Q.Give a brief account of the counter current mechanism.

The counter current mechanism is the arrangement that lets the kidney build a steep concentration gradient in the medulla and so form concentrated urine.

• It depends on two structures lying side by side: Henle's loop and the vasa recta, the capillary running parallel to the loop.
• In the two limbs of Henle's loop the filtrate flows in opposite directions, forming a counter current; blood in the two limbs of the vasa recta flows in a counter current pattern too.
• Sodium chloride carried out by the ascending limb of the loop is exchanged with the descending vasa recta and returned to the interstitium by its ascending part.
• Small amounts of urea likewise pass into the interstitium and are recirculated, adding to the gradient. …

Mammals stand out in their ability to produce a urine far more concentrated than their own body fluids, and in the human kidney this feat rests on two structures working together: Henle's loop and the vasa recta, the slender U-shaped capillary that runs alongside the loop.

The heart of the idea is the direction of flow. In the two limbs of Henle's loop the filtrate travels in opposite directions, which is what "counter current" means. Blood in the two limbs of the vasa recta also flows in a counter current pattern. Because the loop and the vasa recta lie close together, both carrying their fluids in opposing directions, they can build up and then hold a concentration that rises the deeper one goes into the medulla.

That gradient is real and steep. The osmolarity climbs from about 300 milliosmoles per litre out in the cortex to roughly 1200 milliosmoles per litre in the inner medulla. Two substances produce it — sodium chloride and urea — and they are shuffled between tubule and vessel in a coordinated way:

• Sodium chloride is transported out by the ascending limb of Henle's loop and exchanged with the descending limb of the vasa recta.
• The ascending portion of the vasa recta then returns that sodium chloride to the interstitium rather than washing it away.
• In much the same fashion, small amounts of urea enter the thin ascending segment of the loop and are carried back to the interstitium by the collecting tubule. …

Method: Describing a Counter-Current / Gradient-Building Mechanism

Any question asking you to give an account of a physiological mechanism that builds and maintains a concentration gradient (like the counter-current system, or an ion gradient elsewhere in the body) can be answered with the same four-part structure.

Steps

Step 1: Name the paired structures and their geometry

State which two tubes or vessels lie side by side and carry fluid in opposite directions — here, the two limbs of Henle's loop and the two limbs of the vasa recta running parallel to it. The "counter" in counter-current always refers to this opposite-direction, side-by-side arrangement.

Step 2: State what is being exchanged and in which direction

Name the specific solutes moved between the tube and the vessel — sodium chloride pumped out by the ascending limb, urea entering the thin segment — and describe the hand-off: out of one tube, into the interstitium, into the parallel vessel, and partly recycled back.

Step 3: State the resulting gradient, with its numerical range if known

Describe the outcome as a gradient across a defined region — the medullary osmolarity rising from about 300 milliosmoles per litre in the cortex to about 1200 milliosmoles per litre in the inner medulla — since a mechanism question is judged partly on whether you can quantify what it achieves. …