Q.18 Which is the correct sequence of events in urine formation?

1. Filteration, secretion and reabsortpion

2. Secretion, reabsorption and filteration

3. Filteration, reabsorption and secretion

4. Reabsorption, filteration and secretion

Urine Formation Sequence: Filtration, Reabsorption, Secretion

Urine formation in nephrons follows a precise sequence of glomerular filtration first, then tubular reabsorption and secretion. The correct answer is 3. Filtration, reabsorption and secretion.

Correct Answer

Glomerular filtration in Bowman’s capsule produces filtrate from blood plasma, followed by selective reabsorption of water/nutrients in proximal tubule and Loop of Henle, then secretion of wastes/ions in distal tubule and collecting duct. This sequence ensures waste removal while conserving essentials, yielding ~1-2 liters daily urine.

Option Explanations

1. Filtration, secretion and reabsorption (Incorrect)

Secretion cannot precede reabsorption; initial bulk reabsorption (65-70% water/solutes) occurs before fine-tuned secretion of H+/K+ in later tubules.

2. Secretion, reabsorption and filtration (Incorrect)

Filtration initiates the process at glomerulus; secretion/reabsorption occur downstream in renal tubules, not preceding blood filtering.

3. Filtration, reabsorption and secretion (Correct)

Standard nephron physiology: filtration (GFR ~125 ml/min), obligatory reabsorption (proximal), then conditional reabsorption/secretion (distal/collecting duct under hormonal control).

4. Reabsorption, filtration and secretion (Incorrect)

Reabsorption requires prior filtrate from filtration; cannot precede glomerular pressure-driven ultrafiltration.