C h a p t e r 1 8
Disorders of Blood Flow and Blood Pressure
421
Sympathetic
activity
Baroreceptors
Angiotensin II
Vagal and
sympathetic activity
Heart rate
Stroke volume
Heart
Venous return
Blood volume
Salt and water
retention
Aldosterone
Kidney
Renin-angiotensin
mechanism
Adrenal gland
Cardiac output
Arterial blood pressure
Peripheral vascular resistance
FIGURE 18-13.
Mechanisms of
blood pressure regulation.The solid
lines represent the mechanisms for
renal and baroreceptor control of
blood pressure through changes in
cardiac output and peripheral vascular
resistance.The dashed lines represent
the stimulus for regulation of blood
pressure by the baroreceptors and the
kidneys.
stimulation produces an increase in heart rate and car-
diac contractility. Blood vessels are mainly innervated
by the sympathetic nervous system, which produces
constriction of the small arteries and arterioles with a
resultant increase in peripheral vascular resistance.
The ANS control of blood pressure is mediated
through both intrinsic and extrinsic cardiovascular
reflexes, as well as higher neural control centers.
26
The
intrinsic reflexes,
including the baroreceptor and che-
moreceptor reflexes, respond to stimuli originating from
within the cardiovascular system and are essential to
rapid and short-term regulation of blood pressure. The
extrinsic reflexes
mediate the cardiovascular response to
stimuli originating from outside the cardiovascular sys-
tem. They mediate blood pressure responses associated
with factors such as pain and temperature changes. The
neural pathways for these reactions are more diffuse,
and their responses are less consistent than those of the
intrinsic reflexes. Many of these responses are channeled
through the hypothalamus, which plays an essential role
in the control of sympathetic nervous system responses.
The baroreceptors are pressure-sensitive receptors
located in the walls of blood vessels and the heart. They
can be classified as high-pressure and low-pressure
baroreceptors based on the type of blood vessel in which
they are located. The high-pressure carotid and aortic
baroreceptors are located in strategic position between
the heart and the brain (Fig. 18-14). They respond to
changes in the stretch of the vessel wall by sending
impulses to cardiovascular centers in the brain stem
to effect appropriate changes in heart rate and vascu-
lar smooth muscle tone. For example, the fall in blood
pressure that occurs on moving from the lying to the
standing position produces a decrease in the stretch of
the baroreceptors with a resultant increase in heart rate
and sympathetically induced vasoconstriction that pro-
duces an increase in peripheral vascular resistance. The
low-pressure baroreceptors, which are located in large
systemic veins, pulmonary vessels, and walls of the right
atrium and ventricles of the heart, have both circulatory
and renal effects. They produce changes in antidiuretic
hormone (ADH) secretion, resulting in profound effects
on the retention of salt and water.
The arterial chemoreceptors are cells that monitor
the oxygen, carbon dioxide, and hydrogen ion con-
tent of the blood. They are located in the carotid bod-
ies, which lie in the bifurcation of the two common
carotids, and in the aortic bodies of the aorta
26
(see
Fig. 18-14). Because of their location, these chemore-
ceptors are always in close contact with the arterial
blood. Although the main function of the chemorecep-
tors is to regulate ventilation, they also communicate
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