The Kidney performs regulatory functions by different processes filtration, reabsorption, tubular secretions, and countercurrent exchange. These processes are carried on the level of each nephron.
- Ultra Filtration:
In the first stage, non-selective ultra filtration (filtration at molecular level) of blood occurs while passing through the glomerulus ultra filtration. Both the capillary walls of glomerulus and the walls of Bowmen’s capsule are sieve-like in nature. Blood pressure in the glomerulus is high. Thus, a hydrostatic pressure is generated which forces the smaller molecules of blood from glomerulus into the Bowman’s capsule.
Active reabsorption of the glomerular filtrate occurs in the proximal convoluted tubule. Therefore, a large amount of filtrate is reabsorbed and returned to circulation. Here glucose amino acid, water, and salts are reabsorbed.
Reabsorption of water also takes place when the filtrate from proximal convoluted tubule passes down into the descending limb of the loop of Henle. Unlike the descending limb, the ascending limb is impermeable to water. Here reabsorption of sodium and chloride ions takes place through active transport. Sodium and chloride ions move out of filtrate and enter into the interstitial fluid where they are taken up by blood. The filtrate being almost urine now is passed into the collecting duct where the concentration of water in the filtrate is finally adjusted.
- Tubular Secretion:
At the level of both proximal as well as distal convoluted tubules, some poisonous substances along with nitrogenous substances such as ammonia, urea, uric acid, creatinine are secreted from the blood directly into the filtrate by active transport. Nitrogenous wastes added in this way are actually those molecules, which escaped the filtration in gomeruli. Hydrogen and Ammonium ions secretion from the interstitial fluid into the distal convoluted tubule plays important role in the maintenance of the acid-based percentage in the blood.
- Countercurrent Exchange:
There are two countercurrent exchange systems operating in the medulla, which are involved in the exchange of solutes and water. One of the countercurrent is termed as countercurrent multiplier which is situated in the loop of Henle. This countercurrent multiplier is involved in developing concentration of ions across the medulla. While passage of isotonic filtrate goes through the descending limb, the reabsorption of water turns the filtrate gradually hypertonic to the tissue fluid. Subsequently, in the the ascending limb, the sodium and chloride ions are actively removed from the filtrate, thus, reducing the hepertonicity and turning it into hypotonic. Consequently, urine that passes outside the body is hypertonic to body fluid.
The other countercurrent system is composed of vasa recta, the blood vessels that run parallel to the loop of Henle. Only about 10% of the blood that flows through the kidney passes through the vasa recta at a very slow speed. The blood supply to vasa recta provides sufficient nourishment and oxygen to the cells of medulla and also takes away the water absorbed from the filtrate, without affecting the concentration gradient of medulla.