Urine formation
Process of Urine formation
The basic filtration units are the nephrons. Each kidney has a million Nephrons packed together.
Urine formation occurs in three steps:
1. Filtration
2. Reabsorption
3. Secretion
Filtration
· Filtration of blood takes place in the Glomerulus
· Termed as Glomerular filtration
· Blood reaches glomerulus by Afferent arteriole
· Increased pressure of blood in glomerulus causes 3-layer filtration of blood
· Blood is filtered through 3 layers:
· Endothelium of glomerular blood vessels
· Epithelium of Bowman’s capsule (Podocytes)
· Basement membrane
· Fine filtration of blood through the 3-layered membranes is termed as ‘Ultra filtration’
· 1100-1200 ml of blood is filtered by the kidneys per minute
· Amount of the filtrate formed by the kidneys per minute is called Glomerular Filtration Rate (GFR)
Reabsorption
· Almost 99% of the filtrate is reabsorbed by renal tubules
· Useful substances are reabsorbed by epithelial cells of tubules & sent back to blood
· Wastes are absorbed from blood & sent to the filtrate
· Water moves under osmotic pressure
· Role of different tubules
2. Proximal Convoluted Tubule (PCT):
· Large amount of reabsorption of nutrients, water & electrolytes occur here
· Presence of brush border epithelium increases surface area for absorption
3. Henle’s loop
· Minimum reabsorption occurs here
· Important role in osmoregulation
4. Distal Convoluted Tubule (DCT)
· Conditional reabsorption occurs here
5. Collecting duct
· Large amount of reabsorption of water occurs here
Secretion
1. Cells of the tubules secrete substances into the filtrate
2. Maintains ionic & acid-base balance of body fluids
3. Role of different tubules in Secretion:
- Proximal Convoluted Tubule (PCT)
· Selective secretion of H+, NH3, K+
- Henle’s loop
· Descending limb: Transport of water
· Ascending limb: Active/ passive transport of electrolytes
- Distal Convoluted Tubule (DCT)
· Selective secretion of H+, NH3, K+
- Collecting duct
· Selective secretion of H+, K+.