Function of Kidneys
Kidney performs two important functions. These are!
1- Urine Formation:
The main function of kidney is urine formation. Urine formation takes place in three steps. These are!
- Pressure Filtration: When blood enters the kidney via renal artery, it goes to afferent arteriole and then to the glomerulus. Here blood pressure is very high and so most of the water, salts, glucose and urea of blood is forced out of glomerulus capillaries. The material passes into the Bowman’s Capsule and now it is called glomerulus filtrate.
- Selective Re-absorption: In this step, 99% of glomerulus is reabsorbed in the blood capillaries surrounding the renal tubule via diffusion, osmosis and active transport. Some water and most of the glucose is reabsorbed in the first Convoluted tubule which Proximal Convoluted Tubule. Here salts are reabsorbed by active transport and then water follows by osmosis. In descending loop of Henle, water is reabsorbed while the ascending limb of of loop of Henle allows the re-absorption of salts. Water is again reabsorbed in the last portion of renal tubule called distal convoluted tubule.
- Tubular Secretion: This is the third step. In this step, different ions, creatinine, urea etc are secreted from blood into the filtrate in renal tubule. Its main purpose is to maintain the pH of Blood(7.35-7.45). So, urine is formed dilute.
Kidney plays important role in osmoregulation by regulating the water contents in the blood. When there is excess water in body fluids, kidneys form dilute urine. For this purpose, kidneys filter more water from glomerulus capillaries into Bowman’s Capsule.
When there is shortage of water in body fluids, kidney filter less water from glomerulus capillaries into the Bowman’s Capsule. So urine is formed concentrated.