Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 34Circulation Through Special Regions 581

around the injury. The initial redness is due to capillary dilation,
a direct response of the capillaries to pressure. The swelling
(wheal) is local edema due to increased permeability of the cap-
illaries and postcapillary venules, with consequent extravasa-
tion of fluid. The redness spreading out from the injury (flare)
is due to arteriolar dilation. This three-part response—the red
reaction, wheal, and flare—is called the triple response and is
part of the normal reaction to injury (see Chapter 3). It persists
after total sympathectomy. On the other hand, the flare is absent
in locally anesthetized skin and in denervated skin after the sen-
sory nerves have degenerated, but it is present immediately after
nerve block or section above the site of the injury. This, plus
other evidence, indicates that it is due to an axon reflex, a re-
sponse in which impulses initiated in sensory nerves by the in-
jury are relayed antidromically down other branches of the
sensory nerve fibers (Figure 34–14). This is the one situation in
the body in which there is substantial evidence for a physiologic
effect due to antidromic conduction. The transmitter released at
the central termination of the sensory C fiber neurons is sub-
stance P (see Chapter 7), and substance P and CGRP are present
in all parts of the neurons. Both dilate arterioles and, in addi-
tion, substance P causes extravasation of fluid. Effective non-
peptide antagonists to substance P have now been developed,
and they reduce the extravasation. Thus, it appears that these
peptides produce the wheal.

REACTIVE HYPEREMIA

A response of the blood vessels that occurs in many organs but
is visible in the skin is reactive hyperemia, an increase in the
amount of blood in a region when its circulation is reestab-
lished after a period of occlusion. When the blood supply to a
limb is occluded, the cutaneous arterioles below the occlusion
dilate. When the circulation is reestablished, blood flowing

into the dilated vessels makes the skin become fiery red. O 2 in

the atmosphere can diffuse a short distance through the skin,

and reactive hyperemia is prevented if the circulation of the

limb is occluded in an atmosphere of 100% O 2. Therefore, the

arteriolar dilation is apparently due to a local effect of hypoxia.

GENERALIZED RESPONSES

Noradrenergic nerve stimulation and circulating epinephrine

and norepinephrine constrict cutaneous blood vessels. No

known vasodilator nerve fibers extend to the cutaneous ves-

sels, and thus vasodilation is brought about by a decrease in

constrictor tone as well as the local production of vasodilator

metabolites. Skin color and temperature also depend on the

state of the capillaries and venules. A cold blue or gray skin is

one in which the arterioles are constricted and the capillaries

dilated; a warm red skin is one in which both are dilated.

Because painful stimuli cause diffuse noradrenergic dis-

charge, a painful injury causes generalized cutaneous vaso-

constriction in addition to the local triple response. When the

body temperature rises during exercise, the cutaneous blood

vessels dilate in spite of continuing noradrenergic discharge in

other parts of the body. Dilation of cutaneous vessels in

response to a rise in hypothalamic temperature overcomes

other reflex activity. Cold causes cutaneous vasoconstriction;

however, with severe cold, superficial vasodilation may super-

vene. This vasodilation is the cause of the ruddy complexion

seen on a cold day.

Shock is more profound in patients with elevated tempera-

tures because of cutaneous vasodilation, and patients in shock

should not be warmed to the point that their body tempera-

ture rises. This is sometimes a problem because well-meaning

laymen have read in first-aid books that “injured patients

should be kept warm,” and they pile blankets on accident vic-

tims who are in shock.

PLACENTAL & FETAL

CIRCULATION

UTERINE CIRCULATION

The blood flow of the uterus parallels the metabolic activity of
the myometrium and endometrium and undergoes cyclic
fluctuations that correlate with the menstrual cycle in non-
pregnant women. The function of the spiral and basilar arter-
ies of the endometrium in menstruation is discussed in
Chapter 25. During pregnancy, blood flow increases rapidly as
the uterus increases in size (Figure 34–15). Vasodilator metab-
olites are undoubtedly produced in the uterus, as they are in
other active tissues. In early pregnancy, the arteriovenous O 2
difference across the uterus is small, and it has been suggested
that estrogens act on the blood vessels to increase uterine
blood flow in excess of tissue O 2 needs. However, even though
FIGURE 34–14 Axon reflex. uterine blood flow increases 20-fold during pregnancy, the

Orthodromic conduction

Antidromic conduction

Direction taken by impulses

Spinal cord Sensory neuron

Endings

in skin

Endings

near arteriole