Question

Answer each of the following:

a) Briefly describe the major systems involved in the regulation of blood volume and pressure.

b) Describe how countercurrent exchange in the loops of Henle work.

c) Describe the purpose and mechanism of action for vasopressin.

Thank you

Answer 1

b. Describe how countercurrent exchange in the loops of Henle work.

Ans. The countercurrent mechanism is responsible for the generation of an osmotic gradient in the renal medulla. The exchange of a substance or heat between two fluids flowing in opposite direction is known as countercurrent flow. from the structure of the urinary tubules, it is seen that decending, ascending and collecting tubule are very close to each other. The flow in the ascending tubule runs counter to that in the decending and the collecting tubule.Sodium and other ions are transported out of the filtrate in the proximal tubule where water also moves out. By the time urine reaches loop of Henle, about 80% of toatl filtrate is out of the tubule. The loop of Henle absorbs lot of sodium making the urine concentrated or hyperosmolar.

In the ascending tubule and the loop of Henle , there is no movement of water consequent upon the active transport of sodium , making the filtrate less concentrated. The fluid returned to the distal tubule is hypotonic. This segment is able to is able to reabsorb water and the fluid reaching the collecting tubules is isotonic there is further transport of sodium from the collecting duct , but as the fluid passes through medulla, water difuses out of the filtrate and the fluid reaching the pelvis of the kidney is concentrated form of urine.

The countercurrent mechanism is also maintained by vasa rectae. In the vasa recta the blood runs counter to the flow of urine. The flow of blood in the medullary tissue is downward and in the cortex, it is upward.As the blood flows through the medulla, water diffuses out and sodium diffuses in, whereas the reverse movement takes place in cortex, leaving most of the sodium to remain in the intersitial fluid circulation.

3 Describe the purpose and mechanism of action for vasopressin.

Antidiuretic hormone (vasopressin) released by the posterior lobe of pituitary plays role in water reabsorption at the collecting duct When ADH is present more water is reabsorbed (blood voulme and pressure rise and a decreased amount of urine results. Blood -borne ADH binds with its receptor present on the basolateral membrane of a cell in the distal or collecting tubule. This binding activates the cAMP seconadry messenger system within the cell.cAMP increases the opposite luminal mebrane's permeability to water by promoting the insertion of ADH- regulated water channels into the membrane. This membrane is impermeable to water in the absence of ADH. water enters the tubular cell from the tubular lumen through the inserted water channels. Water exits the cell through a different, always open water channel premanently poistioned at the basolateral border, and then enters the blood, in this way being reabsorbed.