Essentials of Anatomy and Physiology

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of a gas, measured in mmHg, is the pressure it exerts
within a mixture of gases, whether the mixture is actu-
ally in a gaseous state or is in a liquid such as blood.
The partial pressures of oxygen and carbon dioxide in
the atmosphere and in the sites of exchange in the
body are listed in Table 15–1. The abbreviation for
partial pressure is “P,” which is used, for example, on
hospital lab slips for blood gases and will be used here.
The partial pressures of oxygen and carbon dioxide
at the sites of external respiration (lungs) and internal
respiration (body) are shown in Fig. 15–8. Because
partial pressure reflects concentration, a gas will dif-
fuse from an area of higher partial pressure to an area
of lower partial pressure.
The air in the alveoli has a high PO 2 and a low PCO 2.
The blood in the pulmonary capillaries, which has just
come from the body, has a low PO 2 and a high PCO 2.
Therefore, in external respiration, oxygen diffuses
from the air in the alveoli to the blood, and carbon
dioxide diffuses from the blood to the air in the alveoli.
The blood that returns to the heart now has a high PO 2
and a low PCO 2 and is pumped by the left ventricle into
systemic circulation.
The arterial blood that reaches systemic capillaries
has a high PO 2 and a low PCO 2. The body cells and tis-


sue fluid have a low PO 2 and a high PCO 2 because cells
continuously use oxygen in cell respiration (energy
production) and produce carbon dioxide in this
process. Therefore, in internal respiration, oxygen dif-
fuses from the blood to tissue fluid (cells), and carbon
dioxide diffuses from tissue fluid to the blood. The
blood that enters systemic veins to return to the heart
now has a low PO 2 and a high PCO 2 and is pumped by
the right ventricle to the lungs to participate in exter-
nal respiration.
Disorders of gas exchange often involve the lungs,
that is, external respiration (see Box 15–6: Pulmonary
Edema and Box 15–7: Pneumonia).

TRANSPORT OF GASES
IN THE BLOOD

Although some oxygen is dissolved in blood plasma
and does create the PO 2 values, it is only about 1.5%
of the total oxygen transported, not enough to sustain
life. As you already know, most oxygen is carried in the
blood bonded to the hemoglobinin red blood cells
(RBCs). The mineral iron is part of hemoglobin and
gives this protein its oxygen-carrying ability.

The Respiratory System 355

Table 15–1 PARTIAL PRESSURES AND OXYGEN SATURATION

Site PO 2 (mmHg) PCO 2 (mmHg) Hemoglobin Saturation (SaO 2 )
Atmosphere 160 0.15 —
Alveolar air 104 40 —
Systemic venous blood 40 45 70–75%
(to pulmonary arteries)
Systemic arterial blood 100 40 95–100%
(from pulmonary veins)
Tissue fluid 40 50 —

Partial pressure is calculated as follows:
% of the gas in the mixture total pressure PGAS
Example:O 2 in the atmosphere
21%760 mmHg 160 mmHg (PO 2 )
Example:CO 2 in the atmosphere
0.04%760 mmHg 0.15 mmHg (PCO 2 )

Notice that alveolar partial pressures are not exactly those of the atmosphere. Alveolar air contains significant amounts of
water vapor and the CO 2 diffusing in from the blood. Oxygen also diffuses readily from the alveoli into the pulmonary capillaries.
Therefore, alveolar PO 2 is lower than atmospheric PO 2 , and alveolar PCO 2 is significantly higher than atmospheric PCO 2.
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