474 Chapter 14
and large area of the skin (1.0 to 1.5 mm thick; 1.7 to 1.8 square
meters in surface area) make it an effective radiator of heat
when the body temperature rises above the ambient tempera-
ture. The transfer of heat from the body to the external envi-
ronment is aided by the flow of warm blood through capillary
loops near the surface of the skin.
Blood flow through the skin is adjusted to maintain deep-
body temperature at about 37 8 C (98.6 8 F). These adjustments are
made by variations in the degree of constriction or dilation of
ordinary arterioles and of unique arteriovenous anastomoses
( fig. 14.23 ). These latter vessels, found predominantly in the
fingertips, palms of the hands, toes, soles of the feet, ears,
nose, and lips, shunt (divert) blood directly from arterioles to
deep venules, thus bypassing superficial capillary loops. Both
the ordinary arterioles and the arteriovenous anastomoses are
innervated by sympathetic nerve fibers. When the ambient
temperature is low, sympathetic nerves stimulate cutaneous
vasoconstriction. Cutaneous blood flow is thus decreased, so
that less heat will be lost from the body. Because the arterio-
venous anastomoses also constrict, the skin may appear rosy
because the blood is diverted to the superficial capillary loops.
In spite of this rosy appearance, however, the total cutane-
ous blood flow and rate of heat loss is lower than under usual
conditions.
Skin can tolerate an extremely low blood flow in cold
weather because its metabolic rate decreases when the ambi-
ent temperature decreases. In cold weather, therefore, the skindioxide concentration rises as a result of inadequate ventila-
tion (hypoventilation), the cerebral arterioles dilate. This is
believed to be due to decreases in the pH of cerebrospinal fluid
rather than to a direct effect of CO 2 on the cerebral vessels.
Conversely, when the arterial CO 2 falls below normal during
hyperventilation, the cerebral vessels constrict. The resulting
decrease in cerebral blood flow is responsible for the dizziness
that occurs during hyperventilation.
Metabolic Regulation
Although the mechanisms just described maintain a relatively
constant total cerebral blood flow, the particular brain regions
that are most active receive an increased blood flow. Indeed,
active brain regions are hyperemic —their blood flow actually
exceeds the aerobic requirements of the active neurons.
This shunting of blood between different brain regions
occurs because the cerebral arterioles are exquisitely sensi-
tive to local changes in metabolic activity, so that those brain
regions with the highest metabolic activity receive the most
blood. Indeed, areas of the brain that control specific processes
have been mapped by the changing patterns of blood flow that
result when these areas are activated. Visual and auditory stim-
uli, for example, increase blood flow to the appropriate sen-
sory areas of the cerebral cortex ( fig. 14.22 ), whereas motor
activities, such as movements of the eyes, arms, and organs of
speech, result in different patterns of blood flow.
The mechanisms by which increased activity within a brain
region causes increased blood flow to that region are complex
and not completely understood. Active neurons release many
substances that stimulate vasodilation, including K^1 , adenosine,
nitric oxide (NO), and others. The close association of astro-
cytes with both neurons and cerebral vessels (chapter 7; see
fig. 7.10) suggests that they may also play a role. Indeed,
astrocytes have been shown to secrete vasodilator chemicals
(including prostaglandin E 2 and carbon monoxide) when
stimulated by the neurotransmitter glutamate, released into
the synapse by neurons. Molecules released by astrocytes and
active neurons could also stimulate the endothelial cells of
the arterioles to produce vasodilators, including nitric oxide.
In this way, neurons, astrocytes, and arterioles function
together—a process termed neurovascular coupling —so that
increased neuronal activity in a local brain region is accom-
panied by an increased cerebral blood flow to that region.
Because of this functional hyperemia (increased blood flow in
response to activity), the active neurons receive more oxygen
and glucose for their increased needs.
Cutaneous Blood Flow
The skin is the outer covering of the body and as such serves
as the first line of defense against invasion by disease-causing
organisms. The skin, as the interface between the internal and
external environments, also helps to maintain a constant deep-
body temperature despite changes in the ambient (external)
temperature—a process called thermoregulation. The thinness
Figure 14.22 A functional MRI with a BOLD (blood
oxygenation level dependent) image of the brain. The
colors indicate increased blood flow to the brain areas stimulated
when the subject views a screen displaying images that change
at 30 second intervals.