Human Physiology, 14th edition (2016)

Respiratory Physiology 551

resistance. The pulmonary circulation, in other words, is a low-

resistance, low-pressure pathway. The low pulmonary blood

pressure produces less filtration pressure (see chapter 14,

fig. 14.9) than that produced in the systemic capillaries, and

thus affords protection against pulmonary edema. This is a

dangerous condition in which excessive fluid can enter the

interstitial spaces of the lungs and then the alveoli, impeding

ventilation and gas exchange. Pulmonary edema occurs when

there is pulmonary hypertension, which may be produced by

left ventricular heart failure.

Pulmonary arterioles constrict when the alveolar P O 2 is low

and dilate as the alveolar P O 2 is raised. This response is oppo-

site to that of systemic arterioles, which dilate in response to

low tissue P O 2 (chapter 14, section 14.3). Dilation of the sys-

temic arterioles when the P O 2 is low helps to supply more blood

and oxygen to the tissues; constriction of the pulmonary arteri-

oles when the alveolar P O 2 is low helps decrease blood flow to

alveoli that are inadequately ventilated.

The response of pulmonary arteries to hypoxia is auto-

matic. Mitochondria in the smooth muscle cells of pulmo-

nary arterioles sense the low P O 2 and initiate a sequence of

events that causes depolarization and an opening of voltage-

gated Ca^2 1 channels in the plasma membrane. This stimu-

lates muscle contraction and pulmonary vasoconstriction,

producing a reduced blood flow matched to reduced alveolar

artery to the aorta through the ductus arteriosus (chapter 13,
section 13.3). After birth, the foramen ovale and ductus arterio-
sus close, and the vascular resistance of the pulmonary circula-
tion falls sharply. This fall in vascular resistance at birth is due
to (1) opening of the vessels as a result of the subatmospheric
intrapulmonary pressure and physical stretching of the lungs
during inspiration and (2) dilation of the pulmonary arterioles
in response to increased alveolar P O 2.
In the adult, the right ventricle (like the left) has a car-
diac output of about 5.5 L per minute. The rate of blood flow
through the pulmonary circulation is thus equal to the flow
rate through the systemic circulation. Blood flow, as described
in chapter 14, is directly proportional to the pressure differ-
ence between the two ends of a vessel and inversely propor-
tional to the vascular resistance. In the systemic circulation,
the mean arterial pressure is 90 to 100 mmHg and the pressure
of the right atrium is 0 mmHg; therefore, the pressure differ-
ence is about 100 mmHg. The mean pressure of the pulmonary
artery, by contrast, is only 15 mmHg and the pressure of the
left atrium is 5 mmHg. The driving pressure in the pulmonary
circulation is thus 15  2  5, or 10 mmHg.
Because the driving pressure in the pulmonary circula-
tion is only one-tenth that of the systemic circulation and yet
the flow rates are equal, it follows that the pulmonary vascu-
lar resistance must be one-tenth that of the systemic vascular

Figure 16.22 Partial pressures of gases in blood. The P (^) O 2 and P (^) CO 2 values of blood are a result of gas exchange in the lung
alveoli and gas exchange between systemic capillaries and body cells.
From
pulmonary
artery
To
pulmonary
Alveoli vein
= 105
P = 40
Systemic
arteries
Systemic
veins
Body
cells
CO 2
Capillaries
PO 2 = 40
PCO 2 = 46
PO 2 = 100
PCO 2 = 40
PO 2 = 40 PCO 2 = 46 PO 2 = 100 PCO 2 = 40
O 2
Right
atrium
and
ventricle
Left
atrium
and
ventricle
CO 2
PO 2
O 2
CO 2