BiologyCBSE Class 11 · Excretory Products and their Elimination

Structure of the Nephron and the Three Steps of Urine Formation

Updated 17 October 2026 · Fact-checked

The nephron is the functional unit of the kidney: Malpighian body (glomerulus and Bowman's capsule), proximal convoluted tubule, Henle's loop, distal convoluted tubule and collecting duct. Urine forms by glomerular filtration (about 125 mL per minute, 180 litres per day), selective tubular reabsorption and tubular secretion.

Understand Nephron Structure and Urine Formation

Kidney. A pair of reddish-brown, bean-shaped organs between the levels of the last thoracic and third lumbar vertebra, 10 to 12 cm long, 5 to 7 cm wide, 2 to 3 cm thick, 120 to 170 g. The concave hilum gives passage to the ureter, blood vessels and nerves. Inside: cortex (outer) and medulla (inner), with medullary pyramids projecting into calyces and extensions of cortex (columns of Bertini) between them; renal pelvis with calyces.

Nephron (about a million per kidney). Parts:

Malpighian (renal) corpuscle: glomerulus (tuft of capillaries formed by the afferent arteriole; blood leaves by the efferent arteriole) enclosed by the cup-like Bowman's capsule.
Proximal convoluted tubule (PCT): lined with simple cuboidal brush-border epithelium (increases surface area).
Henle's loop: descending limb and ascending limb.
Distal convoluted tubule (DCT): simple cuboidal epithelium.
Collecting duct: several DCTs open into it; it runs to the renal pelvis through the medulla.
Peritubular capillaries around the tubules; the vasa recta (a minute vessel parallel to Henle's loop) is absent or reduced in cortical nephrons.
Cortical nephrons: short loop of Henle, mostly in the cortex; juxtamedullary nephrons: long loop extending deep into the medulla, with vasa recta.

Urine formation: three processes.
1. Glomerular filtration: in the glomerulus, blood is filtered through three layers: endothelium of glomerular blood vessels, the epithelium of Bowman's capsule (podocytes with filtration slits) and a basement membrane between them. All plasma constituents except proteins pass into the capsule (ultrafiltration). GFR ≈ 125 mL per minute = 180 litres per day. The juxtaglomerular apparatus (JGA) regulates GFR (see regulation).
2. Selective reabsorption: about 99 per cent of the filtrate is reabsorbed. PCT reabsorbs nearly all essential nutrients, and 70-80 per cent of electrolytes and water. Henle's loop: minimum reabsorption but plays a major role in maintaining the high osmolarity of the medullary interstitial fluid. DCT: conditional reabsorption of Na⁺ and water, and reabsorption of HCO₃⁻. Collecting duct: large amounts of water reabsorbed to produce concentrated urine.
3. Tubular secretion: tubular cells secrete H⁺, K⁺ and ammonia into the filtrate; important for maintaining the ionic and acid-base balance of body fluids (PCT and DCT).

Urine. Volume about 1 to 1.5 litres per day; pale yellow, slightly acidic (pH 6.0); odour due to urea and other waste.

Draw and label: a nephron: glomerulus, Bowman's capsule, PCT, loop of Henle, DCT, collecting duct, afferent and efferent arterioles.

Nephron Structure and Urine Formation: key facts

GFR: $125 m L p er min = 180 L p er d a y$; Filtrate
Reabsorbed share: $(F i l t r a t e - u r in e) / f i l t r a t e x 100∣ ab o u t 99$; Per day
Nephron: Malpighian body
PCT
Henle's loop
DCT
collecting duct; In order
Steps: $F i l t r a t i o n ∣ r e ab sor pt i o n ∣ secr e t i o n$; Urine formation

How to answer Nephron Structure and Urine Formation questions

Questions give filtrate and urine volumes or ask for the site of a process.

1Name the step: filtration, reabsorption or secretion.
2Compute the amount reabsorbed by subtracting urine from filtrate.
3Convert per minute to per day by multiplying by 60 x 24.
4Match the segment: PCT bulk reabsorption; loop concentration gradient; DCT conditional; collecting duct water.
5Include units.

Nephron Structure and Urine Formation in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Water and urine volume

Suppose a person produces 1.5 litres of urine per day while the kidneys filter 180 litres.

What fraction of the filtrate is reabsorbed?

Try it first, then show the solutionSolution

Reabsorbed = 180 - 1.5 = 178.5 L.
Fraction = 178.5 / 180 x 100.
= 99.17 per cent.

Answer: About 99.2 per cent is reabsorbed.

Memory hook: Most of it returns.

Tricks to remember Nephron Structure and Urine Formation

Short memory aids. Say each one aloud once, then use it on a question the same day.

Fil-Rea-Sec
Filtration, Reabsorption, Secretion.
Use it when: Order
PCT = bulk
PCT reabsorbs most.
Use it when: Reabsorption
125 to 180
125 mL per min is 180 L per day.
Use it when: GFR

Nephron Structure and Urine Formation: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: GFR · 2 marks · Very short answerBoard level

Convert a GFR of 125 mL per minute into litres per day. Show working.

Try it first, then show the solutionSolution

125 x 60 x 24 = 180,000 mL (1 mark).
= 180 litres per day (1 mark).

Answer: 180 L per day.

