Maintenance of the Body
When blood encounters either an abrupt change in vessel
diameter or rough or protruding areas of the tube wall (such as
the fatty plaques of atherosclerosis), the smooth laminar blood
flow is replaced by
turbulent flow
, that is, irregular fluid motion
where blood from the different laminae mixes. Turbulence dra-
matically increases resistance.
Relationship Between Flow, Pressure,
and Resistance
Now that we have defined these terms, let’s summarize the rela-
tionships between them.
Blood flow (
) is
proportional to the difference in
blood pressure (Δ
) between two points in the circulation,
that is, the blood pressure, or hydrostatic pressure, gradient.
±us, when Δ
increases, blood flow speeds up, and when
decreases, blood flow declines.
Blood flow is
proportional to the peripheral resis-
tance (
) in the systemic circulation; if
increases, blood
flow decreases.
We can express these relationships by the formula
Of these two factors influencing blood flow,
is far more im-
portant than Δ
in influencing local blood flow because
easily be changed by altering blood vessel diameter. For example,
when the arterioles serving a particular tissue dilate (thus decreas-
ing the resistance), blood flow to that tissue increases, even though
the systemic pressure is unchanged or may actually be falling.
Check Your Understanding
List three factors that determine resistance in a vessel. Which
of these factors is physiologically most important?
Suppose vasoconstriction decreases the diameter of a vessel to
one-third its size. What happens to the rate of flow through
that vessel? Calculate the expected size of the change.
For answers, see Appendix H.
Systemic Blood Pressure
Describe how blood pressure differs in the arteries,
capillaries, and veins.
Any fluid driven by a pump through a circuit of closed channels
operates under pressure, and the nearer the fluid is to the pump,
the greater the pressure exerted on the fluid. Blood flow in blood
vessels is no exception, and blood flows through the blood vessels
along a pressure gradient, always moving from higher- to lower-
pressure areas. Fundamentally,
the pumping action of the heart gen-
erates blood flow. Pressure results when flow is opposed by resistance.
As illustrated in
Figure 19.6
, systemic blood pressure is
highest in the aorta and declines throughout the pathway to
finally reach 0 mm Hg in the right atrium. ±e steepest drop in
blood pressure occurs in the arterioles, which offer the greatest
Unless stated otherwise, the term
blood pressure
means sys-
temic arterial blood pressure in the largest arteries near the
heart. ±e pressure gradient—the
in blood pressure
within the vascular system—provides the driving force that
keeps blood moving, always from an area of higher pressure to
an area of lower pressure, through the body.
is opposition to flow and is a measure of the amount
of friction blood encounters as it passes through the vessels. Be-
cause most friction is encountered in the peripheral (systemic)
circulation, well away from the heart, we generally use the term
peripheral resistance
±ere are three important sources of resistance: blood vis-
cosity, vessel length, and vessel diameter.
Blood Viscosity
±e internal resistance to flow that exists in
all fluids is
ĭ-te) and is related to the thickness
or “stickiness” of a fluid. ±e greater the viscosity, the less eas-
ily molecules slide past one another and the more difficult it is
to get and keep the fluid moving. Blood is much more viscous
than water. Because it contains formed elements and plasma
proteins, it flows more slowly under the same conditions.
Blood viscosity is fairly constant, but conditions such as poly-
cythemia (excessive numbers of red blood cells) can increase
blood viscosity and, hence, resistance. On the other hand, if the
red blood cell count is low, as in some anemias, blood is less
viscous and peripheral resistance declines.
Total Blood Vessel Length
±e relationship between total
blood vessel length and resistance is straightforward: the longer
the vessel, the greater the resistance. For example, an infant’s
blood vessels lengthen as he or she grows to adulthood, and so
both peripheral resistance and blood pressure increase.
Blood Vessel Diameter
Because blood viscosity and vessel
length are normally unchanging, the influence of these factors
can be considered constant in healthy people. However, blood
vessel diameter changes frequently and significantly alters pe-
ripheral resistance. How so? ±e answer lies in principles of fluid
flow. Fluid close to the wall of a tube or channel is slowed by fric-
tion as it passes along the wall, whereas fluid in the center of the
tube flows more freely and faster. You can verify this by watching
the flow of water in a river. Water close to the bank hardly seems
to move, while that in the middle of the river flows quite rapidly.
In a tube of a given size, the relative speed and position of fluid
in the different regions of the tube’s cross section remain constant,
a phenomenon called
laminar flow
. ±e smaller
the tube, the greater the friction, because relatively more of the
fluid contacts the tube wall, where its movement is impeded.
Resistance varies
with the
fourth power
of the ves-
sel radius (one-half the diameter). ±is means, for example, that
if the radius of a vessel doubles, the resistance drops to one-
sixteenth of its original value (
16 and
1/16). For this reason, the large arteries close to the heart,
which do not change dramatically in diameter, contribute little
to peripheral resistance. Instead, the small-diameter arterioles,
which can enlarge or constrict in response to neural and chemi-
cal controls, are the major determinants of peripheral resistance.
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