What is counter current exchange mechanism in nephron
Counter current
exchange:
The loop of the
nephron increases the efficiency of re-absorption by counter current flow with
water and ions being reabsorbed. Generally, the longer the loop of the nephron,
the more water and ions that can be reabsorbed. It follows that desert rodents
(e.g. kangaroo rat) that form highly concentrated urine have vary long nephron
loops, similarly amphibians that are closely associated with aquatic habitats
have nephrons that lack a loop.
Figure shows the
counter current flow mechanisms for concentrating urine. The process of
re-absorption in the proximal convoluted tubule removes some salt (NaCl) and
water from the glomerular filtrate and reduces its volume by 25%. However the
concentration of salt and urea are still iso osmotic with extra cellular fluid.
As the filtrate passes into the ascending limb, sodium (Na+) ions
are actively transported out of the filtrate into extra cellular fluid, with
chlorine (Cl-) ions following passively. Water cannot flow out of
ascending limb because the cells of the ascending limb are impermeable to
water.
Thus the salt concentration of the extra cellular fluid becomes very
high. The salt flows passively into the descending loop, only to move out again
in the ascending loop, creating a recycling of salt through the loop and the
extra cellular fluid. Because the flows in the descending limbs are in opposite
direction, a counter current gradient in salt is set up. The osmotic pressure
of extra cellular brine bath is made even higher because of abundance of urea
that moves out of collecting ducts. Finally the distal convoluted tubule
empties into the collecting duct which is permeable to urea and concentrated
urea in the filtrate diffuses out into surrounding extra cellular fluid. High
urea concentration in extra cellular fluid, coupled with high concentration of
salt, form, urio brine bath that causes water to move out of the filtrate by
osmosis as it moves down the descending limb. Finally many peritubular
capillaries surrounding each nephron collect the water and return it to the
systematic circulation.
Renal pelvis of mammalian kidney is continuous with
ureter that carries urine to urinary bladder. Urine from two ureters (one from
each kidney) accumulates in the urinary bladder. Urine leaves the body through
single tube, the urethra which opens at the body surface at the end of the
Penis (in human male) or just in front of vaginal entrance (in human female).
As the urinary bladder fills with urine tension increases in its smooth muscle
walls. In response to this tension, a reflex response relaxes sphincter muscles
at the entrance to the urethra. This response is called urination. Two kidneys,
two ureters, urinary bladder and urethra constitute urinary system of mammals.
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