Cutaneous Blood Flow and Thermoregulation
Effects of Cutaneous Blood Flow
Dogs and other homeothermic animals use regulation of cutaneous blood
flow as a method of thermoregulation. The most effective sites for
thermoregulation by regulation of cutaneous blood flow are those areas
less insulated. In dogs these areas include the ears, abdominal and
genital regions, which are less covered by fur. Cutaneous blood flow
can be regulated by vasodilation or vasoconstriction of blood vessels near
the skin's surface. Vasodilation of cutaneous blood vessels increases
cutaneous blood flow while vasoconstriction decreases cutaneous blood flow.
Increased cutaneous blood flow causes a loss of heat because more blood
is flowing to the surface of the body where it loses heat more rapidly
to the environment than blood near the body core. Vasoconstriction
has the opposite effect due to decreased blood flow to the surface of the
and Riedel 1982).
Adrenoceptors and Cutaneous
Hypothalamic Thermal Stimulation
and Cutaneous Blood Flow
Adrenaline and noradrenaline are hormones secreted
by the adrenal medulla. Adrenaline affects what is known as the fight-or-flight
syndrome. Its effects include increased heart rate and blood pressure,
increased blood sugar, increased blood flow in heart muscle, lungs and
skeletal muscle, and vasoconstriction and decreased blood flow in smooth
muscle, skin, and the digestive tract. Noradrenaline has many of
the same effects as adrenaline, although it has a lesser effect on heart
rate and a greater effect on vasoconstriction in some organs than adrenaline.
Adrenaline and noradrenaline act through receptor sites known as adrenoceptors
Adrenoceptors are membrane bound receptors located
throughout the body. They mediate many responses to adrenaline and
noradrenaline. Adrenoceptors can be divided into two groups, alpha
and beta, that can be further divided into subtypes. Alpha-adrenoceptors
are found pre and post synaptically in the central and peripheral nervous
system, and on vascular and non-vascular smooth muscle, the heart, and
the liver. In the central nervous system alpha-adrenoceptors play
a role in regulation of neurotransmitter release, blood pressure, hypothermia,
pupil diameter, and central cardiovascular responses. In the peripheral
nervous system their functions include contraction of vascular smooth muscle,
inhibition of lypolysis, and hyperpolarization of sympathetic ganglia.
They also increase muscle contraction in the heart and stimulate glycogen
phosphorylation in the liver. Beta-adrenoceptors are
found in the central nervous system, in the heart and on vascular, uterine,
and airway smooth muscle. Activation of beta-adrenoceptors results
in increases in heart rate and force of contraction of cardiac muscle,
vasodilation, and bronchodilation. Adrenoceptors affect thermoregulation
primarily through their effects on changes in cutaneous blood flow.
Generally alpha-adrenoceptors cause decreased cutaneous blood flow through
vasoconstriction and beta-adrenoceptors cause increased cutaneous blood
flow through vasodilation (Robinson
and Hudson 1998). Dogs infused with beta-adrenoceptor
agonists show increased heart rate and cutaneous blood flow and decreased
arterial blood pressure. These effects are due to vasodilation caused
by stimulation of beta-adrenoceptors (Berlan
et al. 1994).
Cutaneous blood flow can also be regulated by thermal stimulation of
the hypothalamus. This regulation occurs without adrenaline or noradrenaline.
In dogs heating of the hypothalamus can cause vasodilation which leads
to an increase in cutaneous blood flow and a decrease in cutaneous vascular
resistance. Hypothalamic cooling can cause a reduction in cutaneous
blood flow caused by vasodilation (Peter
and Riedel 1982).
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