Loop of Henle
After reabsorption in the proximal convoluted tubule, only 20% of the glomerular filtrate is left to enter the descending loop of Henle with 15% of water, 5% urea plus uric acid and some mineral salts.
Along the U-shaped loop of Henle, sodium chloride is reabsorbed back into the blood stream through active transport under hormonal influence.
Low concentration of sodium chloride in the blood stimulates the adrenal glands to secrete the hormone aldosterone.
Adrenal glands are located on upper surface of the kidneys.
Aldosterone stimulates the cells lining the tubules to actively pump sodium and chloride ions from the filtrate back into the blood stream across the walls of the capillaries.
High concentration of sodium chloride in the blood reduces the secretion of aldosterone, and no more pumping of sodium and chloride occurs.
The high concentration of sodium chloride in the blood increases the blood osmotic pressure which in turn causes reabsorption of water from the filtrate into the blood through osmosis to balance the osmotic pressure in blood.
Adaptations to enable efficient reabsorption;
- Cells lining the tubules have numerous microvilli to increase the surface area for reabsorption.
- Long to increase the surface area for reabsorption.
- Cells lining the tubules have numerous mitochondria to produce more energy for active reabsorption of materials back to the bloodstream.
- It is U-shaped to allow counter current flow system hence create steep concentration gradient for rapid reabsorption.
- It is highly vascularized to transport reabsorbed salts;
- Have thin-epithelium blood capillaries to reduce diffusion distance.