Question

Describe how the kidneys help in osmoregulation in humans.

Answer 1

Osmoregulation is the process which regulates the concentration and osmotic pressure of blood by regulating the water contents of the blood plasma. Human kidney excretes large amount of hypotonic urine when water intake is very high and excretes small amount of hypertonic urine when water is deficient and needs to be conserved.

Excessive water in body fluids decreases osmotic pressure and increases the blood volume. This signals posterior pituitary to stop production of vasopressin which results in decreased resorption of water by the distal convoluted tubules and collecting duct. In deficiency of water, ADH is produced by posterior pituitary which increases absorption of water by tubules. ADH also causes constriction of blood vessels and increase in blood pressure and glomerular blood flow and thereby increasing the filtration rate.

A fall in glomerular blood flow stimulates production of renin from JG cells which converts angiotensin to I and II. Angiotensin II is powerful constrictor of blood vessels. It increases glomerular blood pressure and blood filtration. Angiotensin II also activates adrenal cortex to release aldosterone, which activates resorption of sodium ions and water from distal tubules.

An increase in blood flow to atria of the heart can cause the release of Atrial Natriuretic Factor (ANF ). ANF can cause vasidilatation and thereby decrease blood pressure. It also acts as check on renin-angiotensin mechanism.