Question

5. Explain how the renal medullary osmotic gradient is generated and maintained. Describe how this is important for the regulation of water balance and urine volume. Include the terms Loop of Henle, aquaporin and antidiuretic hormone. Long answer

Answer 1

The main function of kidneys is to excrete out nitrogenous waste from the body in urine concentrated enough to prevent the loss of excess water.

Kidney is divided into outer cortex and inner medulla. Kidney is made of nephrons which are the basic unit. These nephrons filter the plasma and the filtrate is then passed through renal tubules i.e PCT, loop of henle, DCT and collecting duct where essential substances are reabsorbed while the excretory products are secreted as per the requirements of the body.

Various studies have proven that the amount of urine excreted per day (1.5-2 L) is far less than the amount of filtrate formed per day(150-180L). This means that most of the filterate is reabsorbed to concentrate the urine. This is due to medullary osmotic gradient which means that the osmolarity of the renal interstitium increases gradually from the cortex(300mosm/L) to medulla(1200mosm/L) due to increasing concentration of sodium and urea. Sodium concentration is generated by counter current mechanism (flow of fluid in loop of henle and vasa recta in the opposite directions and different concentrations) while the urea is generated by urea recycling.

This medullary osmotic gradient depends on the physiological needs of the body to preserve water. If there is a need for preserving water, ADH( anti diuretic hormone) is secreted which increases this gradient and also increases the number of aquaporins (water channels) in the collecting duct, so thatore concentrated urine is excreted.