Page 1033 - Fundamentals of anatomy physiology

1020 Unit 5 Environmental Exchange
called countercurrent multiplication. Countercurrent refers to
the fact that the exchange takes place between fluids moving in
opposite directions: Tubular fluid in the descending limb flows
toward the renal pelvis, while tubular fluid in the ascending limb
flows toward the cortex. Multiplication refers to the fact that the ef- able to water but not to solutes. As tubular fluid travels deeper
fect of the exchange increases as movement of the fluid continues.
The two parallel limbs of the nephron loop have very dif-
ferent permeability characteristics. The thin descending limb is
permeable to water but relatively impermeable to solutes. The
thick ascending limb is relatively impermeable to both water
and solutes, but it contains active transport mechanisms that
pump sodium and chloride ions from the tubular fluid into the
peritubular fluid of the medulla.
A quick overview of countercurrent multiplication will
help you make sense of the details:
Sodium and chloride ions are pumped out of the thick
ascending limb and into the peritubular fluid. This dilutes
the tubular fluid.
The pumping action increases the osmotic concentration in
the peritubular fluid around the thin descending limb.
This creates a small concentration difference between the
tubular fluid and peritubular fluid in the renal medulla.
The concentration difference results in an osmotic flow of
water out of the thin descending limb and into the peritu-
bular fluid. As a result, the solute concentration increases
in the thin descending limb.
The arrival of the highly concentrated solution in the thick
ascending limb speeds up the transport of sodium and
chloride ions into the peritubular fluid.
26 Solute pumping at the ascending limb leads to higher sol-
ute concentrations in the descending limb, which then brings
about increased solute pumping in the ascending limb. Notice
that this process is a simple positive feedback loop that multi-
plies the concentration difference between the hypotonic tubu-
lar fluid in the ascending limb and the hypertonic peritubular
fluid in the renal medulla.
We can now take a closer look at the mechanics of the pro-
cess. Figure 26–13a diagrams ion transport across the epithe-
lium of the thick ascending limb. Active transport at the apical
surface moves sodium, potassium, and chloride ions out of the
tubular fluid. The carrier is called a Na–K/2 Cl transporter,
because each cycle of the pump carries a sodium ion, a potas-
sium ion, and two chloride ions into the tubular cell. Then
cotransport carriers pump potassium and chloride ions into the
peritubular fluid. However, potassium ions are removed from
the peritubular fluid as the sodium–potassium exchange pump
pumps sodium ions out of the tubular cell. The potassium ions
then diffuse back into the lumen of the tubule through potas-
sium leak channels. The net result is that Na+ and CI– enter the
peritubular fluid of the renal medulla.
The removal of sodium and chloride ions from the tubu-
lar fluid in the ascending limb raises the osmotic concentra-
tion of the peritubular fluid around the thin descending limb
(Figure 26–13b). Recall that the thin descending limb is perme-
into the medulla within the thin descending limb, osmosis
moves water into the peritubular fluid. Solutes remain behind.
As a result, the tubular fluid at the turn of the nephron loop has
a higher osmotic concentration than it did at the start.
The pumping mechanism of the thick ascending limb is
highly effective. Almost two-thirds of the sodium and chloride
ions that enter it are pumped out of the tubular fluid before
that fluid reaches the DCT. In other tissues, differences in sol-
ute concentration are quickly resolved by osmosis. However,
osmosis cannot take place across the wall of the thick ascending
limb, because the epithelium there is impermeable to water. So,
as Na+ and CI– are removed, the solute concentration in the
tubular fluid decreases. Tubular fluid arrives at the DCT with
an osmotic concentration of only about 100 mOsm/L. This
value is one-third the concentration of the peritubular fluid of
the renal cortex.
The rate of ion transport across the thick ascending limb
is proportional to an ion’s concentration in tubular fluid. As
a result, more sodium and chloride ions are pumped into the
medulla at the start of the thick ascending limb, where NaCl
concentrations are highest, than near the cortex. This regional
difference in the rate of ion transport is the basis of the concen-
tration gradient within the medulla.
The Concentration Gradient of the Medulla
Normally, the maximum solute concentration of the per-
itubular fluid near the turn of the nephron loop is about
1200 mOsm/L (Figure 26–13b). Sodium and chloride ions
pumped out of the loop’s ascending limb make up about two-
thirds of that gradient (750 mOsm/L). The rest of the concen-
tration gradient results from the presence of urea.
To understand how urea arrives in the medulla, let’s look
ahead to events in the last segments of the collecting system
(Figure 26–13c). The thick ascending limb of the nephron loop,
the DCT, and the collecting ducts are all impermeable to urea.
As water is reabsorbed, the concentration of urea gradually
increases in the tubular fluid. When the tubular fluid reaches
the papillary duct, it typically contains urea at a concentration
of about 450 mOsm/L. Because the papillary ducts are perme-
able to urea, the urea concentration in the deepest parts of the
medulla also averages 450 mOsm/L.
Benefits of Countercurrent Multiplication
The countercurrent mechanism performs two functions:
1. It efficiently reabsorbs solutes and water before the tubular
fluid reaches the DCT and collecting system.