Regulation of urine concentration and volume

One crucial renal function is to keep the solute concentration of body fluids constant. we use osmolality to measure the amount of solutes in the body fluids. A solution's osmolality is the number of solute particles dissolved in 1 kg of water and reflects the solution's ability to cause osmosis. For any solution interfacing with a selectively permeable membrane, this ability, called osmotic activity, is determined only by the number of solute particles unable to pass through the membrane (called non penetrating solute particles) and is independent of their type. For example, 10 sodium ions have the same osmotic activity as 10 glucose molecules or 10 amino acids in the same volume of water.

Because 1 osmol (equivalent to 1 mole of particles) is a fairly  large unit, the milliosmol (mOsm) equal to 0.001 osmol, is generally used.

The kidneys keep the solute load of body fluids constant at about 300 mOsm, the osmotic concentration of blood plasma, by regulating urine concentration and volume. The kidneys accomplish this feat using countercurrent mechanism. In the kidneys, the term countercurrents means that fluid flows in opposite directions through adjacent segments of the same tube connected by a hairpin turn. These countercurrent mechanism are (1)the interaction between the flow of filtrate through the ascending and decending limbs of the long loops of Henle of juxtamedullary nephrons (the counter current multiplier), and (2)the flow of blood through the ascending and decending portions of the vasa recta blood vessels (the countercurrent exchanger). These countercurrent mechanisms establish and maintain an osmotic gradient extending from the cortex through the depths of the medulla. This gradient allow the kidneys to vary urine concentration dramatically.

The osmolality of the filtrate entering the proximal convoluted tubule is identical to that of plasma, about 300 mOsm, Because of proximal convoluted tubule reabsorption of water and solutes, the filtrate is still isoosmotic with plasma by the time it reaches the decending limb of the loop of Henle. However, its osmolality increases from 300 to about 1200 mOsm in the deepest part of medulla.

How does this increase in concentration occur? The answer lies in the unique workings of the long loop      of the juxtamedullary nephrons, and the vasa recta.