Explain the tubular reabsorption process of urine formation.

Explanation

This is the second step in process of urine formation Tubular Reabsorption:
1. This involves movement of the filtrate back into the circulation.
2. The volume of filtrate formed per day is around 170-180 L and the urine released is around 1.5 L per day, i.e., nearly 99% of the glomerular filtrate that has to be reabsorbed by the renal tubules as it contains certain substances needed by the body. This process is called selective reabsorption.
3. Reabsorption takes place by the tubular epithelial cells in different segments of the nephron either by active transport or passive transport, diffusion and osmosis.
4. Proximal convoluted Tubule (PCT)- Glucose. lactate, amino acids, Na+ and water in the filtrate is reabsorbed in the PCT. Sodium is reabsorbed by active transport through sodium- potassium (Na* - K*) pump in the PCT. Small amounts of urea and uric acid are also reabsorbed.
5. Descending limb of Henle's loop is permeable to water due the presence of aquaporins, but not permeable to salts. Water is lost in the descending limb, hence Na+ and Cigels concentrated in the filtrate.
6. Ascending limb of Henle's loop is impermeable to water but permeable to solutes such as Na, Cland K.
7. The distal convoluted tubule recovers water and secretes potassium into the tubule. Na+, Cl and water remains in the filtrate of the DCT.
8. Most of the reabsorption from this point is dependent on the body's need and is regulated by hormones. Reabsorption of bicarbonate (HCO,) takes place to regulate the blood pH. Homeostasis of K+ and Nat in the blood is also regulated in this region.
Collecting duct is permeable to water, secretes K* (potassium ions are actively transported into the tubule) and reabsorbs Na* to produce concentrated urine. The change in permeability to water is due to the presence of number of water-permeable channels called aquaporins.