Excretion – Urine Formation: NEET MCQs with Answers

Explanation: ADH (vasopressin) promotes water reabsorption from the DCT and collecting duct, preventing diuresis and making urine concentrated (hypertonic), so the statement that it helps in water elimination and makes urine hypotonic is incorrect. Henle's loop concentrates urine, the glomerular filtrate is plasma minus proteins, and glomerular filtration is non-selective.

Explanation: Glucose is reabsorbed from the glomerular filtrate in the proximal convoluted tubule by active transport. Active transport involves the expenditure of energy, because the molecules are moved from a region of lower concentration to one of higher concentration.

Explanation: Net filtration pressure equals the glomerular capillary hydrostatic pressure minus the opposing colloid osmotic pressure of plasma and the capsular hydrostatic pressure, which works out to about 20 mm Hg. This net pressure drives fluid from the glomerulus into Bowman's capsule.

Explanation: Nearly 99% of the glomerular filtrate is reabsorbed by the renal tubules, which is why only about 1.5 litres of urine is produced from around 180 litres of filtrate daily. The other statements are wrong: the descending limb is permeable to water but impermeable to electrolytes; the DCT does absorb bicarbonate and secrete protons (or the reverse) to maintain acid–base balance; and the ascending limb is impermeable to water but permeable to electrolytes.

Explanation: Glomerular blood pressure of about 70 mm Hg drives fluid outward, and it is opposed by the colloid osmotic pressure of the plasma proteins and by the hydrostatic pressure of the filtrate already within the capsule. The balance leaves a net outward gradient of the order of 20 mm Hg.

Explanation: About 99% of the glomerular filtrate (≈180 L/day filtered, ≈1.5 L excreted) is reabsorbed by the renal tubules.

Explanation: With no protein intake, deamination falls, so less urea is formed and excreted. During prolonged starvation body protein is eventually catabolised, but among the options the expected NEET answer is reduced urea in urine.

Explanation: Cardiac output is heart rate multiplied by stroke volume, so 80 × 70 mL = 5600 mL per minute. One-fifth of 5600 mL is 1120 mL. This agrees with the usual figure of 1100 to 1200 mL of blood filtered by the kidneys each minute.

Explanation: A day has 1440 minutes, so the filtrate formed is 100 mL/min × 1440 min = 144,000 mL, or 144 L. If 99% is reabsorbed, only 1% remains as urine. One per cent of 144 L is 1.44 L, which fits the normal adult output of 1 to 1.5 L per day.

Explanation: Glomerular blood pressure forces almost all plasma constituents except proteins through the filtration membrane, so glucose, amino acids, salts and urea all enter the filtrate. This makes filtration the least selective step. The tubules then reclaim substances selectively, taking back nearly all glucose and amino acids but only part of the urea.

Explanation: Glomerular filtration is the least selective step of urine formation, because it separates plasma components mainly by size. Almost everything except proteins passes into Bowman's capsule, useful solutes included. The tubules are selective, and substances such as glucose and amino acids are almost completely reabsorbed. This explains a solute that is present in the filtrate but absent from urine.