362
Chapter 12
arteries has a high oxygen content. As this blood fl ows through
the capillaries of peripheral tissues and organs, some of this
oxygen leaves the blood to enter and be used by cells, resulting
in the lower oxygen content of systemic venous blood.
As shown in Figure 12–2, blood can pass from the sys-
temic veins to the systemic arteries only by fi rst being pumped
through the lungs. Thus the blood returning from the body’s
peripheral organs and tissues via the systemic veins is oxygen-
ated before it is pumped back to them.
Note that the lungs receive all the blood pumped by the
right side of the heart, whereas the branching of the systemic
arteries results in a parallel pattern so that each of the periph-
eral organs and tissues receives only a fraction of the blood
pumped by the left ventricle (see the three capillary beds
shown in Figure 12–2). This arrangement guarantees that all
systemic tissues receive freshly oxygenated blood, and allows
for independent variation in blood fl ow through different tis-
sues as their metabolic activities change. For reference, the
typical distribution of the blood pumped by the left ventricle
in an adult at rest is given in
Figure 12–3
.
Finally, there are some exceptions to the usual ana-
tomical pattern described in this section for the systemic cir-
culation, notably the liver and the pituitary gland. In those
organs, blood passes through two capillary beds, arranged in
series, before returning to the heart. As described in Chapter
11, this pattern is known as a
portal system.
Pressure, Flow, and Resistance
An important feature of the cardiovascular system is the rela-
tionship among blood pressure, blood fl ow, and the resistance
to blood fl ow. As applied to blood, these factors are collec-
tively referred to as
hemodynamics.
In all parts of the sys-
tem, blood fl ow (
F
) is always from a region of higher pressure
to one of lower pressure. The pressure exerted by any fl uid is
called a
hydrostatic pressure,
but this is usually shortened
simply to “pressure” in descriptions of the cardiovascular sys-
tem, and it denotes the force exerted by the blood. This force
is generated in the blood by the contraction of the heart, and
its magnitude varies throughout the system for reasons later
sections will describe. The units for the rate of fl ow are volume
per unit time, usually liters per minute (L/min). The units for
the pressure difference (
P) driving the fl
ow are millimeters
of mercury (mmHg) because historically blood pressure was
measured by determining how high the blood pressure could
drive a column of mercury. It is not the absolute pressure at
any point in the cardiovascular system that determines fl ow
rate, but the difference in pressure between the relevant points
(
Figure 12–4
).
Knowing only the pressure difference between two points
will not tell you the fl ow rate, however. For this, you also need
to know the
resistance
(
R
)
to fl ow—that is, how diffi cult it is
for blood to fl ow between two points at any given pressure dif-
ference. Resistance is the measure of the friction that impedes
fl ow. The basic equation relating these variables is:
F
=
P/R
(12–1)
In words, fl
ow rate is directly proportional to the pressure dif-
ference between two points and inversely proportional to the
resistance. This equation applies not only to the cardiovascular
system, but to any system in which liquid or air moves by bulk
fl ow (e.g., in the urinary and respiratory systems).
Figure 12–3
Distribution of systemic blood fl ow to the various organs and tissues
of the body at rest. Figure 12–61 shows blood fl ow changes during
exercise.
Adapted from Chapman and Mitchell.
Brain
Organ
650 (13%)
Heart
215 (4%)
Skeletal
muscle
1030 (20%)
Skin
430 (9%)
Kidneys
950 (20%)
Abdominal
organs
1200 (24%)
Other
525 (10%)
Total
5000 (100%)
Flow at rest
ml/min
P
1
P
2
P
1
= 100 mmHg
P
2
= 10 mmHg
Flow rate = 10 ml/min
P
1
= 500 mmHg
P
2
= 410 mmHg
Flow rate = 10 ml/min
P
1
P
2
P
= 90 mmHg
P
= 90 mmHg
Figure 12–4
Flow between two points within a tube is proportional to the
pressure difference between the points. The fl ows in these two
identical tubes are the same (10 mL/min) because the pressure
differences
are the same.
previous page 390 Vander's Human Physiology The Mechanisms of Body Function read online next page 392 Vander's Human Physiology The Mechanisms of Body Function read online Home Toggle text on/off