There are three major process involve -
(1) Glomerular filtration (ultrafiltration)
(2) Tubular reabsorption (selective reabsorption)
(3) Tubular secretion (augmentation)
(1) Glomerular filtration :-
- It is a non selective process by which blood in glomerular capillary passes through the filtration bed and reaches the capsular space now called glomerular filtrate / ultrafiltrate.
- All the blood get filters through the process except blood cells and plasma protein (albumin and globulin) because they have large diameter than the size of pores.
- So, glomerular filtrate consists of H2O, salt, glucose, a.a(amino acid), urea, uric acid, hippuric acid, creatinine, vit.,hormone etc.
Glomerular filtrate = Blood - [Blood cells + plasma protein]
Glomerular filtrate = plasma - proteins
Glomerular filtrate = deproteinized plasma
- Blood in glomerular capillary is exerted with some net pressure so that it can reach the capsular space.
• RBS ( Renal blood supply) is 1100-1200 ml/minute i.e. approximately 1/5th of Cardiac output.
• Only 1% of GFR forms urine i.e. about 1ml urine formed per minute.
Remaining 99% of GFR selectively gets reabsorbed.
(2) Selective Reabsorption :-
It is a selective process by which useful substances are taken back into the blood circulation so the remaining waste is now called urine.
(i) In PCT -
- Maximum reabsorption occur here, active as well as passive.
- About 70 to 80% water gets reabsorbed and this reabsorption in PCT is obligatory.
- About 70% Na+, 75% K+, 70% Cl- , some HCO3- , 100% glucose and 100% amino acids also get reabsorbed.
- Reabsorption of Na+, K+, glucose and amino acids is active but reabsorption of water Cl- and HCO3- is passive.
(ii) In Loop of Henle -
- Cells of descending limb are permeable for reabsorption of water and cells of ascending limb are permeable only for salts.
- When ascending limb reabsorb electrolyte like Na+ , K+ , Cl- etc. into medullary interstitial fluid osmolarity of fluid increase.
- To maintain this osmolarity descending limb causes reabsorption of water but this water is reabsorbed into blood.
- So, Loop of Henle shows facultative reabsorption.
Note :- Reabsorption of Na+ from thin segment of ascending limb occurs passively.
(iii) In DCT & collecting duct -
• Cells are themselves impermeable in nature so can become permeable only under influence of hormone.
• So, both these parts show facultative reabsorption.
• They show reabsorption of water calcium Ion, chlorine ion, sodium ion and some HCO3-
(I) ADH (Anti Diuretic Hormone) / Vasopressin :-
It is secreted by hypothalamus but release into blood by posterior pituitary.
It induces reabsorption of water in DCT and collecting duct (along with some salt).
(II) Aldosterone :-
It is secreted by adrenal cortex gland.
It induces reabsorption of salt in DCT and collecting duct (along with some water).
(3) Active secretion / Augmentation :-
• This process occur actively in PCT and DCT parts of Nephron.
• If the blood capillaries surrounding nephron still consist of uric acid, cretenine, ammonium ion or excess K+ is present, it is taken from blood into the filtrate
• Finally the term urine can now be used in collecting duct.