When we compare volume of the filtrate formed per day with that of the urine released per day, we find that nearly 99 per cent of the filtrate has been reabsorbed by the renal tubules. This is called reabsorption. During urine formation, the tubular cells also secrete certain into the filtrate. Tubular secretion is crucial as it helps in the maintenance of ionic balance in the body fluids.
Proximal Convoluted Tubule has simple cuboidal epithelium that increases the surface area for reabsorption. Nearly all of the essential nutrients, and 70-80 per cent of electrolytes and water are reabsorbed by this segment. It also maintains the pH and ionic balance of the body fluids by selective secretion.
Henle’s Loop: The process of reabsorption is very little in its ascending limb. The descending limb is permeable to water but almost impermeable to electrolytes. This concentrates the filtrate or urine.
Distal Convoluted Tubule is where conditional reabsorption of sodium ions and water takes place. It is also responsible for reabsorption of bicarbonate ions and selectively secreting hydrogen and potassium ion to maintain the ionic balance and pH.
Collecting Duct: From this long duct, large amounts of water could be reabsorbed to produce concentrated urine. It plays a role in the maintenance of pH and ionic balance of blood by the selective secretion of ions as well as maintain osmolarity.
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