The Cardiovascular System: Blood Vessels
When blood pressure or blood volume is low, water is con-
served and returned to the bloodstream, and blood pressure
). As blood volume goes, so
goes the arterial blood pressure.
Indirect Renal Mechanism
Te kidneys can also regulate blood pressure
. When arterial
blood pressure declines, certain cells in the kidneys release the
into the blood. Renin enzymatically cleaves
, a plasma protein made by the liver, converting
. In turn,
angiotensin converting enzyme
converts angiotensin I to
. 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
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
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
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.
Mean arterial pressure (MAP)
Fluid loss from
Activation of vasomotor and cardio-
acceleratory centers in brain stem
water by kidneys
Factors that increase MAP.