Excretory System ICSE Class 10: Kidney, Nephron and Urine Formation

What Excretory System is about

Weightage: CISCE publishes no chapter or unit weightage for ICSE Biology. Excretory System is in the 3. Human Anatomy and Physiology (ii) part of the 2027 syllabus and can be asked in both Section A and Section B of the 80-mark paper.Source: CISCE ICSE Biology (Science Paper 3) syllabus, examination year 2027

Key concepts

Excretion and the excretory organs

Excretion: the removal of metabolic wastes, especially nitrogenous wastes such as urea, from the body.

Organ	What it excretes
Kidneys	urea, excess water and salts (as urine)
Lungs	carbon dioxide and water vapour
Skin	water, salts and a little urea (as sweat)
Liver	forms urea from excess amino acids; excretes bile pigments (from broken-down haemoglobin) through the intestine

The kidneys also regulate the water and salt balance of the blood (osmoregulation).

Worked example

Distinguish between excretion and egestion.

Try it first, then show the solutionSolution

1. Excretion removes metabolic wastes produced by cells, such as urea and CO₂.
2. Egestion removes undigested food (faeces), which was never part of the body's metabolism.

Answer: Metabolic wastes versus undigested food.

The urinary system

• Two kidneys: dark red, bean-shaped, on the back wall of the abdomen, one on each side of the backbone; the right kidney is slightly lower than the left (the liver is above it). The inner, concave side has a notch, the hilum, where vessels and the ureter join.
• Renal artery (from the dorsal aorta) brings oxygenated blood with wastes into each kidney; the renal vein takes filtered blood to the inferior vena cava.
• Two ureters: narrow muscular tubes carrying urine from the kidneys to the bladder by peristalsis.
• Urinary bladder: a muscular, elastic bag that stores urine.
• Urethra: the tube through which urine passes out; guarded by sphincter muscles.

Micturition: the process of passing out urine from the urinary bladder through the urethra, when the bladder's muscular wall contracts and the sphincter muscles relax (under voluntary control in adults).

Worked example

Explain micturition.

Try it first, then show the solutionSolution

1. Urine collects in the urinary bladder.
2. When the bladder is full, its muscular wall contracts and the sphincters around the urethra relax.
3. Urine is expelled through the urethra.

Answer: Expulsion of urine from the bladder through the urethra by bladder contraction and sphincter relaxation.

Internal structure of the kidney

A longitudinal section of the kidney shows:

• Capsule: a thin, tough outer covering.
• Cortex: the outer, darker region; contains the Malpighian capsules (glomeruli and Bowman's capsules) and convoluted tubules.
• Medulla: the inner, paler region with cone-shaped renal pyramids, made of loops of Henle and collecting ducts.
• Pelvis: a funnel-shaped cavity that collects urine from the collecting ducts and leads into the ureter.

The renal artery and vein enter and leave at the hilum.

Worked example

In which region of the kidney are the Malpighian capsules found?

Try it first, then show the solutionSolution

1. Malpighian capsules (glomerulus + Bowman's capsule) lie in the outer region.
2. That region is the cortex.

Answer: Cortex.

Structure of a nephron

The nephron (kidney tubule) is the structural and functional unit of the kidney; each kidney has about a million.

1. Bowman's capsule: a double-walled cup at the start of the tubule.
2. Glomerulus: a knot of capillaries inside Bowman's capsule. Together they form the Malpighian capsule (renal corpuscle).
3. Afferent arteriole (a branch of the renal artery) brings blood into the glomerulus; it is wider. Efferent arteriole carries blood out; it is narrower. This difference creates high pressure in the glomerulus.
4. Proximal convoluted tubule, loop of Henle, distal convoluted tubule: the coiled and looped tubule, surrounded by a network of peritubular capillaries formed from the efferent arteriole.
5. Collecting duct: receives urine from many nephrons and leads to the pelvis.

Worked example

Differentiate between the afferent and efferent arterioles in terms of diameter and direction of blood flow.

Try it first, then show the solutionSolution

1. Afferent: wider; carries blood into the glomerulus.
2. Efferent: narrower; carries blood out of the glomerulus.

Answer: Afferent wider (in); efferent narrower (out).

Formation of urine

1. Ultrafiltration (in the Malpighian capsule): blood in the glomerulus is under high pressure (afferent wider than efferent), so water and small dissolved substances (glucose, amino acids, salts, urea) are forced out through the thin capillary walls into Bowman's capsule. Blood cells and plasma proteins are too large and stay in the blood. The liquid in the capsule is the glomerular filtrate.

2. Selective reabsorption (mainly in the proximal convoluted tubule): useful substances are taken back into the surrounding capillaries: all the glucose and amino acids, most of the water and many salts. Urea is reabsorbed little.

3. Tubular secretion: cells of the tubule actively secrete some substances from the blood into the filtrate, such as potassium and hydrogen ions, creatinine and some drugs.

