Question

In kidneys, urine is produced by three processes

• A. Dialysis, ultrafiltration and tubular secretion
• B. Ultrafiltration, dialysis and tubular secretion
• C. Ultrafiltration, tubular reabsorption and tubular secretion
• D. Tubular reabsorption and tubular secretion

Answer 1

The correct option is C Ultrafiltration, tubular reabsorption and tubular secretion

Urine formation in humans takes place by three processes - ultrafiltration, tubular reabsorption and secretion. It takes place in the Malpighian corpuscle, Henle’s loop PCT and DCT. Blood is filtered so finely through the membranes of bowman's capsule, that almost all the constituents of the plasma except the proteins pass onto the lumen of the Bowman’s capsule. Therefore, it is considered as a process of ultra filtration. A comparison of the volume of the filtrate formed per day (180 litres per day) with that of the urine released (1.5 litres), suggest that nearly 99 per cent of the filtrate has to be reabsorbed by the renal tubules. This process is called reabsorption. The tubular epithelial cells in different segments of nephron perform this either by active or passive mechanisms. During urine formation, the tubular cells secrete substances like $H^{+}$, $K^{+}$ and ammonia into the filtrate. Tubular secretion is also an important step in urine formation as it helps in the maintenance of ionic and acid-base balance of body fluids.