Human Physiology, 14th edition (2016)

550 Chapter 16

delivery to the tissues, although it would have little effect on

the total oxygen content of blood.

An electrode that produces a current in response to dis-

solved carbon dioxide is also used, so that the P^ CO 2 of blood

can be measured together with its P^ O 2 Blood in the systemic

veins, which is delivered to the lungs by the pulmonary arter-

ies, usually has a P^ O 2 of 40 mmHg and a P^ CO 2 of 46 mmHg.

After gas exchange in the alveoli of the lungs, blood in the

pulmonary veins and systemic arteries has a P^ O 2 of about

100 mmHg and a P^ CO 2 of 40 mmHg ( fig. 16.22 ). The values in

arterial blood are relatively constant and clinically significant

because they reflect lung function. Blood gas measurements

of venous blood are not as useful because these values are far

more variable. For example, venous P^ O 2 is much lower and

P^ CO 2 much higher after exercise than at rest, whereas arterial

values are not significantly affected by moderate physical

activity.

If the oxygen electrode is next inserted into an unknown
sample of blood, the P^ O 2 of that sample can be read directly
from the previously calibrated scale. Suppose, as illustrated in
figure 16.21 , the blood sample has a P^ O 2 of 100 mmHg. Alveo-
lar air has a P^ O 2 of about 105 mmHg, so this reading indicates
that the blood is almost in complete equilibrium with the alve-
olar air.
The oxygen electrode responds only to oxygen dissolved
in water or plasma; it cannot respond to oxygen that is bound
to hemoglobin in red blood cells. Most of the oxygen in blood,
however, is located in the red blood cells attached to hemoglo-
bin. The oxygen content of whole blood thus depends on both
its P^ O 2 and its red blood cell and hemoglobin content. At a P^ O 2 o f
about 100 mmHg, whole blood normally contains almost 20 ml
O 2 per 100 ml blood; of this amount, only 0.3 ml of O 2 is dis-
solved in the plasma and 19.7 ml of O 2 is found within the red
blood cells (see fig. 16.31 ). Because only the 0.3 ml of O 2 affects
the P^ O 2 measurement, this measurement would be unchanged if
the red blood cells were removed from the sample.

Significance of Blood P O

2

and P CO

2

Measurements

Because blood P^ O 2 measurements are not directly affected by
the oxygen in red blood cells, the P^ O 2 does not provide a mea-
surement of the total oxygen content of whole blood. It does,
however, provide a good index of lung function. If the inspired
air has a normal P^ O 2 but the arterial P^ O 2 is below normal, for
example, you could conclude that gas exchange in the lungs
is impaired. Measurements of arterial P^ O 2 thus provide valu-
able information in treating people with pulmonary diseases,
in performing surgery (when breathing may be depressed by
anesthesia), and in caring for premature babies with respiratory
distress syndrome.
When the lungs are functioning properly, the P^ O 2 of sys-
temic arterial blood is only 5 mmHg less than the P^ O 2 o f
alveolar air. At a normal P^ O 2 of about 100 mmHg, hemoglo-
bin is almost completely loaded with oxygen, indicated by an
oxyhemoglobin saturation (the percentage of oxyhemoglobin
to total hemoglobin) of 97%. Given this high oxyhemoglobin
saturation, an increase in blood P^ O 2 —produced, for example,
by breathing 100% oxygen from a gas tank—cannot signifi-
cantly increase the amount of oxygen contained in the red
blood cells. It can, however, significantly increase the amount
of oxygen dissolved in the plasma (because the amount dis-
solved is directly determined by the P^ O 2 ). If the P^ O 2 doubles, the
amount of oxygen dissolved in the plasma also doubles, but the
total oxygen content of whole blood increases only slightly.
This is because the plasma contains relatively little oxygen
compared to the red blood cells.
Since the oxygen carried by red blood cells must first dis-
solve in plasma before it can diffuse to the tissue cells, how-
ever, a doubling of the blood P^ O 2 means that the rate of oxygen
diffusion to the tissues would double under these conditions.
For this reason, breathing from a tank of 100% oxygen (with
a P^ O 2 of 760 mmHg) would significantly increase oxygen

CLINICAL APPLICATION

A pulse oximeter is a device that clips on a fingertip or pinna

and noninvasively measures the oxyhemoglobin saturation.

Because it is noninvasive and fast, it is commonly used in

hospitals for many purposes when not all of the information

that can be obtained from blood gas machines is needed.

The pulse oximeter has two light-emitting diodes (LEDs):

one emits red light (with a wavelength of 600–750 nm), and

the other emits infrared light (of 850–1,000 nm). Oxyhemo-

globin absorbs relatively more infrared light, allowing more of

the red light to pass through the tissue to a sensor, whereas

deoxyhemoglobin absorbs more of the red and passes more

of the infrared light. From this information, the device deter-

mines the percent oxyhemoglobin saturation.

Clinical Investigation CLUES

Peter measured a normal blood oxygenation level for that

altitude on a pulse oximeter at the hospital.

• What does a pulse oximeter measure and how does
  it work?


• How could Peter have a normal percent
  oxyhemoglobin saturation for that altitude when his
  arterial PO 2 was lower there than at sea level?

Pulmonary Circulation and

Ventilation/Perfusion Ratios

In a fetus, the pulmonary circulation has a high vascular resis-

tance because the lungs are partially collapsed. This high vas-

cular resistance helps to shunt blood from the right to the left

atrium through the foramen ovale, and from the pulmonary