Question

Briefly state the mechanism of urine formation in human kidney.

Answer 1

Mechanism of urine formation:

It has four steps. 1. Ultra filtration 2. Reabsorption 3. Secretion 4. Concentration of urine

1. Ultra filtration : As the blood passes through the afferent arteriole it gets ultra filtrated under pressure. All small volume solutes and water are filtered out and enter the Bowman's capsule. This product is called glomerular filtrate.

2. Reabsorption : Glomerular filtrate passes through the pct which is surrounded by peritubular capillaries. The latter reabsorb all the useful components of glomerular filtrate like glucose, amino acids, Na+, cl- , bicarbonate, water(75%). Selective reabsorption also occurs in the region of dct.

3. Secretion : It occurs in dct mostly and a little in pct. It is the process by which waste materials are absorbed from peritubular capillaries and secreted into the lumen of the nephron. Urea, uric acid, extra salts, k+, h+ ate secreted into urine to maintain the pH.

4. Concentration of urine : 75% of water is reabsorbed from glomerular filtrate in pct. About 10% is reabsorbed by osmosis in the area of loop of Henley. Anti diuretic hormone helps in reabsorption of water in dct. In absence of adh reabsorption doesn't take place in dct. Deficiency of ash causes excessive, repeated dilute urination. This process of passage of urine is called micturition.