168
CARDIOVASCULAR PHYSIOLOGY CONCEPTS
during exercise can contribute to the increase
in blood flow associated with exercise. There
is no convincing evidence, however, for simi-
lar active, neurogenic vasodilator mechanisms
existing in humans.
Cutaneous Circulation
The nutrient and oxygen requirements of the
skin are quite low relative to other organs;
therefore, cutaneous blood flow does not pri-
marily serve a metabolic support role. Instead,
the primary role of blood flow to the skin is to
allow heat to be exchanged between the blood
and the environment to help regulate body tem-
perature. Therefore, the cutaneous circulation
is primarily under the control of hypothalamic
thermoregulatory centers that adjust the sym-
pathetic outflow to the cutaneous vasculature.
MICROVASCULAR ORGANIZATION
OF THE SKIN
The microvascular network that supplies skin
is unique among organs and varies depending
on the type of skin. Small arteries arising from
the subcutaneous tissues give rise to arterioles
that penetrate into the dermis and give rise to
capillaries that loop underneath the epidermis
(Fig. 7.15). Blood flows from these capillary
loops into venules and then into an extensive,
interconnecting venous plexus, in which most
of the cutaneous blood volume is found. The
blood in the venous plexus is also responsi-
ble for skin coloration in lightly pigmented
individuals. In the skin of the nose, lips, ears,
palms, toes, sole of the feet, and fingers (espe-
cially the fingertips), blood flows directly to the
venous plexus from the small subcutaneous
arteries through special interconnecting vessels
called arteriovenous (AV) anastomoses.
The resistance vessels supplying the subepi-
dermal capillary loops and the AV anastomoses
are richly innervated by sympathetic adrener-
gic fibers. Constriction of these vessels during
sympathetic activation decreases blood flow
through the capillary loops and the venous
plexus.
Although
the AV anastomoses
are
almost exclusively controlled by sympathetic
influences, the resistance vessels respond to
both metabolic influences and sympathetic
influences and therefore demonstrate local reg-
ulatory phenomena such as reactive hyperemia
and
autoregulation.
These
local
regulatory
responses, however, are relatively weak com-
pared to those observed in most other organs.
EFFECTS OF TEMPERATURE ON SKIN
BLOOD FLOW
At normal body and ambient temperatures, sym-
pathetic adrenergic activity contributes to a high
degree of vascular tone, and skin blood flow
represents about 4% of the cardiac output (see
Table 7-1). In times of severe cold stress, skin
Epidermis
Dermis
Subcutaneous
Tissue
Capillary
Vein
Venous Plexus
AV anastomosis
Artery
■ FIGURE 7.15 Microvascular anatomy of the cutaneous circulation. Arteries w ithin the subcutaneous tis-
sue give rise to either arterioles that travel into the dermis and give rise to capillary loops or AV anasto-
moses that connect to a plexus of small veins in the subdermis. The venous plexus also receives blood
from the capillary loops. Sym pathetic stim ulation constricts the resistance vessels and AV anastomoses,
thereby decreasing dermal blood flow.
previous page 181 Cardiovascular Physiology Concepts  2nd Edition read online next page 183 Cardiovascular Physiology Concepts  2nd Edition read online Home Toggle text on/off