Describe the process of urine formation in kidneys.

Mechanism of Urine Formation: 
It has four components — glomerular filtration, selective reabsorption, tubular secretion and concentration.

1. Glomerular Filtration: Blood flows in 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 Bowman’s capsule. The product is called nephric or glomerular filtrate. Its volume is 125 ml/min (180 litres/day).
2. Reabsorption: Nephric filtrate is also called primary urine. It passes into proximal convoluted tubule. The peritubular capillaries around PCT reabsorb all the useful components of nephric filtrate, e.g, glucose, amino acids, vitamins C, calcium, potassium, sodium, chloride, bicarbonate and water (75%). Selective reabsorption also occurs in the region of distal convoluted tubule. The amount of water absorption depends upon amount of excess water present in the body and the amount of dissolved waste to be excreted.
3. Tubular Secretion (Augmentation): It occurs mostly in the distal convoluted tubule which is also surrounded by peritubular capillaries. Smaller amount of tubular secretion also takes place in the area of proximal convoluted tubule. Tubular secretion is active secretion of waste products by the blood capillaries into the urinary tubule. It ensures 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.
4. 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 tubes in the presence of hormone called antidiuretic hormone (ADH) or vasopressin. The hormone is secreted only when concentrated urine is to be passed out. It is not secreted when a person drinks a lot of water. 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).