C h a p t e r 1 7
Control of Cardiovascular Function
377
In the pulmonary circulation, the blood volume
(approximately 450 mL in the adult) can vary from as
low as 50% of normal to as high as 200% of normal.
An increase in intrathoracic pressure, such as occurs
when exhaling against a closed glottis, impedes venous
return to the right heart. This can produce a transient
shift from the pulmonary to the systemic circulation of
as much as 250 mL of blood as blood backs up in the
vena cava and large veins. Body position also affects the
distribution of blood volume. In the recumbent posi-
tion, approximately 25% to 30% of the total blood
volume is in the pulmonary circulation. On standing,
gravity causes a rapid displacement of this blood to the
lower part of the body. Because the volume of the sys-
temic circulation is approximately seven times that of
the pulmonary circulation, a shift of blood from one
system to the other has a much greater effect on the
pulmonary than on the systemic circulation.
The movement of blood from the arterial to the
venous side of the circulation depends on a pressure dif-
ference or gradient, moving from an area of higher pres-
sure to one of lower pressure. The pressure distribution
in different parts of the circulation is almost an inverse
of the volume distribution (see Fig. 17-2). The pressure
in the arterial side of the circulation, which contains
only approximately one sixth of the blood volume, is
much higher than the pressure on the venous side of the
circulation, which contains approximately two thirds of
the blood. This pressure and volume distribution is due
in large part to the structure and relative elasticity of
the arteries and veins. The pressure difference between
the arterial and venous sides of the circulation (approxi-
mately 84 mm Hg) provides the driving force for the
flow of blood in the systemic circulation. The pulmo-
nary circulation has a similar arterial–venous pressure
difference, albeit it of a lesser magnitude, that facilitates
blood flow.
0
0
20
40
60
20
40
60
80
100
120
Pul.
art.
Rt.
vent.
Veins
Caps.
Aorta
Arte-
rioles
Sm.
art.
Lg.
art.
Lt.
vent.
4%
4%
4%
16%
64%
Pressure (mm Hg)
Total blood
volume (%)
FIGURE 17-2.
Pressure and volume
distribution in the systemic circulation.
The graphs show the inverse relation
between internal pressure and
volume in different portions of the
circulatory system. (From Smith JJ,
Kampine JP. Circulatory Physiology:
The Essentials. 3rd ed. Baltimore, MD:
J.B.Lippincott; 1990.)
SUMMARY CONCEPTS
■■
The cardiovascular or circulatory system, which
consists of a pump (heart), a series of distributing
(arteries and arterioles) and collecting (veins and
venules) blood vessels, and an extensive system
of exchange vessels (capillaries), functions mainly
as a transport system that circulates nutrients and
other materials to and removes waste products
from tissues throughout the body.
■■
The circulatory system can be divided into two
parts: the right heart and pulmonary circulation,
which moves blood through the lungs and
creates a link with the gas exchange function
of the respiratory system; and the left heart
and systemic circulation, which moves blood
throughout all the other tissues of the body.
■■
Blood flow in the circulatory system depends on
a blood volume that is sufficient to fill the blood
vessels and a pressure difference across the
system that provides the force that is needed to
move blood forward. The venous system, which
is a low pressure system designed to collect
and return blood to the heart, contains about
two-thirds of the blood; the arterial system,
which is a high pressure distributive system,
contains about one-sixth of the blood; and the
capillaries, which have the lowest pressure
and function as an exchange system for gases,
nutrients, and wastes, contain the least amount
of blood.
