Active Hyperemia
Active hyperemia is the
increase in organ blood flow (hyperemia) that is associated with increased
metabolic activity of an organ or tissue. An example of active hyperemia is
the increase in blood flow that accompanies
muscle contraction, which is also called
exercise or functional
hyperemia in skeletal muscle. Blood flow increases because the increased oxygen consumption of
during muscle contraction stimulates the production of vasoactive substances
that dilate the resistance vessels in the skeletal muscle. Other examples
include the increase in gastrointestinal blood flow during digestion of food,
the increase in coronary blood flow when heart rate is increased, and the
increase in cerebral blood flow associated with increased neuronal activity in
the brain. The figure shows that there is a resting flow associated with the
basal oxygen consumption of the tissue. As the oxygen consumption increases,
there is generally a near-linear increase in blood flow until the vessels begin
to achieve a maximally dilated state.
The magnitude of active hyperemia responses differ among organs because of
the relative changes in metabolic activity from rest and their vasodilatory
capacity. Active hyperemia can result in up to a 50-fold increase in muscle
blood flow with maximal exercise, whereas cerebral blood flow may only increase
2-fold with increased neuronal activity.
Active hyperemia can also be influenced by competing vasoconstrictor
mechanisms. For example, sympathetic
activation during exercise can reduce the maximal skeletal muscle active
hyperemia compared to what would occur in the absence of sympathetic activation.
Active hyperemia may be due to
a combination of tissue hypoxia and the generation of
vasodilator metabolites such as potassium ion, carbon dioxide, nitric oxide,
and adenosine.
Revised 03/28/2007
