708
UNIT 4
Maintenance of the Body
19
As we noted earlier, blood volume is a major determinant
of cardiac output (via its influence on venous return, EDV, and
stroke volume). An increase in blood volume is followed by a rise
in blood pressure, and anything that increases blood volume—
such as excessive salt intake, which promotes water retention—
raises MAP because of the greater fluid load in the vascular tree.
By the same token, decreased blood volume translates to a fall
in blood pressure. Blood loss and the garden-variety dehydra-
tion that occurs during vigorous exercise are common causes of
reduced blood volume. A sudden drop in blood pressure oFen
Long-Term Regulation: Renal Mechanisms
Unlike short-term controls of blood pressure that alter periph-
eral resistance and cardiac output, long-term controls alter blood
volume. Renal mechanisms mediate long-term regulation.
Although baroreceptors respond to short-term changes in
blood pressure, they quickly adapt to prolonged or chronic epi-
sodes of high or low pressure. Tis is where the kidneys step in
to restore and maintain blood pressure homeostasis by regu-
lating blood volume. Although blood volume varies with age,
body size, and sex, renal mechanisms usually keep it close to 5 L.
Arterial pressure
Blood volume
Aldosterone
Mean arterial pressure
Blood volume
Mean arterial pressure
Filtration by kidneys
Urine formation
Arterial pressure
Inhibits baroreceptors
Sympathetic nervous
system activity
Water intake
Water reabsorption
by kidneys
Sodium reabsorption
by kidneys
ADH release by
posterior pituitary
Vasoconstriction;
peripheral resistance
Thirst via
hypothalamus
Adrenal cortex
Angiotensin II
Angiotensin converting
enzyme (ACE)
Secretes
Initial stimulus
Physiological response
Result
Direct renal mechanism
Indirect renal mechanism (renin-angiotensin-aldosterone)
Angiotensin I
Angiotensinogen
Renin release
from kidneys
Figure 19.10
Direct and indirect (hormonal) mechanisms for renal control of blood
pressure.
Low blood pressure also triggers other actions not shown here that increase BP:
additional mechanisms of renin release (described in Chapter 25) and short-term actions of the
sympathetic nervous system.
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