Excretion – Function of Tubules: NEET MCQs with Answers

Explanation: In the PCT, HCO₃⁻, NaCl and H₂O are reabsorbed, while H⁺ and NH₃ are secreted into the filtrate. In the DCT, conditional reabsorption of NaCl and H₂O takes place, HCO₃⁻ is reabsorbed, and K⁺ and H⁺ are secreted. The correct diagram shows exactly these movements. The wrong diagrams show H₂O moving into the filtrate in the PCT, NaCl moving into the filtrate in the DCT, or NH₃ reabsorbed and H₂O secreted in the DCT.

Explanation: The ascending limb of Henle's loop is impermeable to water and only transports electrolytes, which dilutes the filtrate. The descending limb and PCT are permeable to water, and the DCT conditionally reabsorbs water.

Explanation: Both statements are true: the ascending limb of Henle's loop is impermeable to water but transports electrolytes actively or passively. Because electrolytes move out into the medullary fluid while water cannot follow, the filtrate becomes diluted as it ascends, so R correctly explains A.

Explanation: According to NCERT, the ascending limb of Henle's loop is impermeable to water but allows transport of electrolytes actively or passively, so the filtrate becomes dilute as it moves up. The descending limb is the reverse – permeable to water and almost impermeable to electrolytes – so the other options are wrong.

Explanation: The main function of Henle's loop is to absorb water from the tubular lumen, which concentrates the urine. If the loop of Henle were absent, this concentrating mechanism would be lost and the urine would become more dilute.

Explanation: From Bowman's capsule the glomerular filtrate enters the proximal convoluted tubule, which is surrounded by a network of peritubular capillaries and is the seat of reabsorption. About 75% of the glomerular filtrate — including glucose, amino acids, vitamins, sodium, potassium, chlorides, phosphates, bicarbonates, most of the water and some urea — is reabsorbed in the PCT before the filtrate reaches the loop of Henle.

Explanation: If the proximal convoluted tubule were removed, the kidney would malfunction. In the absence of the reabsorption normally carried out there, far more water and solute would remain in the filtrate and the urine would be much more dilute.

Explanation: The proximal convoluted tubule is the site of active reabsorption of sodium. The majority — about 70% — of the filtered sodium is reabsorbed here, into the cytosol of the epithelial cells of the nephron.

Explanation: The ascending limb of the loop of Henle is impermeable to water but permeable to K⁺, Cl⁻ and Na⁺ and partially permeable to urea. Because sodium, potassium, calcium, magnesium and chloride are reabsorbed here, the filtrate becomes hypotonic. The descending limb is the opposite — highly permeable to water but much less so to urea and ions.

Explanation: The descending limb of Henle's loop is permeable to water only (1-ii). The proximal convoluted tubule reabsorbs ions, water and organic nutrients (2-iv). The ascending limb reabsorbs salts only, being impermeable to water (3-i). The distal convoluted tubule carries out the conditional reabsorption of sodium ions and water (4-iii).

Explanation: Both statements are false. The descending limb of the loop of Henle is permeable to water but almost impermeable to electrolytes — exactly the reverse of what Statement I says. And the PCT is lined by simple cuboidal brush-border epithelium, not columnar, although it is true that this increases the surface area for reabsorption.

Explanation: If tubular reabsorption of water failed, huge volumes of dilute urine would be lost and the blood would become concentrated. Water would then be drawn osmotically out of the tissues, so the cells would shrink and the body would become severely dehydrated.

Explanation: About 180 litres of filtrate are formed daily but only about 1.5 litres of urine are released, so nearly 99 per cent is reabsorbed. The descending limb of Henle's loop is permeable to water but almost impermeable to electrolytes, while the ascending limb is the reverse, and the DCT can reabsorb bicarbonate.

Explanation: Glucose is entirely reclaimed in the proximal tubule, so that normal urine contains none; its appearance signals either hyperglycaemia beyond the renal threshold or tubular damage. Salts are reabsorbed selectively according to need, while urea and uric acid are excreted.

Explanation: Glucose is reabsorbed against its concentration gradient by a sodium-linked cotransporter in the proximal tubule, the energy coming indirectly from the sodium-potassium pump. Because the carriers are limited in number, glucose appears in the urine once the blood level exceeds the renal threshold.