Question

Write short notes on selective reabsorption.

Answer 1

Hint: Excretion is removal of nitrogenous wastes from the body in the form of urine. The nitrogenous wastes along with useful substances reach the kidney. The body cannot remove the wastes at the expense of water and electrolytes which are useful for the body. This process makes sure that the useful substances are back into the blood and only the waste is excreted.

Complete Answer:
Urine formation takes place in three steps. Ultrafiltration, Selective reabsorption and Tubular secretion. The urine formation takes place in the different parts of the nephron.

Ultrafiltration: It is the first step that takes place in the bowman's capsule. The blood gets filtered by the glomerulus and is collected in the Bowman’s capsule. All the substances except proteins are filtered in the capsule. Thus the blood is filtered very finely and hence the process is Ultrafiltration
Selective Reabsorption/ Tubular reabsorption: The filtrate that is formed per day is 180 litres/day. However, the urine released is around 1.5 litres/day. This means around 99% of the filtrate is reabsorbed by the tubules. This is an important step because the filtrate contains many useful substances that need to be sent to the blood so that those substances are used by the body. The filtrate was formed by filtration of the blood and this step again passes the substances back to the blood. Hence it is Reabsorption. Different parts of tubules bring about reabsorption of different substances and thus it is Selective Reabsorption.
Tubular Secretion: The phenomenon of secretion of metabolic wastes by the tubular cells into the filtrate is known as Tubular secretion. It includes secretion of potassium, hydrogen, ammonia and some wastes like hippuric acid etc.
Diagram showing substances that are reabsorbed and secreted by the Renal tubule

  Substances Reabsorbed by the renal tubules
- PCT reabsorbs nearly all the nutrients and 70-80% of electrolytes and water.
- The descending limb of loop of Henle is permeable to water while ascending limb is permeable to electrolytes
- Conditioned reabsorption of Sodium and water takes place through DCT. It also reabsorbs bicarbonate ions.
- Large amounts of water is also reabsorbed by the collecting ducts

Note: The main advantage of this process is that the majority of substances are reabsorbed and are back into the blood. Water too is reabsorbed and thus we urinate only 1.5 litres per day.