Human Physiology, 14th edition (2016)

466 Chapter 14

Parasympathetic Control of Blood Flow

Parasympathetic endings in arterioles are always cholinergic

and always promote vasodilation. Parasympathetic innerva-

tion of blood vessels, however, is limited to the digestive tract,

external genitalia, and salivary glands. Because of this lim-

ited distribution, the parasympathetic system is less important

than the sympathetic system in the control of total peripheral

resistance.

The extrinsic control of blood flow is summarized in

table 14.4.

Paracrine Regulation of Blood Flow

Paracrine regulators are molecules produced by one tissue that

help to regulate another tissue of the same organ (chapter 11,

section 11.7). Blood vessels are particularly subject to para-

crine regulation. Specifically, the endothelium of the tunica

interna produces a number of paracrine regulators that cause the

smooth muscle of the tunica media to either relax or contract.

Smooth muscle relaxation results from the local effects of

a number of molecules produced by the vessel endothelium,

including bradykinin, nitric oxide, and several prostaglandins,

particularly prostaglandin I 2 (prostacyclin). The relaxation

of vascular smooth muscle produces vasodilation, which is an

effect that can be medically useful. For example, the vasodi-

lation induced by nitric oxide explains why nitroglycerin and

related drugs (which can be converted to nitric oxide) are ben-

eficial for the treatment of angina pectoris. As another exam-

ple, people with pulmonary hypertension—a disease in which

increased vascular resistance in the pulmonary circulation can

lead to failure of the right ventricle—are sometimes treated by

intravenous administration of prostacyclin (prostaglandin I 2 ).

The endothelium of arterioles contains an enzyme, endo-

thelial nitric oxide synthase (eNOS), which produces nitric

of ADH is not believed to be significant under physiological
conditions in humans.

Regulation by Sympathetic Nerves

Stimulation of the sympathoadrenal system produces an
increase in the cardiac output (as previously discussed) and
an increase in total peripheral resistance. The latter effect is
due to alpha-adrenergic stimulation (chapter 9; see fig. 9.10)
of vascular smooth muscle by norepinephrine and, to a lesser
degree, by epinephrine. This produces vasoconstriction of the
arterioles in the viscera and skin.
Even when a person is calm, the sympathoadrenal system
is active to a certain degree and helps set the “tone” of vascular
smooth muscles. In this case, adrenergic sympathetic fibers
(those that release norepinephrine) activate alpha-adrenergic
receptors to cause a basal level of vasoconstriction throughout
the body. During the fight-or-flight reaction, an increase in the
activity of adrenergic fibers produces vasoconstriction in the
digestive tract, kidneys, and skin.
Arterioles in skeletal muscles receive cholinergic
sympathetic fibers, which release acetylcholine as a neurotrans-
mitter. During the fight-or-flight reaction, the activity of these
cholinergic fibers increases. This causes vasodilation. Vasodila-
tion in skeletal muscles is also produced by epinephrine secreted
by the adrenal medulla, which stimulates beta-adrenergic recep-
tors. During the fight-or-flight reaction, therefore, blood flow is
decreased to the viscera and skin because of the alpha-adrenergic
effects of vasoconstriction in these organs, whereas blood flow to
the skeletal muscles is increased. This diversion of blood flow to
the skeletal muscles during emergency conditions may give these
muscles an “extra edge” in responding to the emergency. Once
exercise begins, however, the blood flow to skeletal muscles
increases far more due to other mechanisms (described shortly
under Intrinsic Regulation of Blood Flow).

Figure 14.17 A diagram

of the systemic and pulmonary

circulations. Notice that with few

exceptions (such as blood flow in the

renal circulation) the flow of arterial

blood is in parallel rather than in series

(arterial blood does not usually flow

from one organ to another).

Hepatic artery

Splenic artery

Mesenteric artery

(from intestine)

Lungs

Liver
Hepatic
vein

Kidney

Hepatic

portal vein

Vena

cava

Renal afferent

arterioles

Renal efferent

arterioles