Question
A student is asked to trace the path of filtrate through a nephron, starting from the glomerulus. Arrange the following in the correct sequence and state the primary function of each:
Bowman’s capsule → PCT → Loop of Henle → DCT → Collecting duct
Also: Which part of the nephron is responsible for the concentration of urine, and what mechanism makes it possible?
(NEET 2024 pattern question — structure + function combined)
Solution — Step by Step
Blood enters the glomerulus under high pressure (because the afferent arteriole is wider than the efferent arteriole — this pressure difference is the driving force). Water, glucose, urea, salts, and small molecules are forced out into the Bowman’s capsule as glomerular filtrate. Large proteins and blood cells stay behind — they’re too big to pass.
About 70–80% of the filtrate is reabsorbed here. The PCT actively reabsorbs glucose, amino acids, and most Na⁺ ions. The epithelial cells of PCT have a brush border (microvilli) to increase surface area — this is a structural adaptation NEET loves to test. Urea and some water follow by osmosis.
The loop descends into the medulla and creates a concentration gradient in the surrounding tissue. The descending limb is permeable to water (water leaves passively). The ascending limb is impermeable to water but actively pumps out Na⁺ and Cl⁻. This countercurrent arrangement is what allows urine to be concentrated — without it, you’d lose massive amounts of water daily.
The DCT handles the fine adjustments under hormonal control. Aldosterone causes Na⁺ reabsorption here; ADH (vasopressin) increases water permeability. This is where blood pH and ionic balance get regulated — not a major reabsorption site, but critical for homeostasis.
The collecting duct passes through the high-concentration medullary gradient created by the loop. When ADH is present, water is drawn out osmotically, concentrating the urine. The final urine leaving the collecting duct is hypertonic compared to plasma. It drains into the renal pelvis → ureter → bladder.
Why This Works
The nephron is essentially a filtration + selective recovery machine. The body filters ~180 litres of plasma per day but excretes only 1–2 litres as urine. That 99% recovery happens because the tubule actively reclaims everything useful from the filtrate.
The concentration of urine specifically depends on the countercurrent multiplier (loop of Henle) and countercurrent exchanger (vasa recta blood vessels running alongside). The loop builds the gradient; the collecting duct exploits it under ADH control. Remove ADH and you get dilute, high-volume urine — that’s exactly what happens in diabetes insipidus.
For NEET: The loop of Henle is longest in desert mammals (like kangaroo rat) because they need maximum water conservation. This structural-function link is a classic MCQ. Juxtamedullary nephrons (near the medulla) have longer loops than cortical nephrons — and only about 15% of human nephrons are juxtamedullary.
Alternative Method — “Function First” Approach
Instead of memorising the sequence as anatomy, map each segment to its function category:
| Segment | Primary Function | Hormone |
|---|---|---|
| Bowman’s capsule | Ultrafiltration | None |
| PCT | Bulk reabsorption | None (always active) |
| Loop of Henle | Gradient creation | None |
| DCT | Ionic fine-tuning | Aldosterone |
| Collecting duct | Water conservation | ADH |
When an MCQ asks “where does aldosterone act?”, you immediately know it’s DCT — no need to recall anatomy, just the hormone-segment pairing. NEET 2023 and 2024 both had one direct question on this mapping.
Common Mistake
Students confuse PCT and DCT reabsorption. PCT handles the bulk of glucose and amino acid reabsorption (obligatory, no hormonal control). DCT is for fine hormonal regulation. A very common error is writing that “aldosterone acts on PCT” — it does not. Aldosterone acts specifically on the DCT and the collecting duct. If you write PCT in an answer, you lose the mark even if the rest is correct.
Second trap: saying the ascending limb of loop of Henle is permeable to water. It is not — it is impermeable to water, which is the whole point of building the osmotic gradient.