Kidneys- counter current exchange mechanisms
Counter-current multiplier which takes place in the loop of Henle
The end result is to make the environment of the loop of Henle to be hypertonic. This environment is placed in the pyramids. As urine enters the descending limb of Henle's loop it carries both water and solute. It looses its water in this limb and when it gets to the counter limb at the thin ascending limb of Henle's loop it looses its solute rather than water because the membrane of this limb is impermeable to water. At the thick segment to the ascending limb of Henle's loop, the membrane is impermeable to both water and solute (Na+). Carrier transport mechanism then takes over to pump solute mainly Na+ ions outside using Na+- K+ ATPase.
Vasa recta and exchanger
The counter-current exchanger mechanism which takes place in the vasa recta is simple enough and its main aim is to keep the hypertonic nature of the loop of Henle's environment. It works in the pyramids which already have become hypertonic. The ascending vasa recta absorbs the solutes using concentration gradient. The solutes are passed into the descending vasa recta and from there back into the pyramidal environment. Hence this counter-current system maintains the hypertonic nature of the environment of the pyramids thereby encouraging osmotic gradient to allow water to flow from the collecting ducts to the same environment. In effect it concentrates urine which otherwise would have been dilute and loose water.
Cross section through the kidney x40
Counter-current multiplier system in the loop of Henle: H - water Na represents Na+ and any other solute.
School of Medicine
Creator: Oluwole Ogunranti