Question

Describe the reabsorption of filtered sodium and water in the major tubular segments.

Answer 1

Tubular reabsorption is the process by which water and other substances are transported from renal tubules back to the blood. When the glomerular filtrate flows through the tubular portion of nephron, both quantitative and qualitative changes occur.

Large quantity of water (more than 99%), electrolytes and other substances are reabsorbed by the tubular epithelial cells. The reabsorbed substances move into the interstitial fluid of renal medulla. And, from here, the substances move into the blood in peritubular capillaries. Since the substances are taken back into the blood from the glomerular filtrate, the entire process is called tubular reabsorption.

•SITE OF REABSORPTION- Reabsorption of the substances occurs in almost all the segments of tubular portion of nephron.

1).Substances reabsorbed from Proximal Convoluted Tubule - About 7/8 of the filtrate (about 88%) is reabsorbed in proximal convoluted tubule. The brush border of epithelial cells in proximal convoluted tubule increases the surface area and facilitates the reabsorption. Substances reabsorbed from proximal convoluted tubule are glucose, amino acids, sodium, potassium, calcium, bicarbonates, chlorides, phosphates, urea, uric acid and water.

2).Substances reabsorbed from Loop of Henle-Substances reabsorbed from loop of Henle are sodium and chloride.

3).Substances reabsorbed from Distal convoluted tubule- Sodium, calcium, bicarbonate and water are reabsorbed from distal convoluted tubule.

**Reabsorption of Sodium-From the glomerular filtrate, 99% of sodium is reabsorbed. Two thirds of sodium is reabsorbed in proximal convoluted tubule and remaining one third in other segments (except descending limb) and collecting duct.

Sodium reabsorption occurs in three steps: 1.Transport from lumen of renal tubules into the tubular epithelial cells 2.Transport from tubular cells into the interstitial fluid 3.Transport from interstitial fluid to the blood.

**Reabsorption of water-  Reabsorption of water occurs from proximal and distal convoluted tubules and in collecting duct.

•Reabsorption of water from proximal convoluted tubule – {obligatory water reabsorption} = Obligatory reabsorption is the type of water reabsorption in proximal convoluted tubule, which is secondary (obligatory) to sodium reabsorption. When sodium is reabsorbed from the tubule, the osmotic pressure decreases. It causes osmosis of water from renal tubule.

•Reabsorption of water from distal convoluted tubule and collecting duct – {facultative water reabsorption} = =Facultative reabsorption is the type of water reabsorption in distal convoluted tubule and collecting duct that occurs by the activity of antidiuretic hormone (ADH). Normally, the distal convoluted tubule and the collecting duct are not permeable to water. But in the presence of ADH, these segments become permeable to water, so it is reabsorbed.