506
Chapter 14
is almost completely reversed. Thus, the hairpin-loop struc-
ture of the vasa recta minimizes excessive loss of solute from
the interstitium by
diffusion.
At the same time, both the salt
and water being reabsorbed from the loops of Henle and col-
lecting ducts are carried away in equivalent amounts by
bulk
ow,
as determined by the usual capillary Starling forces. This
maintains the steady-state countercurrent gradient set up by the
loops of Henle. Because of NaCl and water reabsorbed from the
loop of Henle and collecting ducts, the amount of blood fl ow
leaving the vasa recta is at least two-fold higher than the blood
fl ow entering the vasa recta. Finally the total blood fl ow going
through all of the vasa recta is a small percentage of the total
renal blood fl ow. This helps to minimize the washout of the
hypertonic interstitium of the medulla.
The Recycling of Urea Helps to Establish
a Hypertonic Medullary Interstitium
As was just described, the countercurrent multiplier establishes
a hypertonic medullary interstitium that the vasa recta help to
preserve. We already learned how the reabsorption of water in
the proximal tubule mediates the reabsorption of urea by dif-
fusion. As urea passes through the remainder of the nephron,
it is reabsorbed, secreted into the tubule, and then reabsorbed
again (
Figure 14–19
). This traps urea, an osmotically active
molecule, in the medullary interstitium, thus increasing its
osmolarity. In fact, as shown in Figure 14–17, urea contributes
to the total osmolarity of the renal medulla.
Urea is freely fi ltered in the glomerulus. Approximately
50 percent of the fi
ltered urea is reabsorbed in the proxi-
mal tubule, and the remaining 50 percent enters the loop
of Henle. In the thin descending and ascending limbs of
the loop of Henle, urea that has accumulated in the med-
ullary interstitium is secreted back into the tubular lumen
by facilitated diffusion. Therefore, virtually all of the urea
that was originally fi ltered in the glomerulus is present in
the fl uid that enters the distal tubule. Some of the origi-
nal urea is reabsorbed from the distal tubule and cortical
collecting duct. Thereafter, about half of the urea is reab-
sorbed from the
medullary
collecting duct, whereas only 5
percent diffuses into the vasa recta. The remaining amount
is secreted back into the loop of Henle. Fifteen percent of
the urea originally fi
ltered remains in the collecting duct
and is excreted in the urine. This recycling of urea through
the medullary interstitium and minimal uptake by the vasa
recta traps urea there and contributes to the high osmolarity
shown in Figure 14–17.
Renal Sodium Regulation
In healthy individuals, urinary sodium excretion increases
when there is an excess of sodium in the body and decreases
when there is a sodium defi cit. These homeostatic responses
are so precise that total-body sodium normally varies by only
a few percent despite a wide range of sodium intakes and the
occasional occurrence of large losses via the skin and gastroin-
testinal tract.
As we have seen, sodium is freely fi lterable from the glo-
merular capillaries into Bowman’s space and is actively reab-
sorbed, but not secreted. Therefore:
Sodium excreted = Sodium fi ltered – Sodium reabsorbed
The body can adjust sodium excretion by changing both
processes on the right of the equation. Thus, for example,
when total-body sodium decreases for any reason, sodium
excretion decreases below normal levels because sodium reab-
sorption increases.
The fi rst issue in understanding the responses control-
ling sodium reabsorption is to determine what inputs initiate
them; that is, what variables are receptors actually sensing?
Surprisingly, there are no important receptors capable of
detecting the total amount of sodium in the body. Rather, the
responses that regulate urinary sodium excretion are initiated
mainly by various cardiovascular baroreceptors, such as the
carotid sinus, and by sensors in the kidney that monitor the
fi l t e r e d l o a d o f s o d i u m .
As described in Chapter 12, baroreceptors respond to
pressure changes within the cardiovascular system and initiate
refl exes that rapidly regulate these pressures by acting on the
heart, arterioles, and veins. The new information in this chap-
ter is that
regulation of cardiovascular pressures by baroreceptors
also simultaneously achieves regulation of total-body sodium.
Sodium is the major extracellular solute constituting,
along with associated anions, approximately 90 percent of
these solutes. Thus, changes in total-body sodium result in
similar changes in extracellular volume. Because extracellu-
Glomerulus
50%
reabsorbed
30%
reabsorbed
Cortical
collecting
duct
55%
reabsorbed
100%
50%
70%
15%
100%
50% facilitated
diffusion
5% removed
Proximal
tubule
Medullary
collecting
duct
Distal
tubule
Loop of
Henle
Urea recycling
Cortex
Outer
medulla
Inner
medulla
Figure 14–19
Urea recycling. The recycling of urea “traps” urea in the inner
medulla, which increases osmolarity and helps to establish and
maintain hypertonicity.
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