How is water balance maintained?

Fluid gain must be equal to fluid loss if the body composition of fluids is to remain constant.

The level of water in the plasma and tissue fluid is regulated by varying the amount of water taken into the body (drinking) and the amount reabsorbed from the filtrate in the nephron tubules.

Osmoreceptors in the hypothalamus detect the osmotic potential of the blood. The hypothalamus also controls the secretion of the anti-diuretic hormone (ADH) from the posterior pituitary.

Water is mostly reabsorbed from the filtrate in the distal convoluted tubule (DCT) and the collecting duct (CD). Some of the water moves naturally because of concentration gradients. Some water reabsorption is controlled by the level of ADH in the blood

How does osmotic pressure work?

Water levels in the body fluids change the osmotic pressure of the blood.

If there are high water concentration plasma levels, then there will be a lower osmotic pressure.

If there are low water concentration plasma levels, then there will be a higher osmotic pressure.