#biology# class12
Describe the structure of the urine formation and it's mechanism.
Answer:
Urine is the liquid waste product of the human body. It contains urea, uric acid, salts, water and other waste products that are the result of various metabolic processes occurring in the body.The structural and functional unit of the kidneys is called the nephrons. Millions of nephrons are involved in the process of urine formation.
-The formation process occurs in 3 steps or phases.
1)Glomerular Filtration
2)Tubular Reabsorption
3) Tubular Secretion
>Glomerular Filtration
This process occurs in the glomerular capillaries. The process of filtration leads to the formation of an ultrafiltrate. The blood enters into these capillaries with high pressure and gets filtered across the thin capillary walls of glomerulus. Everything except the blood cells and proteins are pushed into the capsular space of the Bowman’s capsule to form the ultrafiltrate. The glomerular filtration rate (GFR) is 125ml/min or 180 Litres/day.
>Tubular Reabsorption/ selective reabsorption
During glomerular filtration, all substances except blood cells and proteins are pushed through the capillaries at high pressure. At the level of the Proximal Convoluted Tubule(PCT), some of the substances from the filtrate are reabsorbed. These include sodium chloride, potassium, glucose, amino acids, bicarbonate, and 75% of water.
Absorption of some substances is passive, some substances are actively transported while others are co-transported. The absorption depends upon the permeability of different parts of the nephron. The distal convoluted tubule shows selective absorption. The substances and water which is reabsorbed are taken up by the peritubular capillaries to be returned to the blood.
>Tubular Secretion
In the renal tubule, with reabsorption, secretion also occurs. Secretion of harmful toxic materials like ammonia, creatinine, urea, hippuric acid, drugs, etc., is actively secreted from the DCT. Mostly, secretion occurs in DCT. Some secretions also occur in the loop of Henle and DCT. Urea is secreted in the loop of Henle, while in this DCT, potassium, ammonia is reabsorbed.
Thus mechanism of urine formation is completed in 3 steps.
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