170
CARDIOVASCULAR PHYSIOLOGY CONCEPTS
submucosa from which arterioles and capil-
laries arise to supply blood to the intestinal
villi. Water and nutrients transported from the
intestinal lumen into the villi enter the blood
and are carried away by the portal venous cir-
culation.
Intestinal blood flow is closely coupled to
the primary function of the intestine, which
is the absorption of water, electrolytes, and
nutrients from the intestinal lumen. There-
fore, intestinal blood flow increases when food
is present within the intestine. In an adult
human, blood flow to the intestine (superior
mesenteric artery) in the fasted state is about
300 mL/min, and increases two- to threefold
following a meal. This functional (absorptive)
hyperemia is stimulated by gastrointestinal
hormones such as gastrin and cholecysto-
kinin, as well as by glucose, amino acids, and
fatty acids that are absorbed by the intestine.
Evidence exists that submucosal arteriolar
vasodilation during functional hyperemia is
mediated by hyperosmolarity and nitric oxide.
The intestinal circulation is strongly influ-
enced by the activity of sympathetic adren-
ergic nerves. Increased sympathetic activity
during exercise or in response to decreased
baroreceptor firing (e.g., during hemorrhage
or standing) constricts both arterial resist-
ance vessels and venous capacitance vessels.
Because the intestinal circulation receives such
a large fraction of cardiac output, sympathetic
stimulation of the intestine causes a substan-
tial increase in total systemic vascular resist-
ance. Additionally, the large blood volume
contained within the venous vasculature is
mobilized during sympathetic stimulation to
increase central venous pressure. The spleen
is also an important venous reservoir, and in
some species (e.g., dogs), this organ stores
hemoconcentrated blood. Stressful conditions
in the dog (e.g., blood loss) can cause splenic
contraction, which can substantially increase
circulating blood volume and hematocrit.
Parasympathetic activation of the intestine
increases motility and glandular secretions,
which is associated with an increase in blood
flow. This may involve metabolic mechanisms
or local paracrine influences such as the
formation of bradykinin and nitric oxide.
HEPATIC CIRCULATION
Venous blood leaving the gastrointestinal tract,
spleen, and pancreas drains into the hepatic
portal vein, which supplies approximately 75%
of the hepatic blood flow. The remainder of the
hepatic blood flow is supplied by the hepatic
artery, which is a branch of the celiac artery.
Note that in this arrangement, most of the liver
circulation is in series with the gastrointestinal,
splenic, and pancreatic circulations. Therefore,
changes in blood flow in these vascular beds
have a significant influence on hepatic flow.
Terminal vessels from the hepatic portal
vein and hepatic artery form sinusoids within
the liver, which function as capillaries. The
pressure within these sinusoids is very low,
just a few mm Hg above central venous pres-
sure. This is important because hepatic sinu-
soids are very permeable (see Chapter 8).
Changes in central venous and hepatic venous
pressure are almost completely transmitted to
the sinusoids. Therefore, elevations in central
venous pressure during right ventricular fail-
ure can cause substantial increases in sinusoid
pressure and fluid filtration, leading to hepatic
edema and accumulation of fluid within the
abdominal cavity (ascites).
The liver circulation does not show classical
autoregulation; however, decreases in hepatic
portal flow result in reciprocal increases in
hepatic artery flow, and vice versa. Sympathetic
nerve activation constricts vessels derived from
both the hepatic portal system and hepatic
artery. The most important effect of sympathetic
activation is on venous capacitance vessels,
which contain a significant fraction (~15%) of
the venous blood volume in the body. The liver,
like the gastrointestinal circulation, functions
as an important venous reservoir.
Renal Circulation
Approximately 20% of the cardiac output
perfuses the kidneys although the kidneys
represent only about 0.4%
of total body
weight. Renal blood flow, therefore, is about
400 mL/min per 100 g of tissue weight, which
is the highest of any major organ within the
body (see Table 7-1). Only the pituitary and
carotid bodies have higher blood flows per
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