Example 2: Three layers · 3 marks · Short answerBoard level

Name the three layers of the filtration barrier and what cannot pass.

Try it first, then show the solutionSolution

Endothelium of glomerular capillaries (1 mark).
Epithelium of Bowman's capsule with podocytes and the basement membrane (1 mark).
Proteins and blood cells are retained; the rest of plasma filters (1 mark).

Answer: Three layers; proteins stay.

Example 3: Reabsorption calculation · 3 marks · Short answerHarder board level

If 180 L is filtered and 1.5 L of urine is excreted, find the volume and per cent reabsorbed. Where is most water reabsorbed first?

Try it first, then show the solutionSolution

Reabsorbed = 180 - 1.5 = 178.5 L (1 mark).
Per cent = 178.5 / 180 x 100 = 99.17 per cent (1 mark).
Bulk (70-80 per cent) is reabsorbed in the PCT (1 mark).

Answer: 178.5 L; 99.2 per cent; PCT.

Example 4: Segments · 4 marks · Short answerHarder board level

State the main function in each segment: PCT, Henle's loop, DCT, collecting duct.

Try it first, then show the solutionSolution

PCT: reabsorption of nutrients and most water and electrolytes (1 mark).
Henle's loop: maintains medullary gradient (1 mark).
DCT: conditional reabsorption of Na+, water; HCO3- (1 mark).
Collecting duct: water reabsorption for concentrated urine (1 mark).

Answer: Bulk, gradient, conditional, concentration.

Example 5: Blocked efferent arteriole · 5 marks · Long answerHOTS / competency

Suppose the efferent arteriole of a nephron narrows. Predict the glomerular pressure and filtration, and explain what happens to the urine volume if the whole kidney were affected for a long time.

Try it first, then show the solutionSolution

Narrowing raises resistance to outflow and glomerular pressure (1 mark).
Filtration rises initially (1 mark).
Prolonged high pressure damages the filter, letting proteins leak (1 mark).
Damaged nephrons filter less and GFR falls over time (1 mark).
Urine volume and waste removal decrease, causing waste build-up (1 mark).

Answer: Pressure up, then damage, then fall in filtration.

Where students lose marks in Nephron Structure and Urine Formation

The slips examiners see most often, and what to write instead.

Mistake: Reabsorption in glomerulus
Steps mixed.
Wrong working
Reabsorption takes place in the glomerulus.
Right working
Filtration occurs there; reabsorption in tubules.
Do this: Filtration at glomerulus.
Usually costs: 1 mark.
Mistake: 180 mL per day
Units.
Wrong working
GFR is 180 mL per day.
Right working
180 litres per day.
Do this: Litres.
Usually costs: 1 mark.
Mistake: Proteins in filtrate
Selectivity.
Wrong working
Proteins appear in the filtrate.
Right working
Proteins stay in blood.
Do this: No proteins.
Usually costs: 1 mark.

Exam tips

Show unit conversions.
Name each segment and process.
Draw a labelled nephron.
Quote the 99 per cent reabsorption.

Practice questions on Nephron Structure and Urine Formation

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
Most of the reabsorption of water and nutrients takes place in
(a) loop of Henle (b) PCT (c) DCT (d) collecting duct
Hint
Bulk.
Answer
(b).
Q2 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Tubular secretion helps maintain acid-base balance.
Reason (R): Tubular cells secrete H+ and K+ into the filtrate.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Ions.
Answer
(a).
Q3 · 2 marks · Very short answerBoard level
Differentiate cortical and juxtamedullary nephrons.
Hint
Loop length.
Answer
Cortical nephrons have a short loop of Henle and lie mainly in the cortex; juxtamedullary nephrons have a long loop extending deep into the medulla and are accompanied by vasa recta.
Q4 · 4 marks · Case-basedHarder board level
Read and answer.
Suppose filtrate is made at 100 mL per minute for 10 hours.
(a) Find volume filtered. (1)
(b) If 99 per cent is reabsorbed, find the urine volume. (1)
(c) Name the step of urine formation that causes the large change. (2)
Hint
100 x 600.
Answer
(a) 100 x 600 = 60,000 mL = 60 L. (b) 1 per cent of 60 L = 0.6 L. (c) Selective tubular reabsorption.
Q5 · 3 marks · Short answerHOTS / competency
Suppose a drug blocks the secretion of H+ in the tubules. Predict the effect on blood pH and explain.
Hint
Acid-base balance.
Answer
Less H+ would be removed in urine, so blood would become more acidic. The kidney normally helps regulate pH by secreting H+ and reabsorbing HCO3-.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.GFR?Show answer
125 mL per min.
2.Daily urine?Show answer
1 to 1.5 litres.
3.Brush border is in?Show answer
PCT.
4.What is the vasa recta?Show answer
Capillary parallel to Henle's loop.
5.Site of filtration?Show answer
Glomerulus.
6.pH of urine?Show answer
About 6.0.

Nephron Structure and Urine Formation: frequently asked questions

What is a nephron?

Functional unit of the kidney.

What is ultrafiltration?

Filtration of plasma without proteins.

What is the Malpighian body?

Glomerulus plus Bowman's capsule.

What is secretion?

Active addition of substances to filtrate.

How much urine is formed?

About 1 to 1.5 litres a day.