Ganong's Review of Medical Physiology, 23rd Edition

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 capillaries
and postcapillary venules, with consequent extravasation
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 sensory
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 response
in which impulses initiated in sensory nerves by the injury
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 substance
P (see Chapter 7), and substance P and CGRP are present
in all parts of the neurons. Both dilate arterioles and, in addition,
substance P causes extravasation of fluid. Effective nonpeptide
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 reestablished
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
Spinal cord
Orthodromic conduction
Antidromic conduction
Direction taken by impulses
FIGURE 34–14 Axon reflex.
Sensory neuron
Endings
in skin
Endings
near arteriole
CHAPTER 34 Circulation Through Special Regions 581
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 vessels,
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 discharge,
a painful injury causes generalized cutaneous vasoconstriction
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 supervene.
This vasodilation is the cause of the ruddy complexion
seen on a cold day.
Shock is more profound in patients with elevated temperatures
because of cutaneous vasodilation, and patients in shock
should not be warmed to the point that their body temperature
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 victims
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 nonpregnant
women. The function of the spiral and basilar arteries
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 metabolites
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
uterine blood flow increases 20-fold during pregnancy, the