Question

Describe how urine is formed in the nephron through filtration, reabsorption and secretion.

Answer 1

Urine formation takes place in the following manner-

1. Glomerular Filtration: This process occurs in the glomerular capillaries. The process of filtration leads to the formation of an ultrafiltrate. The blood gushes into these capillaries with high pressure and gets filtered across the thin capillary walls. Everything except the blood cells and proteins is pushed into the capsular space of the Bowman’s capsule to form the ultrafiltrate. The glomerular filtration rate (GFR) is 125ml/min or 180 Litres/day.

2. Tubular Reabsorption: During glomerular filtration, all substances except blood cells and proteins are pushed through the capillaries at high pressure. At the level of proximal convoluted tubule(PCT), some of the substances from the filtrate are reabsorbed. These include sodium chloride, potassium, glucose, amino acids, bicarbonate, and 75% of water.

Absorption of some substances is passive, some substances are actively transported while others are co-transported. The absorption depends upon the permeability of different parts of the nephron. The distal convoluted tubule shows selective absorption. The substances and water which is reabsorbed are taken up by the peritubular capillaries to be returned to the blood.

3. Tubular Secretion: The peritubular capillaries that help in transporting the reabsorbed substances into the bloodstream, also help in actively secreting substances like H+ ions, K+ ions. Whenever excess K+ is secreted into the filtrate, Na+ ions are actively reabsorbed to maintain the Na-K balance. Some drugs are not filtered in the glomerulus and so are actively secreted into the filtrate during the tubular secretion phase.