The remaining fluid is urine: mostly water, with urea, salts and creatinine; its yellow colour is due to the pigment urochrome.

ADH (antidiuretic hormone) from the pituitary increases water reabsorption from the tubules; if it is lacking, large amounts of dilute urine are passed.

Worked example

Glucose is present in the glomerular filtrate but not in normal urine. Explain.

Try it first, then show the solutionSolution

1. Glucose is small, so it passes into the filtrate during ultrafiltration.
2. All of it is reabsorbed into the blood in the tubule (selective reabsorption).

Answer: It is completely reabsorbed in the tubule.

Comparing blood plasma, filtrate and urine

Substance	Blood plasma	Glomerular filtrate	Urine
Plasma proteins	present	absent	absent
Glucose	present	present	absent (normally)
Urea	low	low	high (concentrated)
Water	high	high	less (most reabsorbed)
Salts	present	present	present (amount regulated)

Two useful conclusions:

• Proteins absent from filtrate → ultrafiltration holds back large molecules.
• Glucose absent from urine → selective reabsorption returns it.
• Urea concentrated in urine → water reabsorbed, urea left behind.

Worked example

Why is the concentration of urea higher in urine than in the glomerular filtrate?

Try it first, then show the solutionSolution

1. Most of the water in the filtrate is reabsorbed in the tubules.
2. Little urea is reabsorbed.
3. So urea becomes more concentrated.

Answer: Water is reabsorbed but most urea is not.

Excretory System in real life

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

Topics in this chapter

• Structure of a Nephron and the Malpighian Capsule
• Formation of Urine: Ultrafiltration, Reabsorption, Secretion
• Human Urinary System and Structure of the Kidney

Excretory System: key facts

Path of urine

$\text{nephron}\to \text{collecting duct}\to \text{pelvis}\to \text{ureter}\to \text{bladder}\to \text{urethra}$

Micturition at the end.

Blood through the nephron

$\text{renal artery}\to \text{afferent arteriole}\to \text{glomerulus}\to \text{efferent arteriole}\to \text{peritubular capillaries}\to \text{renal vein}$

Afferent wider than efferent.

Three steps of urine formation

$\text{ultrafiltration}\to \text{selective reabsorption}\to \text{tubular secretion}$

Capsule, then tubule.

Malpighian capsule

$\text{Malpighian capsule}=\text{Bowman’s capsule}+\text{glomerulus}$

Site of ultrafiltration.

Excretory System: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

How the board asks Excretory System

The question patterns to expect, and the method that works for each.

Tricks to remember Excretory System

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

• Afferent Arrives, Efferent Exits

  A for Arrive (and wider: A is 'All-in'), E for Exit (narrower).

  Use it when: Arteriole questions.

• FRS

  Filter, Reabsorb, Secrete: the three steps in order.

  Use it when: Urine formation.

• Cortex = Capsules

  C for Cortex and Capsules; Medulla = loops (M for 'Many loops').

  Use it when: Kidney regions.

• Big stays, small goes

  In ultrafiltration, big things (cells, proteins) stay in blood; small things go into the filtrate.

  Use it when: Filtrate composition.

Excretory System: board-level solved questions

Ordered from board level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section B style)

Read the situation, then answer every part. In the board paper this is a 6-mark question split into 2 + 2 + 1 + 1 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Excretory System

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

• ✗Mistake: Reversed arteriole diameters

  CISCE examiners' comments, 2024: candidates wrote narrow for the afferent arteriole and wide for the efferent.

  Wrong working

  Afferent arteriole narrow; efferent wide.

  Right working

  Afferent arteriole wider; efferent narrower.

  ✓Do this: Wide in, narrow out = high pressure.

  Usually costs: 1 mark.

• ✗Mistake: Ultrafiltration without the glomerulus

  CISCE examiners' comments, 2024: defining ultrafiltration, candidates did not mention the glomerulus and filtration under high pressure.

  Wrong working

  Ultrafiltration is the filtration of blood.

  Right working

  Filtration of blood under high pressure in the glomerulus into Bowman's capsule.

  ✓Do this: Include site and pressure.

  Usually costs: 1 mark.

• ✗Mistake: Micturition as 'removal of wastes'

  CISCE examiners' comments, 2025: candidates wrote only 'passing urine' or 'removal of wastes' without the bladder and sphincter muscles.

  Wrong working

  Micturition is removal of wastes from the body.

  Right working

  Expulsion of urine from the urinary bladder through the urethra, by bladder contraction and sphincter relaxation.

  ✓Do this: Name the bladder, urethra and sphincters.

  Usually costs: 1 mark.

• ✗Mistake: Excretion confused with egestion

  CISCE examiners' comments, 2025: candidates confused excretion with egestion and defecation, and did not specify nitrogenous waste.

  Wrong working

  Excretion is passing out faeces.

  Right working

  Excretion is removal of metabolic (nitrogenous) wastes such as urea.

  ✓Do this: Metabolic waste, not undigested food.

  Usually costs: 1 mark.

