 |
| Summary |
|
| Excretion is the elimination of waste products from the body. The |
| |
| non - volatile products are mainly excreted in the urine. The urinary system consists of those organs which form, store and void urine. |
| |
| The nitrogenous wastes are mainly produced by catabolism of proteins. Animals are ammonotelic, ureotelic or uricotelic accordingly as they excrete ammonia, urea or uric acid as the principal nitrogeneous waste product. Higher invertebrates and vertebrates have developed specialised tissues for excretion. |
| |
| The mammalian urinary system consists of 2 kidneys, 2 ureters and a urinary bladder. The kidney contains many tubular nephrons which are located partly in the renal medulla. They form urine and drain it through kidney. |
| |
| Each nephron starts from a blind expanded end called the Bowman's capsule. Closely applied to a tuft of capillaries called the glomerulus. |
| |
| The Bowman's capsule is followed by a PCT, a V shaped Henle's loop, a DCT and a collecting tubule. Collecting tubules drain the urine into collecting ducts which open into ducts of Belline which drain the urine into renal pelvis. |
| |
| The human urine contains urea, water, other nitrogenous waste products sodium chloride, mineral salts, some non-nitrogenous organic substances. |
| |
| The urine is primarily formed by the filtration of a protein - free filtrate from the blood of glomerular capillaries into the Bowman's capsule. As the glomerular filtrate flows through the tubules, they reabsorb many substances from it. Some substances such as glucose, amino acids and Na+ are reabsorbed actively and totally. Water is absorbed passively by osmosis and different portions of tubules absorb different substances. Tubules also secrete various substances such as K+, uric acid, creatinine and ammonia into the urine. A combination of glomerular filtration, tubular reabsorption and tubular secretion forms the urine in nephrons. |
| |
| Kidneys also maintain the osmotic concentration of blood and thus have a role in osmoregulation. This is largely due to counter - current mechanisms working in the kidney and the action of the ADH hormone in promoting water reabsorbtion from the tubules. |
| |
| Renal failure is treated with haemodialysis or kidney transplant. |
| |
