172
CARDIOVASCULAR PHYSIOLOGY CONCEPTS
located in the inner cortex near the outer medulla
give rise to very long capillaries (vasa recta) that
loop down deep within the medulla. The capil-
laries are involved with countercurrent exchange
and the maintenance of medullary osmotic gra-
dients. Capillaries eventually form venules and
then veins, which join together to exit the kidney
as the renal vein. Therefore, within the kidney, a
capillary bed (glomerular capillaries) is located
between the two principal sites of resistance
(afferent and efferent arterioles). Furthermore, a
second capillary bed (peritubular capillaries) is in
series with the glomerular capillaries and is sepa-
rated by the efferent arteriole.
RENAL HEMODYNAMICS
The vascular arrangement within the kid-
ney
is
very
important
for
filtration
and
reabsorption functions of the kidney. Changes
in afferent and efferent arteriole resistance
affect not only blood flow, but also the hydro-
static pressures within the glomerular and
peritubular
capillaries.
Glomerular
capil-
lary pressure, which is about 50 mm Hg, is
much higher than that in capillaries found in
other organs. This high pressure drives fluid
filtration (see Chapter 8). The peritubular
capillary pressure, however, is low (about 10
to 20 mm Hg). This is important because it
permits fluid reabsorption to lim it water loss
and urine excretion. About 20% of the plasma
entering the kidney is filtered. If significant
reabsorption did not occur, a high rate of
urine formation would rapidly lead to hypov-
olemia and hypotension and an excessive loss
of electrolytes. Figure 7.17 shows the effects
of afferent and efferent arteriole dilation and
constriction on blood flow and glomerular
A
tF
i P
tP
C
tF
tP
i P
A A
GC
EA
PC
I
f
■ FIGURE 7.17 Effects of renal afferent and efferent arteriole resistances on blood flow and renal capillary
pressures. The following descriptions assume constant aortic pressure. Panel A. Decreased afferent arte-
riole (AA) resistance (R) increases glom erular capillary (GC) and peritubular capillary
(PC)
pressures (P)
and increases flow (F). Panel B. Increased AA resistance decreases GC and PC pressures and decreases
F. Panel C. Decreased efferent arteriole
(EA)
resistance decreases GC pressure, increases PC pressure,
and increases F. Panel D. Increased EA resistance increases GC pressure, decreases PC pressure, and
decreases F. *, arteriole undergoing resistance change.
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