Urine formation process includes:
- Glomerular filtration
- Selective reabsorption
- Active secretion
- Glomerular filtration/ultrafiltration :
- Filtration of blood under pressure is called ultrafiltration
- Takes place in glomerulus
- Hydrostatic pressure of blood in glomerulus is 60 mm Hg which is opposed by osmotic pressure of plasma protein (30 mm Hg)
- Similarly, HP of glomerular filtrate is 20 mm Hg. Thus, net filtration pressure is 10 mm hg which cause filtration of blood and primary urine is formed
- Primary urine or filtrate contains some useful and some harmful substances.
- Selective reabsorption
- Absorption of useful substance from primary urine into blood is called selective reabsorbtion
- This takes place in PCT, DCT, Loop of henle, collecting tube
Proximal convoluted tubule(PCT)
- Most of the useful substance like glucose ,amino acids , vitamins, etc are absorbed from filtrate
- Some harmful substances like urea, uric acid also get absorbed.
- Absorb water (90%)
- Minerals also get absorbed.
- Isotonic filtrates enter descending loop of henle.
Descending loop of henle (DLH)
- Meant foe conservation of water
- Impermeable to salt
- Only water is reabsorbed.
- Hypertonic filtrates enter ascending loop of henle.
Ascending loop of henle (ALH)
- Permeable to salts only
- Thus more salt are transported to medullar region and some may enter descending limb so there is cyclic movement of salts in loop of henle.
- Hypotonic urine enters DCT.
DCT ( distal convoluted tubule)
- Permeability is determined by ADH hormone
- Wall is impermeable to salt and water.
Collecting tubule
- Large amount of water is reabsorbed from filtrate into blood making the urine hypertonic.
- Active secretion
- Secretion of harmful substance from blood into filtrate is called active secretion
- Nutrients and water are reabsorbed into blood and waste ions, urea, and uric acids, get secreted into filtrate from blood.
#urine is yellowish in color. Color of urine is determined by urochrome formed in liver.