Describe how counter current multiplier mechanism helps in the formation of concentrated urine in human .

Explanation

Formation of concentrated urine is accomplished by kidneys using counter current mechanisms. The major function of Henle.s loop is to concentrate `Na^(+)` and `Cl^(-)` . There is low osmolarity near the cortex and high osmolarity towards the medulla. This osmolarity in the medulla is due to the presence of the solute transporters and is maintained by the arrangement of the loop of Henle, collecting duct and vasa recta. This arrangement allows movement of solutes from the filtrate to the interstitial fluid. At the transition between the proximal convoluted tubule and the descending loop of Henle the osmolarity of the interstitial fluid is similar to that of the blood - about 300mOsm. Ascending and descending limbs of Henle, create a counter current multiplier ( interaction between flow of filtrate through the limbs of Henle.s and JMN) by active transport. Figure ( a) shows, the counter current multiplier created by the long loops of ijenle of the JM nephrons which creates medullary osmotic gradient. As the fluid enters the descending limb, water moves from the lumen into the interstitial fluid and the osmolarity decreases. To counteract this dilution the region of the ascending limb actively pumps solutes from the lumen into the interstitial fluid and the osmolarity increases to about 1200mOsm in medulla. This mismatch between water and salts creates osmotic gradient in the medulla. The osmotic gradient is also due to the permeability of the collecting duct to urea. The vasa recta, maintains the medullary osmotic gradient via counter current exchanger (the flow of blood through the ascending and descending vasa recta blood -ves.sels) by passive transport. Figure (b) shows counter current exchanger where the vasa recta preserves the medullary gradient while removing reabsorbed water and solutes. This system does not produce an osmotic gradient, but protects the medulla by removal of excess salts from the interstitial fluid and removing reabsorbed water. The vasa recta leave the kidney at the junction between the cortex and medulla. The interstitial fluid at this point is iso-osmotic to the blood. When the blood leaves the efferent arteriole and enters vasa recta the osmolarity in the medulla increases (1200mOsm) and results in passive uptake of solutes and loss of water. As the blood enters the cortex, the osmolarity in the blood decreases (300mOsm) and the blood loses solutes and gains water to fonn concentrated urine (hypertonic ). Human kidneys can produce urine nearly four times concentrated than the initial filtrate formed