counter current mechanism

Описание: 👉Countercurrent Mechanism
is the process by which a progressively increasing osmotic gradient is formed in the interstitial fluid of the renal medulla as a result of countercurrent flow.
Countercurrent multiplication involves the long loops of Henle
of juxtamedullary nephrons. the descending limb of the loop of Henle carries tubular fluid from the
renal cortex deep into the medulla, and the ascending limb carries it in the opposite direction. Since countercurrent flow
through the descending and ascending limbs of the long loop of
Henle establishes the osmotic gradient in the renal medulla, the
long loop of Henle is said to function as a countercurrent multiplier. The kidneys use this osmotic gradient to excrete concentrated urine.

👉Production of concentrated urine by the kidneys occurs in the
following way:-

●1Symporters in thick ascending limb cells of the loop of
Henle cause a buildup of Na and Cl in the renal
medulla. In the thick ascending limb of the loop of Henle,
the Na–K–2Cl symporters reabsorb Na and Cl from
the tubular fluid . Water is not reabsorbed
in this segment, however, because the cells are impermeable
to water. As a result, there is a buildup of Na and Cl ions
in the interstitial fluid of the medulla.

●2 Counter current flow through the descending and ascending limbs of the loop of Henle establishes an osmotic gradient in the renal medulla. Since tubular fluid constantly
moves from the descending limb to the thick ascending limb
of the loop of Henle, the thick ascending limb is constantly
reabsorbing Naand Cl. Consequently, the reabsorbed Na
and Cl become increasingly concentrated in the interstitial
fluid of the medulla, which results in the formation of an osmotic gradient that ranges from 300 mOsm/liter in the outer
medulla to 1200 mOsm/liter deep in the inner medulla. The
descending limb of the loop of Henle is very permeable to
water but impermeable to solutes except urea. Because the osmolarity of the interstitial fluid outside the descending
limb is higher than the tubular fluid within it, water moves
out of the descending limb via osmosis. This causes the osmolarity of the tubular fluid to increase. As the fluid continues along the descending limb, its osmolarity increases even
more: At the hairpin turn of the loop, the osmolarity can be
as high as 1200 mOsm/liter in juxtamedullary nephrons. As
you have already learned, the ascending limb of the loop is
impermeable to water, but its symporters reabsorb Na and
Cl from the tubular fluid into the interstitial fluid of the renal medulla, so the osmolarity of the tubular fluid progressively decreases as it flows through the ascending limb. At
the junction of the medulla and cortex, the osmolarity of the
tubular fluid has fallen to about 100 mOsm/liter. Overall, tubular fluid becomes progressively more concentrated as it
flows along the descending limb and progressively more dilute as it moves along the ascending limb.

●3 Cells in the collecting ducts reabsorb more water and urea.
When ADH increases the water permeability of the principal
cells, water quickly moves via osmosis out of the collecting
duct tubular fluid, into the interstitial fluid of the inner
medulla, and then into the vasa recta. With loss of water, the
urea left behind in the tubular fluid of the collecting duct
becomes increasingly concentrated. Because duct cells deep
in the medulla are permeable to it, urea diffuses from the
fluid in the duct into the interstitial fluid of the medulla.

●4 Urea recycling causes a buildup of urea in the renal
medulla. As urea accumulates in the interstitial fluid, some
of it diffuses into the tubular fluid in the descending and thin
ascending limbs of the long loops of Henle, which also are
permeable to urea (Figure 26.19a). However, while the fluid
flows through the thick ascending limb, distal convoluted
tubule, and cortical portion of the collecting duct, urea
remains in the lumen because cells in these segments are
impermeable to it. As fluid flows along the collecting ducts,
water reabsorption continues via osmosis because ADH is
present. This water reabsorption further increases the concentration of urea in the tubular fluid, more urea diffuses
into the interstitial fluid of the inner renal medulla, and the
cycle repeats. The constant transfer of urea between
segments of the renal tubule and the interstitial fluid of the
medulla is termed urea recycling. In this way, reabsorption
of water from the tubular fluid of the ducts promotes the
buildup of urea in the interstitial fluid of the renal medulla,
which in turn promotes water reabsorption. The solutes left
behind in the lumen thus become very concentrated, and
a small volume of concentrated urine is excreted.