Role Of Kidney In Osmoregulation

Osmo regulation is the process which regulates the concentration and osmotic pressure of blood by regulating the water contents of blood plasma. It is an important process as excessive loss of water may cause dehydration whereas excess of water intake may dilute the body fluids.

The vertebrate kidney is extremely flexible in its working. It excretes large amount of hypotonic urine when water intake is very high, while it excretes small amount of hypertonic urine when water is deficient and needs to be conserved.

Regulation of concentration of body fluids

Excess water in body fluids

When this condition arises, the urine passed out of the body is more dilute or hypotonic than the body fluids to expel the excess of water. This is because excess of water in the body fluids lowers the osmotic pressure of the blood and increases the blood volume. These changes disturbs the exchange of materials between the blood pressure and causes cardiovascular dysfunctioning respectively.

It involves 2 processes. They are:

a) Ultra filtration

b) Decreased reabsorption.

Ultra filtration is increased due to the increase in hydrostatic pressure due to the excess of water. So more nephric filtrate is filtered out from the glomerular capillaries into Bowman's capsule.

Water is mainly reabsorbed through the collecting tubules. The permeability of the wall of the DCT and collecting tubules is controlled by anti - diuretic hormone or ADH or vasopressin hormone which is released from the posterior lobe of the pituitary gland. Excess of water in the body fluids signals to posterior pituitary to stop the release of the hormone vasopressin. Deficiency of this hormone lowers the permeability of the cells of the distal convoluted tubule and the collecting duct, decreasing the reabsorption of Na⁺ from the filtrate continues in these regions of the nephrons. More filtration combined with less reabsorption of water produces abundant dilute urine and this brings down the volume of body fluids to normal.

Shortage of water in body fluids

When the volume of body fluids decreases below normal due to profuse sweating during heavy exercise or high temperature or excessive bleeding or a prolonged delay in fluid intake, the rate of ultra filtration is decreased due to decreased blood volume and low hydrostatic pressure of blood in the glomerular capillaries and rate of reabsorption of water is increased by increasing the permeability of the wall of DCT and collecting tubules due to increased release of ADH from the posterior pituitary. Less ultrafiltration and more reabsorption produce small amount of hypertonic urine which increases body fluid volume to normal.

A deficiency of ADH causes diabetes insipidus which is characterised by diuresis or increased urine output, about 10 times more than the normal rate.

When there is a decrease in blood pressure or blood volume in the afferent arteriole of the glomerulus, the Justaglomerular Apparatus (TGA) that is situated between the afferent and efferent arterioles release an enzyme 'renin' into the blood stream. A plasma protein angiotensinogen is converted to apeptide angiotensin II by renin.

Angiotensin II acts as a hormone.

1) It increases blood pressure by causing arterioles to contract.

2) It stimulates the adrenal gland to produce another hormone aldosterone, which in turn induces the distal convoluted tubule to reabsorb more Na⁺ and water thereby increasing the volume and pressure of blood.

An increase in blood volume and pressure stimulates the walls of the heart (atria) to release another hormone Atrial Natriuretic Factor (ANF) which opposes the working of the Renin - Angiotensin - Aldosterone System (RAAS) .

ANF stops the production of rennin from the JGA (Juxtaglomerular Apparatus) and thereby stops NaCl absorption by the collecting duct, which in turn inhibits the production of the aldosterone from the adrenal gland.

ADH, RAAS and ANF work as a system in maintaining body fluid osmolarity, salt concentration blood pressure and volume.

(a)

(b)

Water and solute balance in (a) fresh water and (b) marine bony fish