Biology · Excretory Products and Their Elimination · NEET
Filtration happens first, at the glomerulus (Bowman's capsule), where blood is pushed through and a filtrate forms. Reabsorption happens next, along the tubule, where useful things (glucose, amino acids, Na+, most water) are taken back from the filtrate into the blood. Filtration removes almost everything from plasma; reabsorption returns nearly 99% of it. So filtration makes the filtrate, reabsorption saves what the body needs.
Glucose, amino acids and Na+ are reabsorbed by ACTIVE transport (they need energy). NCERT is clear on this. In contrast, nitrogenous wastes (like urea) are reabsorbed by PASSIVE transport, and water is also reabsorbed passively in the early parts of the nephron. This active vs passive split is a very common NEET question, so remember: nutrients and sodium = active; wastes and water = passive.
Nearly 99% of the filtrate is reabsorbed by the renal tubules. The kidneys form about 180 litres of filtrate per day but the urine released is only about 1.5 litres. Comparing these two numbers tells you that almost all of the filtrate must be taken back into the blood. This 99% figure is a favourite NEET number.
The proximal convoluted tubule (PCT) does the bulk of reabsorption. It reabsorbs nearly all the essential nutrients (glucose, amino acids) and about 70-80% of the electrolytes and water. The PCT is lined by simple cuboidal brush-border epithelium; the brush border (microvilli) increases the surface area for reabsorption. So if a question asks 'main site of reabsorption', the answer is PCT.
No. Reabsorption moves substances FROM the filtrate INTO the blood (taking back useful things). Tubular secretion is the opposite direction: the tubular cells push substances (H+, K+, ammonia) FROM the blood INTO the filtrate to keep blood pH balanced. Both happen in the tubule, but their direction is opposite. See the tubular secretion page for the next step.
The part of nephron involved in active reabsorption of sodium is
Cuboidal epithelium with brush border of microvilli is found in:
Match the following parts of a nephron with their function: (a) Descending limb of Henle's loop (b) Proximal convoluted tubule (c) Ascending limb of Henle's loop (d) Distal convoluted tubule; (i) Reabsorption of salts only (ii) Reabsorption of water only (iii) Conditional reabsorption of sodium ion and water (iv) Reabsorption of ion, water and organic nutrients.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is the process where the renal tubule takes back useful substances (glucose, amino acids, salts, water) from the filtrate and returns them to the blood, so they are not lost in urine. About 99% of the filtrate is reabsorbed this way.
Because the kidneys filter blood in bulk (180 litres/day) to remove wastes quickly, but the body cannot afford to lose that much water, glucose and salt. Reabsorption saves nearly all of it, leaving only about 1.5 litres as urine.
Glucose, amino acids and Na+ are reabsorbed by active transport. Nitrogenous wastes are reabsorbed passively, and water is reabsorbed passively in the initial segments of the nephron.
Tubular secretion. Here the tubular cells secrete H+, K+ and ammonia into the filtrate to balance blood pH and ions. It is the third main step of urine formation.
