Osmoregulation by Kidney

Maintaining the solute concentration of the body fluids is called osmoregulation. Fine control of the precise amount of water and salt reabsorbed into blood is an important function of the distal convoluted tubules and collecting ducts. Depending Plants and animals on the need of the water in the body, kidneys excrete hypotonic (dilute) or hypertonic (concentrated) urine. Osmoregulation is controlled by the hormones ADH and aldosterone. Feedback circuits regulate their secretion.

(a) When the water content of the body is more, leading to low osmotic pressure, less ADH (anti diuretic hormone) is released. Hence the wall of the DCT and collecting tubules remain less permeable and as a result plenty of dilute urine (hypotonic urine) is excreted.

(b) When water content of the body is low, the posterior pituitary secretes more of ADH. The permeability of the tubules is increased. As a result more water is reabsorbed into the blood and reduced volume of concentrated urine is excreted (hypertonic urine). Diuresis means the production of increased amount of urine, so anti diuresis means reduction of urine volume and hence the name antidiuretic hormone or ADH.

(c) Urine is also concentrated by the counter current system of the descending and ascending limbs of Henle’s loop. About 5% of the water from the filtrate is absorbed in this part.

(d) In response to low sodium ion concentration (or low blood pressure) another hormone, aldosterone is released by the adrenal cortex. It stimulates the kidney tubules to absorb sodium ions in exchange of potassium ions. This leads to reabsorption of water by osmosis. As a result of increased blood volume the blood pressure is increased. Similarly high sodium concentration will inhibit aldosterone release and as a result in would lead to lower sodium ion concentration in blood.