Counter Current Mechanism
The countercurrent mechanism is a process that occurs in the kidneys, enabling
them to concentrate urine and maintain water and electrolyte balance in the
body. It primarily takes place in the loop of Henle, and involves the
countercurrent multiplier system in the loop of Henle and the countercurrent
exchanger in the vasa recta (the
parallel network of capillaries around LOH).
Mammals have the ability to produce concentrated urine.
The Henle's loop and vasa recta play a significant role in
this.
The flow of filtrate in the two limbs of Henle's loop is in
opposite directions and thus forms a counter current.
Large amount of Na+ ions are actively reabsorbed from filtrate into interstitium in ascending limb of loop of Henle and is followed by Passive transport of CI-. this increases osmolarity of medulla region. Thus, more water is reabsorbed from DLOH and more hypertonic nephric filtrate reaches in ascending limb. Thus, more Na+ is reabsorbed from ALOH. which further increases osmolarity of interstitium of medulla. Thus, LOH acts as counter Current multiplier.
The flow of blood through the two limbs of the vasa recta is
also in a countercurrent pattern i.e. in
opposite direction. only about ( 1-2)% of total
renal blood flows through vasa recta. the descending Vasa recta lies in
close aproximity with ascending limb of loop of Henle and receives some Na+ and
CI- while ascending vasa recta is in Close aproximity with descending limb of
loop of Henle and leaves Na+ and CI- in
the interestitium. Thus, it acts as Counter current exchanger.
The proximity between the
Henle’s loop and vasa recta, as well as the countercurrent in them help
in maintaining an increasing osmolarity towards the inner medullary
interstitium, i.e., from 300 mosmol/L in
the cortex to about 1200 mosmol/L in the
inner medulla.
This gradient is mainly caused by NaCl and urea. NaCl is transported by the ascending limb of
Henle's loop which is exchanged with the descending limb of vasa recta. NaCl is
returned to the interstitium by ascending part of the vasa recta.
The filtrate is concentrated as it moves down the descending
limb but is diluted by the ascending limb.
DCT & collecting duct concentrate the filtrate about four times,
i.e., from 300 mOsmol/L to 1200
mOsmol/L an excellent mechanism of
conservation of water.
Similarly, small amounts of urea/electrolyte enter the thin
segment of the ascending limb of Henle’s loop which is transported back to the
interstitium by the distal region of collecting duct. This special arrangement of Henle's loop and
vasa recta is called the countercurrent mechanism.
This mechanism helps to maintain a concentration gradient in
the medullary interstitium. Presence of
such interstitial gradient helps in an easy passage of water from
collecting ducts.
As the filtrate flows down in the collecting duct more and more water moves out of the tubule
by osmosis which makes filtrate hypertonic to blood.
The human kidney can produce urine nearly four times more concentrated
than the initial filtrate formed.