capillaries into the alveoli.
In other words, external respiration in the lungs converts
low-oxygen blood (deoxygenated) into high-oxygen blood
(oxygenated). As the blood flows through the pulmonary
capillaries, it picks up O2
from alveolar air and unloads CO2
into alveolar air.
In the systemic capillaries, where gaseous exchange
with tissue cells (internal respiration) occurs:
is higher in the blood than in the interstitial fluid
systemic capillaries. Therefore,
oxygen diffuses from the systemic capillaries into the
interstitial fluid and on into the body cells.
is lower in the blood than in the interstitial fluid
systemic capillaries. Therefore,
carbon dioxide diffuses from the cells to the interstitial
fluid to the systemic capillaries.
Because cells constantly use O2
to produce ATP and constant-
ly produce CO2
as a by-product, internal respiration converts
high-oxygen (oxygenated) blood into low-oxygen blood (deoxy-
genated). The low-oxygen blood returns to the heart and is
pumped to the lungs for another cycle of external respiration.
Oxygen Is Transported Through
the Blood Attached to Hemoglobin
Gases don’t dissolve well in liquids—that is, they are not
very soluble—except under pressure. Therefore, only a
small percentage of oxygen (1.5%) or carbon dioxide (7%)
gets transported in solution in the blood plasma. The rest
gets transported bound to proteins or in other forms.
382 CHAPTER 13
The Respiratory System