Chapter : 4. Excretion
Mechanism Of Urine Formation
(iii) Secretion (Augmentation) . It occurs mostly in the distal convoluted tubule which is also surrounded by peritubular capillaries. Smaller amount of tubular secretion also takes palce in the area of proximal convoluted tubule. Tubular secretion is active secretion of waste products by the blood capillaries. It cause removal of all the waste products from blood, viz. urea, uric acid, creatinine. Extra salts, K+ and H+ are also secreted into urinary tubule to maintain a proper concentration and pH of the urine.
(iv) Concentration of the Urine. 75% of water content of nephric filtrate is reabsorbed in the region of proximal convoluted tubule. Some 10% of water passes out of the filtrate through osmosis in the area of loop of Henle. It is because loops of Henle are immersed in hyper-osmotic interstitial fluid. Further concentration takes place in the area of collecting tubule in the presence of hormone called antidiuretic hormone (ADH) or vasopressin. Absence of Antidiuretic hormone produces a dilute urine. Hormone action, therefore, maintains osmotic concentration of body fluids. Deficiency of ADH causes excessive, repeated dilute urination (diabetes insipidus).
The process of passes urine in called Micturition.
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