The right answer is regulation of blood pressure and water balance of cells.
The exchanges of water that take place through the membrane of the capillaries are linked:
* At hydrostatic pressure: this pressure is due to the forces exerted by a liquid on the structures that contain this liquid.
The capillary hydrostatic pressure being greater than the interstitial hydrostatic pressure (the difference is called transmural pressure), water passes from the blood compartment to the interstitial compartment through the capillary wall. Each day, about 20 L of water are thus filtered in humans.
* Oncotic pressure: this pressure, also called osmotic colloid pressure, is due to the presence of large molecules (especially proteins) that attract water.
The oncotic pressure of the plasma (25mmHg) being higher than that of the extracellular fluid (0mmHg) because of a higher protein content in the blood, water passes from the interstitial compartment to the blood compartment. Every day, 18 L of water are reabsorbed in humans.
