Excretory System
1. What Is Excretion?
EXCRETION is the process of removing METABOLIC WASTES from the body.
Excretion vs Egestion
| Process | Meaning | Examples |
|---|---|---|
| Excretion | Removal of WASTE PRODUCTS produced by body cells | CO₂, urea, excess water, salts |
| Egestion | Removal of UNDIGESTED food | Faeces |
Waste Products to Be Removed
| Waste Product | Produced From | Removed By |
|---|---|---|
| Carbon dioxide | Respiration | Lungs |
| Urea | Breakdown of EXCESS PROTEINS (in liver) | Kidneys (in urine) |
| Excess water and salts | Various metabolic processes | Kidneys (in urine), Skin (sweat) |
| Bile pigments | Breakdown of old RBCs (in liver) | Liver → Intestines (in faeces) |
2. Organs of the Excretory System
| Organ | Function |
|---|---|
| Kidneys (2) | FILTER blood to produce URINE |
| Ureters (2) | Carry URINE from kidneys to URINARY BLADDER |
| Urinary bladder | STORES urine temporarily |
| Urethra | Carries urine from bladder OUT of the body |
| Skin | Excretes SWEAT (water, salts, some urea) |
| Lungs | Excrete CARBON DIOXIDE and water vapour |
| Liver | Converts ammonia to UREA, breaks down haemoglobin |
3. The Kidney — Internal Structure
The kidney is BEAN-SHAPED, about 10-12 cm long.
Layers (Outside → Inside)
- Cortex (outer layer).
- Medulla (inner region).
- Renal pelvis (central cavity — collects urine).
Path of Urine
Cortex → Medulla → Renal pelvis → Ureter → Bladder → Urethra
Each kidney contains about 1 MILLION NEPHRONS (the functional units).
4. The Nephron — Functional Unit
A NEPHRON is the microscopic FILTERING UNIT of the kidney.
Parts of a Nephron
| Part | Location | Function |
|---|---|---|
| Glomerulus | Cortex | A KNOT of blood capillaries. FILTRATION of blood. |
| Bowman's capsule | Cortex | Cup-shaped structure surrounding glomerulus. Collects FILTRATE. |
| Proximal convoluted tubule (PCT) | Cortex | REABSORPTION of glucose, amino acids, vitamins, most water |
| Loop of Henle | Medulla | REABSORPTION of water and salts (maintains concentration) |
| Distal convoluted tubule (DCT) | Cortex | Further REABSORPTION of salts, secretion of wastes |
| Collecting duct | Medulla | COLLECTS urine from many nephrons. Carries to renal pelvis. |
Glomerular Filtration
- Blood enters the glomerulus through the AFFERENT ARTERIOLE.
- High pressure forces water, urea, glucose, salts, and small molecules OUT of the blood and INTO Bowman's capsule.
- Blood cells and large proteins REMAIN in blood (too large to pass through).
- The filtered fluid is called GLOMERULAR FILTRATE.
5. Urine Formation — Summary
| Process | Where | What Happens |
|---|---|---|
| Filtration | Glomerulus → Bowman's capsule | Water, urea, glucose, salts filter out of blood |
| Reabsorption | PCT, Loop of Henle, DCT | USEFUL substances (glucose, amino acids, most water, some salts) are REABSORBED into blood |
| Secretion | DCT | Additional wastes are SECRETED into the tubule |
| Collection | Collecting duct | Final URINE is collected and sent to ureter |
Final urine contains: Water, urea, excess salts, and some other wastes.
6. The Skin — Another Excretory Organ
Structure of Skin (Quick Overview)
| Layer | Features |
|---|---|
| Epidermis (outer) | Dead cells on top, living cells below. Contains MELANIN (pigment). |
| Dermis (inner) | Contains SWEAT GLANDS, hair follicles, oil glands, blood vessels, nerve endings. |
Sweat Glands
- Secrete SWEAT (water + salts + small amount of urea).
- Sweating also helps REGULATE BODY TEMPERATURE (cooling by evaporation).
- About 2-3 million sweat glands in the human body.
Functions of Skin
- Protection from germs, UV rays, injury.
- Excretion (sweat).
- Temperature regulation (sweating, blood vessel dilation/constriction).
- Sensation (touch, pressure, pain, heat, cold).
7. ICSE Exam Focus
| Topic | Marks | Frequency |
|---|---|---|
| Excretory organs and their functions | 2-3 marks | Very High |
| Nephron structure and function | 3 marks | Very High |
| Urine formation (filtration, reabsorption) | 3 marks | High |
| Skin structure and functions | 2-3 marks | Medium |
| Kidney structure (cortex, medulla, pelvis) | 2 marks | High |
Common Mistakes
- Thinking urination and excretion are the same (excretion INCLUDES urine, sweat, CO₂ — not just urination).
- Confusing Egestion (removing faeces — undigested food) with Excretion (removing metabolic wastes).
- Saying the liver is part of the excretory system — it PRODUCES urea but is not an excretory organ per se (it is part of the digestive system primarily).
- Writing that the glomerulus is in the medulla (it is in the CORTEX).
Self-Test (5 Questions)
Q1. The functional unit of the kidney is: (1 mark)
- A) Neuron
- B) Nephron
- C) Alveolus
- D) Villus
Q2. 'Name the process by which blood is filtered in the nephron.' (1 mark)
Q3. 'Which organ of the body produces urea?' (1 mark)
- A) Kidney
- B) Liver
- C) Lungs
- D) Skin
Q4. 'Name two substances that are reabsorbed into the blood in the nephron.' (2 marks)
Q5. 'What is the role of the skin in excretion?' (2 marks)
Answers
A1. B) Nephron. A2. Glomerular filtration (in the glomerulus-Bowman's capsule). A3. B) Liver (converts excess ammonia from protein breakdown into less toxic urea). A4. Glucose, amino acids, water, salts (any two). A5. The skin has SWEAT GLANDS that secrete SWEAT containing water, salts, and small amounts of urea. Sweating helps excrete these wastes and also regulates body temperature (cooling by evaporation).
