Chapter 19
The Cardiovascular System: Blood Vessels
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19
that movement receive more blood than the adjoining neurons.
Brain tissue is exceptionally sensitive to declining pH, and in-
creased blood carbon dioxide levels (resulting in acidic con-
ditions in brain tissue) cause marked vasodilation. Low blood
levels of oxygen are a much less potent stimulus for autoregula-
tion. However, very high carbon dioxide levels abolish autoreg-
ulatory mechanisms and severely depress brain activity.
Besides metabolic controls, the brain also has a myogenic
mechanism that protects it from possibly damaging changes
in blood pressure. When MAP declines, cerebral vessels dilate
to ensure adequate brain perfusion. When MAP rises, cerebral
vessels constrict, protecting the small, more fragile vessels far-
ther along the pathway from excessive pressure. Under certain
circumstances, such as brain ischemia caused by rising intrac-
ranial pressure (as with a brain tumor), the brain (via the med-
ullary cardiovascular centers) regulates its own blood flow by
triggering a rise in systemic blood pressure.
However, when systemic pressure changes are extreme, the
brain becomes vulnerable. Fainting, or
syncope
(sin
9
cuh-pe;
“cutting short”), occurs when MAP falls below 60 mm Hg. Cer-
ebral edema is the usual result of pressures over 160 mm Hg,
which dramatically increase brain capillary permeability.
The Skin
Blood flow through the skin (1) supplies nutrients to cells, (2)
helps regulate body temperature, and (3) provides a blood res-
ervoir. Autoregulation serves the first function in response to
the need for oxygen, but the other two require neural interven-
tion. Te primary function of the cutaneous circulation is to
help maintain body temperature, so we will concentrate on that
function here.
Below the skin surface are extensive venous plexuses (net-
works of intertwining vessels). Te blood flow through these
plexuses can change from 50 ml/min to as much as 2500 ml/
min, depending on body temperature. Tis capability re-
flects neural adjustments of blood flow through arterioles and
through unique coiled arteriovenous anastomoses. Tese tiny
arteriovenous shunts are located mainly in the fingertips, palms
of the hands, toes, soles of the feet, ears, nose, and lips. Richly
supplied with sympathetic nerve endings (unlike the shunts
of most other capillary beds), they are controlled by reflexes
initiated by temperature receptors or signals from higher CNS
centers. Te arterioles, in addition, respond to metabolic au-
toregulatory stimuli.
When the skin is exposed to heat, or body temperature rises
for other reasons (such as vigorous exercise), the hypothalamic
“thermostat” signals for reduced vasomotor stimulation of the
skin vessels. As a result, warm blood flushes into the capillary
beds and heat radiates from the skin surface. Te arterioles di-
late even more when we sweat, because an enzyme in perspi-
ration acts on a protein in tissue fluid to produce
bradykinin
,
which stimulates the vessel’s endothelial cells to release the po-
tent vasodilator NO.
When the ambient temperature is cold and body tempera-
ture drops, superficial skin vessels strongly constrict. Hence,
blood almost entirely bypasses the capillaries associated with
the arteriovenous anastomoses, diverting the warm blood to
the deeper, more vital organs. Paradoxically, the skin may stay
quite rosy because some blood gets “trapped” in the superficial
capillary loops as the shunts swing into operation. Te trapped
blood remains red because the chilled skin cells take up less O
2
.
The Lungs
Blood flow through the pulmonary circuit to and from the
lungs is unusual in many ways. Te pathway is relatively short,
and pulmonary arteries and arterioles are structurally like veins
and venules. Tat is, they have thin walls and large lumens. Be-
cause resistance to blood flow is low in the pulmonary arterial
system, less pressure is needed to propel blood through those
vessels. Consequently, arterial pressure in the pulmonary cir-
culation is much lower than in the systemic circulation (24/10
versus 120/80 mm Hg).
In the pulmonary circulation, the autoregulatory mechanism
is the
opposite
of what is seen in most tissues: Low pulmonary
oxygen levels cause local vasoconstriction, and high levels pro-
mote vasodilation. While this may seem odd, it is perfectly
consistent with the gas exchange role of this circulation. When
the air sacs of the lungs are flooded with oxygen-rich air, the
pulmonary capillaries become flushed with blood and ready to
receive the oxygen load. If the air sacs are collapsed or blocked
with mucus, the oxygen content in those areas is low, and blood
largely bypasses those nonfunctional areas.
The Heart
Aortic pressure and the pumping activity of the ventricles influ-
ence the movement of blood through the smaller vessels of the
coronary circulation. When the ventricles contract and com-
press the coronary vessels, blood flow through the myocardium
stops. As the heart relaxes, the high aortic pressure forces blood
through the coronary circulation.
Under normal circumstances, the myoglobin in cardiac cells
stores sufficient oxygen to satisfy the cells’ oxygen needs during
systole. However, an abnormally rapid heartbeat seriously re-
duces the ability of the myocardium to receive adequate oxygen
and nutrients during diastole.
Under resting conditions, blood flow through the heart is
about 250 ml/min and is probably controlled by a myogenic
mechanism. Consequently, blood flow remains fairly constant
despite wide variations (50 to 140 mm Hg) in coronary per-
fusion pressure. During strenuous exercise, the coronary vessels
dilate in response to local accumulation of vasodilators (par-
ticularly adenosine), and blood flow may increase three to four
times (see Figure 19.13). Additionally, any event that decreases
the oxygen content of the blood releases vasodilators that adjust
the O
2
supply to the O
2
demand.
Tis enhanced blood flow during increased heart activity is
important because under resting conditions, cardiac cells use
as much as 65% of the oxygen carried to them in blood. (Most
other tissue cells use about 25% of the delivered oxygen.) Con-
sequently, increasing the blood flow is the only way to provide
more oxygen to a vigorously working heart.
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