Mechanisms of urine formation

Formation of urine involves four stages i. Glomerular filtration, ii.Tubular reabsorption, iii. Tubular secretion, iv.Concentration of urine

i) Glomerular filtration

Blood flows from renal artery to glomerulus through afferent arteriole. Observe the fig-7 of glomerular filtration in nephron and try to answer the following questions. Which arteriole has more diameter, afferent or efferent? What are the substances that are filtered into the glomerular capsule?

ii) Tubular Re-absorption

Filtrate from glomerular is also called primary urine which almost equal to blood in chemical composition except the presence of blood cells. It passes into proximal convoluted tubule. Useful substances in primary urine are reabsorbed into peritubular net work. If you drink more water will you pass more urine? What are the substances reabsorbed into peritubular net work from proximal convoluted tubule (PCT)?

iii) Tubular secretion

After reabsorption in PCT region, the urine travels through the loop of Henle into DCT. Here some other wastes like extra salts ions of K+ Na+ Cl – and H+ secretes from peritubular capillaries in to DCT.
It occurs mostly in the distal convoluted tubule, which is also surrounded by peritubular capillaries. This maintains a proper concentration and pH of the urine. Smaller amount of tubular secretion also takes place in the area of proximal convoluted tubule. Observe tubular secretion in fig-7.
What are the substances that secretes into DCT?

iv) Formation of hypertonic Urine

75% of water content of nephric filtrate is reabsorbed in the region of proximal convoluted tubule. 10% of water passes out of filtrate through osmosis in the area of loop of Henle. Further concentration of urine takes place in the area of collecting tubes in the presence of hormone called