Selective reabsorption :
- Out of 180 liters of filtrate formed every day 178.5 liters along with useful materials reabsorbed into the blood through peritubular capillaries leaving 1.5 liters excreted in the form of urine.
- The tubular epithelial cells of different segments of nephron perform these either active or passive mechanisms.
- Substance like glucose, amino acids Na+ absorbed actively.
- Nitrogenous wastes are absorbed by passive transport.
- Reabsorption of water also occurs passively in the initial segments of the nephron.
Tubular secretion :
- The tubular cells adds substances like H+ , K+ and ammonia to the filtrate from the peritubular capillaries.
- Tubular secretion maintains ionic and acid base balance of the body fluids.
FUNCTION OF THE TUBULES :
Proximal convoluted tubule (PCT) :
- PCT is lined by simple Cuboidal brush border epithelium which increases the surface area for absorption.
- All essential nutrients and 70-80% of the electrolytes and water are reabsorbed by this segment.
- PCT also maintain the pH and ionic balance of the body fluids by selective secretion of H+, K+ and ammonia into the filtrate and by absorption of HCO3-.
Henle’s Loop :
- This region plays important role in maintenance of high osmolarity of medullary interstitial fluid.
- The descending limb is permeable to water but impermeable to electrolytes. This concentrates the filtrates as it moves down.
- The ascending limb is permeable to electrolytes but impermeable to electrolytes. Therefore as the concentrated filtrate pass upward, it gets diluted due to active or passive transport of electrolytes to the medullary fluid.
Distal convoluted tubules :
- Selectable reabsorption of Na+ and water takes place in this segment.
- DCT also capable of reabsorption of HCO3- and selective secretion of H+, K+, and NH3 to maintain the pH and sodium-potassium level in blood.
Collecting duct :
- This duct extends from cortex to inner part of the medulla.
- Large amount of water could be reabsorbed from this region to produce concentrated urine.
- This segment allow small amount of urea into the medullary interstitium to keep up the osmolarity.
MECHANISM OF CONCENTRATION OF FILTRATE :
- Mammals have the ability to produce concentrated urine.
- The Henle’s loop and vasa recta plays significant role in concentrating urine.
- The flow of filtrate in two limb of Henle’s loop and blood flow in two limbs of vasa recta are in opposite direction hence form counter current.
- The proximity between the Henle’s loop and vasa recta, as well as the counter current in them help in maintain an increasing osmolarity towards the inner medullary interstitium, i.e. from 300 mOsmolL-1 in the cortex to about 1200 mOsmolL-1 in the inner medulla.
- The gradient is mainly due to NaCl and urea.
- The NaCl actively transported from the ascending limb of Henle’s loop is exchanged by the ascending portion of the vasa recta.
- NaCl is returned to the interstitium by the ascending portion of vasa recta.
- Small amount of urea enters the thin segment of ascending limb of Henle’s loop which is transported back to the interstitium by the collecting tubule.
- This above described transport of substances facilitated by the special arrangement of Henle’s loop and vasa recta is called the counter current mechanism.
- This mechanism helps to maintain a concentration gradient in the medullary interstitium, that promote easy passage of water from the collecting duct, leads to formation of concentrated urine.
