Chapter 19
The Cardiovascular System: Blood Vessels
709
19
When blood pressure or blood volume is low, water is con-
served and returned to the bloodstream, and blood pressure
rises (
Figures 19.10
and
19.11
). As blood volume goes, so
goes the arterial blood pressure.
Indirect Renal Mechanism
Te kidneys can also regulate blood pressure
indirectly
via the
renin-angiotensin-aldosterone mechanism
. When arterial
blood pressure declines, certain cells in the kidneys release the
enzyme
renin
into the blood. Renin enzymatically cleaves
an-
giotensinogen
, a plasma protein made by the liver, converting
it to
angiotensin I
. In turn,
angiotensin converting enzyme
(ACE)
converts angiotensin I to
angiotensin II
. ACE activity is
associated with the capillary endothelium in various body tis-
sues, particularly the lungs.
Angiotensin II acts in four ways to stabilize arterial blood
pressure and extracellular fluid volume (Figure 19.10).
It stimulates the adrenal cortex to secrete
aldosterone
, a
hormone that enhances renal reabsorption of sodium. As
sodium moves into the bloodstream, water follows, which
conserves blood volume. In addition, angiotensin II directly
stimulates sodium reabsorption by the kidneys.
signals internal bleeding and blood volume too low to support
normal circulation.
However, these assertions—increased blood volume increases
BP and decreased blood volume decreases BP—do not tell the
whole story because we are dealing with a dynamic system. In-
creases in blood volume that raise blood pressure also stimulate
the kidneys to eliminate water, which reduces blood volume and
consequently blood pressure. Likewise, falling blood volume trig-
gers renal mechanisms that increase blood volume and blood
pressure. As you can see, blood pressure can be stabilized or main-
tained within normal limits only when blood volume is stable.
Te kidneys act both directly and indirectly to regulate arte-
rial pressure and provide the major long-term mechanisms of
blood pressure control.
Direct Renal Mechanism
Te
direct renal mechanism
alters blood volume independently
of hormones. When either blood volume or blood pressure
rises, the rate at which fluid filters from the bloodstream into the
kidney tubules speeds up. In such situations, the kidneys cannot
reabsorb the filtrate rapidly enough, and more of it leaves the
body in urine. As a result, blood volume and blood pressure fall.
Peripheral resistance
Cardiac output
Mean arterial pressure (MAP)
Activity of
muscular
pump and
respiratory
pump
Fluid loss from
hemorrhage,
excessive
sweating
Crisis stressors:
exercise, trauma,
body
temperature
Vasomotor tone;
bloodborne
chemicals
(epinephrine,
NE, ADH,
angiotensin II)
Dehydration,
high hematocrit
Body size
Diameter of
blood vessels
Blood
viscosity
Heart
rate
Stroke
volume
Venous
return
Activation of vasomotor and cardio-
acceleratory centers in brain stem
Blood
volume
Baroreceptors
Chemoreceptors
Conservation
of Na
+
and
water by kidneys
Blood volume
Blood pressure
Blood pH
O
2
CO
2
Blood vessel
length
Release
of ANP
Initial stimulus
Result
Physiological response
Figure 19.11
Factors that increase MAP.
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