Regulation of urine formation is under hormonal control. Comment.

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Solution

The kidneys are not sensitive to just chemical composition and osmolarity of blood and plasma, but, are also influenced by hormones. The most important hormone that directly regulates kidney function is the Anti-diuretic Hormone(ADH). Apart from ADH, hormonal regulation also takes place with the help of aldosterone and Renin.

Hormone Action
ADH When the water levels in the body fall below a certain level, the receptors known as osmoreceptors present throughout the body are activated. Osmoreceptors are a type of receptors that detect a change in the osmotic pressure of the blood and plasma. Once these osmoreceptors are activated, they send impulses to the posterior lobe of the pituitary gland to stimulate secretion of ADH. ADH stimulates the kidneys to reabsorb water from the urine to restore the water levels in the body.
Renin When there is a fall in blood pressure, the amount of blood passing through the glomeruli is less and so there is less filtration rate. Due to this, a type of cells known as juxtaglomerular cells present around the glomerulus, are activated and they release an enzyme called renin.

Renin in the blood converts angiotensinogen to angiotensin I, which is eventually converted to angiotensin II. Angiotensin II acts on the blood vessels and capillaries to cause their constriction thereby increasing the blood pressure. The angiotensin II also stimulates the adrenal cortex to release aldosterone which has an effect on sodium levels in the blood.
Aldosterone Aldosterone is primarily responsible for reabsorption of sodium from the urine. Reabsorption of sodium leads to increase in water reabsorption as well. Both these together help to restore the blood pressure.

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