Explain the 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.
(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.