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
, that is, irregular ﬂuid motion
where blood from the diﬀerent laminae mixes. Turbulence dra-
matically increases resistance.
Relationship Between Flow, Pressure,
Now that we have deﬁned these terms, let’s summarize the rela-
tionships between them.
Blood ﬂow (
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-
) in the systemic circulation; if
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
, 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
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
±ere are three important sources of resistance: blood vis-
cosity, vessel length, and vessel diameter.
±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
. ±e smaller
the tube, the greater the friction, because relatively more of the
ﬂuid contacts the tube wall, where its movement is impeded.
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 (
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.