C h a p t e r 2 1
Control of Respiratory Function
529
The movement of blood through the pulmonary cap-
illary bed requires that the mean pulmonary arterial
pressure be greater than the mean pulmonary venous
pressure.
Distribution of Pulmonary Blood Flow
As with ventilation, the distribution of pulmonary
blood flow is affected by body position and gravity. In
the upright position, the distance of the upper apices of
the lung above the level of the heart may exceed the per-
fusion capabilities of the mean pulmonary arterial pres-
sure (approximately 12 mm Hg); therefore, blood flow
in the upper part of the lungs is less than that in the base.
In the supine position, the lungs and the heart are at the
same level, and blood flow to the apices and base of the
lungs becomes more uniform. In this position, however,
blood flow to the posterior or dependent portions (e.g.,
bottom of the lung when lying on the side) exceeds flow
in the anterior or nondependent portions of the lungs.
The alveolar concentration of oxygen also affects
pulmonary blood flow. When the concentration of oxy-
gen in air of the alveoli decreases below normal blood,
the adjacent blood vessels constrict in an effort to dis-
tribute blood where it will be most effectively oxygen-
ated. When alveolar oxygen levels drop below 60 mm
Hg, marked vasoconstriction occurs, and at very low
oxygen levels, the local flow may be almost abolished.
In regional hypoxia, as occurs with a localized airway
obstruction (e.g., atelectasis), vasoconstriction is local-
ized to a specific region of the lung. Generalized hypoxia,
such as occurs at high altitudes causes vasoconstriction
throughout all of the vessels of the lung.
Shunt
Shunt refers to blood that moves from the right to the
left side of the circulation without being oxygenated.
As with dead air space, there are two types of shunts:
anatomic and physiologic. In an
anatomic shunt
, blood
moves from the venous to the arterial side of the cir-
culation without moving through the lungs. Anatomic
shunting of blood is most commonly due to congenital
heart defects (see Chapter 19). In a
physiologic shunt
,
there is mismatching of ventilation and perfusion within
the lung, resulting in insufficient ventilation to provide
the oxygen needed to oxygenate the blood flowing
through the alveolar capillaries. Physiologic shunting of
blood usually results from destructive lung disease that
impairs ventilation or from heart failure that interferes
with movement of blood through sections of the lungs.
Mismatching of Ventilation and
Perfusion
The gas exchange properties of the lung depend on
matching ventilation and perfusion, ensuring that equal
amounts of air and blood are entering the respiratory
portion of the lungs. Both dead air space and shunt
produce a mismatching of ventilation and perfusion, as
depicted in Figure 21-16. With shunt (depicted on the
left), there is perfusion without ventilation, resulting in
a low ventilation-perfusion ratio. It occurs in conditions
such as atelectasis in which there is airway obstruction
(see Chapter 23). With dead air space (depicted on the
right), there is ventilation without perfusion, resulting
in a high ventilation–perfusion ratio. It occurs with con-
ditions such as pulmonary embolism, which impairs
blood flow to a part of the lung. The arterial blood leav-
ing the pulmonary circulation reflects mixing of blood
from normally ventilated and perfused areas of the lung
as well as areas that are not perfused (dead air space)
or ventilated (shunt). Many of the conditions that cause
mismatching of ventilation and perfusion involve both
dead air space and shunt. In chronic obstructive lung
disease, for example, there may be impaired ventila-
tion in one area of the lung and impaired perfusion in
another area.
Diffusion
Diffusion takes place in the respiratory portions of the
lung and refers to the movement of gases across the
alveolar–capillary membrane. Diffusion of gases in
the lung is affected by (1) difference in the pressure of
the gas on either side of the membrane, (2) the surface
area that is available for diffusion, (3) the thickness of
the alveolar–capillary membrane through which the
gas must pass, and (4) the diffusion characteristics of
the gas. Administration of high concentrations of oxy-
gen increases the difference in partial pressure between
the two sides of the membrane and increases the diffu-
sion of the gas. Diseases that destroy lung tissue and
the surface area for diffusion and those that increase
the thickness of the alveolar–capillary membrane
adversely influence the diffusing capacity of the lungs.
Perfusion without
ventilation
Venous
blood
Arterial
blood
Airways
Ventilation without
perfusion
Normal
Alveolus
FIGURE 21-16.
Matching of ventilation and perfusion. (Center)
Normal matching of ventilation and perfusion; (left) perfusion
without ventilation (i.e., shunt); (right) ventilation without
perfusion (i.e., dead air space).
