Question
Draw and label the structure of a nephron. Describe the process of urine formation, covering all three major steps.
Solution — Step by Step
The nephron has two main regions: the renal corpuscle (Bowman’s capsule + glomerulus) and the renal tubule (PCT → Loop of Henle → DCT → collecting duct).
Draw a cup-shaped Bowman’s capsule enclosing a knot of capillaries (glomerulus). From it, draw a coiled proximal convoluted tubule (PCT), then a hairpin-shaped Loop of Henle dipping into the medulla, followed by the distal convoluted tubule (DCT) opening into the collecting duct.
Key labels: Afferent arteriole, efferent arteriole, glomerulus, Bowman’s capsule, PCT, descending limb of LoH, ascending limb of LoH, DCT, collecting duct, peritubular capillaries.
Blood enters via the afferent arteriole (wider) and leaves via the efferent arteriole (narrower). This pressure difference creates high glomerular blood pressure (~75 mm Hg), forcing water, ions, glucose, urea, and amino acids out of blood into Bowman’s capsule.
Why does the efferent being narrower matter? It’s like pressing a garden hose nozzle — increases the upstream pressure, forcing filtrate out. The GFR (glomerular filtration rate) is ~125 mL/min, producing ~180 L of filtrate per day.
Blood cells and large proteins do not pass through — the basement membrane and podocytes act as a size filter.
As filtrate moves through the PCT, useful substances are actively transported back into the peritubular capillaries. Glucose, amino acids, ~70% of water, and most Na⁺ are reabsorbed here.
In the Loop of Henle, the descending limb is permeable to water (water leaves by osmosis into the hypertonic medulla) while the ascending limb is impermeable to water but actively pumps out Na⁺ and Cl⁻. This countercurrent mechanism builds a concentration gradient in the medulla — critical for concentrating urine later.
The DCT reabsorbs more Na⁺ and water under hormonal control (aldosterone and ADH).
This is the kidney’s “last cleanup” step. Substances like H⁺, K⁺, NH₃, and certain drugs are actively secreted from peritubular capillaries into the tubular fluid in the DCT and collecting duct.
This step is why the kidney can regulate blood pH — by secreting H⁺ when blood is too acidic. Final urine composition is determined here.
From the collecting duct, urine flows into the renal pelvis → ureter → urinary bladder → urethra. ADH (antidiuretic hormone) controls how much water the collecting duct reabsorbs — drink less water, more ADH, more concentrated urine.
Final urine is hypertonic to blood plasma — contains urea, creatinine, uric acid, salts, and water. ~1.5 L/day is produced from 180 L of filtrate. The rest (178.5 L) is reabsorbed.
Why This Works
The nephron is essentially a filter + selective pump system. The glomerulus doesn’t discriminate much — it dumps almost everything small into the filtrate. Then the tubule carefully takes back what the body needs. This “filter everything, reclaim what you want” strategy is far more efficient than trying to selectively filter from the start.
The Loop of Henle’s countercurrent multiplier is the reason mammals (and birds) can produce concentrated urine. Organisms without this loop — like amphibians — can’t concentrate urine and are therefore tied to water. This is why frogs can’t survive in dry environments the way humans can.
The hormonal control layer (ADH for water, aldosterone for Na⁺) means the kidney is not a fixed filter — it dynamically adjusts output based on body needs, which is why your urine is pale after drinking lots of water and dark yellow when you’re dehydrated.
Alternative Method — Remembering the Nephron for Diagrams
For NEET diagram questions, use the mnemonic “BC → PCT → LoH → DCT → CD” — Bowman’s Capsule, Proximal Convoluted Tubule, Loop of Henle, Distal Convoluted Tubule, Collecting Duct.
For 5-mark NEET diagram questions, examiners specifically check: (1) Bowman’s capsule with glomerulus inside, (2) the hairpin shape of Loop of Henle with descending and ascending limbs labeled separately, (3) the two types of cells in the collecting duct (principal and intercalated). These three details separate a 4-mark answer from a 5-mark one.
When describing urine formation in written answers, structure it as three headings: Filtration → Reabsorption → Secretion. Every mark scheme at CBSE and NEET follows this sequence.
Common Mistake
Most students write that the ascending limb of the Loop of Henle is permeable to water. It is not — it’s impermeable to water and only pumps out salt. The descending limb is permeable to water, not salt. Swapping these two kills the countercurrent mechanism logic entirely and is a classic 1-mark loss in NEET. Draw arrows: water leaving downward limb, salt leaving upward limb — never mix them up.