• ✗Mistake: Urine for urea

  CISCE examiners' comments, 2024: asked to name a nitrogenous waste, candidates wrote urine instead of urea.

  Wrong working

  Nitrogenous waste: urine.

  Right working

  Urea (urine is the liquid carrying it).

  ✓Do this: Urea is the substance; urine is the solution.

  Usually costs: 1 mark.

Practice questions on Excretory System

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

1. Q1 · 1 mark · MCQBoard level

   Ultrafiltration takes place in the:
   (a) loop of Henle
   (b) collecting duct
   (c) Malpighian capsule
   (d) urinary bladder

   Hint

   Glomerulus.

   Answer

   (c)

2. Q2 · 1 mark · MCQBoard level

   Which substance is present in the glomerular filtrate but normally absent in urine?
   (a) Urea
   (b) Glucose
   (c) Water
   (d) Salts

   Hint

   Completely reabsorbed.

   Answer

   (b)

3. Q3 · 1 mark · Very short answerHarder board level

   Arrange in the correct sequence, beginning with the glomerulus: loop of Henle, Bowman's capsule, collecting duct, proximal convoluted tubule, distal convoluted tubule.

   Hint

   Follow the filtrate.

   Answer

   Glomerulus → Bowman's capsule → proximal convoluted tubule → loop of Henle → distal convoluted tubule → collecting duct.

4. Q4 · 1 mark · Assertion-reasonHarder board level

   Assertion (A): Blood cells are not found in the glomerular filtrate.
   Reason (R): Blood cells are reabsorbed in the proximal convoluted tubule.
   (a) Both (A) and (R) are true, and (R) is the correct explanation for (A).
   (b) Both (A) and (R) are true, but (R) is not the correct explanation for (A).
   (c) (A) is true and (R) is false.
   (d) (A) is false and (R) is true.

   Hint

   Do cells ever enter?

   Answer

   (c): blood cells are too large to be filtered at all.

5. Q5 · 3 marks · Very short answerBoard level

   Name: (a) the tube from kidney to bladder; (b) the capillary knot in Bowman's capsule; (c) the outer region of the kidney.

   Hint

   One term each.

   Answer

   (a) Ureter (b) Glomerulus (c) Cortex

6. Q6 · 2 marks · Short answerHarder board level

   What is tubular secretion? Give two substances secreted.

   Hint

   Blood to tubule.

   Answer

   Active transfer of substances from the blood in the capillaries into the filtrate in the tubule; e.g. potassium ions and hydrogen ions (also creatinine, some drugs).

7. Q7 · 2 marks · Short answerHarder board level

   Explain why a person passes more urine on a cold day than on a hot day.

   Hint

   Sweating and ADH.

   Answer

   On a cold day less water is lost as sweat, so the blood has more water; less ADH is released, less water is reabsorbed, and more urine is produced.

8. Q8 · 3 marks · Short answerHarder board level

   Give the functions of: (a) ureter, (b) urinary bladder, (c) sphincter muscles of the urethra.

   Hint

   Carry, store, control.

   Answer

   (a) Carries urine from kidney to bladder by peristalsis. (b) Stores urine. (c) Control the release of urine (keep the urethra closed until micturition).

9. Q9 · 2 marks · Case-basedHOTS / competency

   A sample of fluid from a nephron contains water, glucose, salts and urea but no protein. From which part was it most likely taken, and why?

   Hint

   Glucose still there.

   Answer

   Bowman's capsule (or start of the proximal tubule): it is glomerular filtrate, with glucose not yet reabsorbed and protein filtered out.

10. Q10 · 3 marks · Short answerHOTS / competency

    Why does the efferent arteriole carry blood with less water and urea but more protein concentration than the afferent arteriole?

    Hint

    Filtration removed some.

    Answer

    During ultrafiltration, water, urea and small solutes leave the glomerulus into the capsule, but proteins stay in the blood. So the efferent blood has less water and urea, and its proteins are more concentrated.

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. 1.Functional unit of the kidney?Show answer

   Nephron.

2. 2.Which arteriole is wider?Show answer

   Afferent arteriole.

3. 3.What is the glomerular filtrate?Show answer

   The fluid filtered from the glomerulus into Bowman's capsule.

4. 4.Where is urine stored?Show answer

   Urinary bladder.

5. 5.Pigment giving urine its colour?Show answer

   Urochrome.

6. 6.Organ that converts excess amino acids into urea?Show answer

   Liver.

7. 7.Vessel carrying blood away from the kidney?Show answer

   Renal vein.

8. 8.Hormone increasing water reabsorption?Show answer

   ADH (antidiuretic hormone).

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Can you define excretion, ultrafiltration and micturition?
• ☐Can you draw and label the urinary system and the kidney section?
• ☐Can you draw a Malpighian capsule with the arterioles correctly?
• ☐Can you explain the three steps of urine formation?
• ☐Can you explain why glucose is in the filtrate but not in urine?

Excretory System: frequently asked questions