Human Physiology, 14th edition (2016)

Cardiac Output, Blood Flow, and Blood Pressure 465

Figure 14.15 The relationships between blood flow, vessel radius, and resistance. ( a ) The resistance and blood flow
are equally divided between two branches of a vessel. ( b ) A doubling of the radius of one branch and halving of the radius of the other
produces a sixteenfold increase in blood flow in the former and a sixteenfold decrease of blood flow in the latter.
See the Test Your Quantitative Ability section of the Review Activities at the end of this chapter.

Radius = 1 mm

Resistance = R

Blood flow = F

Radius = 1 mm

Resistance = R

Blood flow = F

Radius = 2 mm

Resistance = 1/16 R

Blood flow = 16 F

Radius = 1/2 mm

Resistance = 16 R

Blood flow = 1/16 F

Arterial

blood

Arterial

blood

(a)

(b)

one organ and vasodilation in another result in a diversion, or
shunting, of blood to the second organ. Because arterioles are
the smallest arteries and can become narrower by vasocon-
striction, they provide the greatest resistance to blood flow
( fig. 14.16 ). Blood flow to an organ is thus largely determined
by the degree of vasoconstriction or vasodilation of its arteri-
oles. The rate of blood flow to an organ can be increased by
dilation of its arterioles and can be decreased by constriction
of its arterioles.

Total Peripheral Resistance

The sum of all the vascular resistances within the systemic cir-

culation is called the total peripheral resistance. The arteries

that supply blood to the organs are generally in parallel rather

than in series with each other. That is, arterial blood passes

through only one set of resistance vessels (arterioles) before

returning to the heart ( fig.  14.17 ). Because one organ is not

“downstream” from another in terms of its arterial supply,

changes in resistance within one organ directly affect blood

flow in that organ only.

Vasodilation in a large organ might, however, significantly

decrease the total peripheral resistance and, by this means,

might decrease the mean arterial pressure. In the absence of

compensatory mechanisms, the driving force for blood flow

through all organs might be reduced. This situation is normally

prevented by an increase in the cardiac output and by vasocon-

striction in other areas. During exercise of the large muscles,

for example, the arterioles in the exercising muscles are dilated.

This would cause a great fall in mean arterial pressure if there

were no compensations. But the blood pressure actually rises

during exercise, primarily because of increased cardiac output

and vasoconstriction in the viscera. Also, sympathetic nerves

produce cutaneous vasoconstriction at the beginning of exercise,

raising blood pressure. However, when exercise is prolonged,

increased metabolic heat production overrides this effect to

increase the flow of warm blood to the skin for improved heat

loss (see table 14.7 ).

Extrinsic Regulation of Blood Flow

The term extrinsic regulation refers to control by the auto-

nomic nervous system and endocrine system. Angiotensin II,

for example, directly stimulates vascular smooth muscle to

produce vasoconstriction. Antidiuretic hormone (ADH) also

has a vasoconstrictor effect at high concentrations; this is why

it is also called vasopressin. However, this vasopressor effect

Figure 14.16 Blood pressure in different vessels of
the systemic circulation. Notice that the pressure generated
by the beating of the ventricles is largely dissipated by the time
the blood gets into the venous system, and that this pressure
drop occurs primarily as blood goes through the arterioles and
capillaries.

Left

ventricle

Large

arteries Capillaries Venules

Large

veins

Small

arteries and

arterioles

Pressure (mmHg)

0

20

40

60

80

100

120

Resistance

vessels

Exchange

vessels

Capacitance

vessels