Why does water move back into the capillary?

The movement of water back into the capillary is mainly influenced by the process known as reabsorption. Reabsorption is the selective process by which essential substances like water, glucose, amino acids, and ions, are reclaimed from the filtrate formed in the renal corpuscle and returned to the bloodstream. The following mechanisms contribute to the reabsorption of water back into the capillary:

- Osmosis: Osmosis is the movement of solvent (water) from a region of lower solute concentration to a region of higher solute concentration through a semipermeable membrane. The peritubular capillaries surrounding the renal tubules have a higher concentration of solutes compared to the filtrate inside the tubules. Therefore, water moves out of the tubules by osmosis into the capillaries, following the solute concentration gradient.

- Active Transport of Sodium: The active transport of sodium ions from the filtrate into the peritubular capillaries creates an osmotic gradient that facilitates water reabsorption. The sodium-potassium pump (Na⁺/K⁺ ATPase) located on the basolateral side (facing away from the lumen) of proximal tubule cells actively pumps sodium ions out of the tubule, into the interstitial fluid, and ultimately into the capillaries. The high sodium concentration in the interstitial fluid draws water out of the proximal tubules through osmosis.

- Hydrostatic Pressure: Hydrostatic pressure, which is the force exerted by a column of fluid, also plays a role in water reabsorption. In the glomerulus, where blood pressure is relatively high, the hydrostatic pressure forces fluid out of the glomerular capillaries into the renal tubule, forming the filtrate. As the filtrate moves along the tubules, the hydrostatic pressure decreases, promoting the reabsorption of water back into the capillaries.

- Loop of Henle: The loop of Henle is a specialized region of the renal tubule involved in the reabsorption of water. The descending limb of the loop of Henle is permeable to water but almost impermeable to sodium ions, allowing water to move out by osmosis. The ascending limb is impermeable to water but actively transports sodium ions out of the tubule into the interstitial fluid. This creates a high osmotic pressure in the medulla, which draws water out of the collecting duct and into the surrounding capillaries.

The combined effect of osmosis, active transport of sodium, hydrostatic pressure, and the loop of Henle mechanism ensures that most of the water filtered in the glomerulus is reabsorbed back into the capillaries, conserving water and maintaining the body's fluid balance and electrolyte concentrations.