The relative distribution of blood flow
to organs is regulated by the vascular
resistance of the individual organs,
which is determined by extrinsic
(neurohumoral) and intrinsic (local
regulatory) mechanisms.
Important local mechanisms regulating
organ blood flow include the following:
(1) tissue factors such as adenosine,
K+, 0 2, C02, and H+; (2) paracrine
hormones such as bradykinin,
histamine, and prostaglandins; (3)
endothelial factors such as nitric oxide,
endothelin-1, and prostacyclin; and (4)
myogenic mechanisms intrinsic to the
vascular smooth muscle.
The following local factors produce
vasodilation in most tissues: adenosine,
K+, H+, C02, hypoxia, bradykinin,
histamine, PGE2, prostacyclin, and nitric
oxide. The following local factors produce
vasoconstriction: endothelin-1 and the
myogenic response to vascular stretch.
Mechanical compression of blood
vessels strongly influences blood flow
in the coronary circulation and in
contracting skeletal muscle.
Flow autoregulation is very important
in organs such as the heart, brain,
and kidneys; the gastrointestinal and
skeletal muscle circulations show
moderate autoregulation.
Blood flow is tightly coupled to
oxidative metabolism particularly in
coronary, cerebral, skeletal muscle, and
gastrointestinal circulations; therefore, an
increase in tissue oxygen consumption
leads to an increase in blood flow
(functional or active hyperemia).
Blood flow in the following organs
is moderately to strongly influenced
by sympathetic vasoconstrictor
mechanisms: resting skeletal muscle,
kidneys, gastrointestinal circulation,
and skin (related to thermoregulation).
Vascular control mechanisms linked
to oxidative metabolism (metabolic
mechanisms) are particularly strong in
the heart, brain, and skeletal muscle.
For each question, choose the one best
Two minutes after perfusion pressure to
a kidney is suddenly reduced from 100
to 70 mm Hg, which of the following
will occur?
a. Afferent arterioles will be dilated.
b. Renal blood flow will be reduced by
c. Renal vascular resistance will be
d. The kidney will become hypoxic.
If a coronary artery is occluded for
1 minute and then the occlusion is
a. A period of active hyperemia follows,
h. Coronary flow increases because of
vasoconstriction occurring during the
c. Endothelial release of nitric oxide will
contribute to the reactive hyperemia.
d. Interstitial adenosine concentrations
will increase and constrict coronary
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