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  Biology (NTSE/Olympiad)  

4. Excretion

Mechanism Of Urine Formation

Mechanism Of Urine Formation
It has four steps - ultra filtration, selective reabsorption, secretion and concentration.

(i) Ultra Filtration. Blood flows inside glomerulus under pressure due to narrowness of efferent arteriole. As a result it undergoes pressure filtration or ultrafiltration. All small volume solutes (e.g., urea, uric acid, amino acids, hormones, glucose, ions, vitamins) and water are filtered out and enter the Bowmans' capsule. The product is called nephric or glomerular filtrate. Its volume is 125 ml/min (180 litres/day).
(ii) Reabsorption. Nephric filtrate is also called primary urine. It passes into proximal convoluted tubule. The same is surrounded by peritubular capillaries. The latter reabsorb all the useful components of nephric filtrate, e.g, glucose, amino acids, vitamins C, calcium, potassium, sodium, chloride, bicarbonate and water (75%). Selective absorption also occurs in the region of distal convoluted tubule.

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