Can someone explain what is meant by a counter-current multiplier is?

Question

First in general, but then in the context of the kidney (loop of henle)

Answer 1

Countercurrent exchange refers to a system in which there is adjacent flow in opposite directions and there is some sort of exchange between two sides.  (If the flow was in the same direction is would be concurrent exchange.)  In a countercurrent system where you have a flow leading to an activity point where the exchanged property is introduced, and return flow across the exchange region, you will get a multiplier effect.  The change at the point will exceed the change without the counter current system.

Imagine air in a straight duct passing a heating element.  The temperature of the air goes up a little.  Now bend the duct over at the heating element.  The outgoing (warmed) air will preheat the incoming air before it gets to the heating element.  This means that the temperature just past the heating element will be higher because of the bend.

In the kidney, the tubules exchange fluid.  This results in an increase in concentration as the fluid reaches the turn where water is resorbed from the fluid which is already concentrated.  The returning fluid is progressively diluted upon return.  This allows a higher maximum concentration for the same energy input.