Human Anatomy — Excretory System
Introduction
The excretory system removes metabolic wastes (mainly urea, uric acid, and creatinine) from the body. In ICSE Class 10 Biology, you study the structure and function of the kidney, the nephron as the functional unit, and the process of urine formation.
Organs of the Excretory System
| Organ | Function |
|---|---|
| Kidneys (pair) | Filter blood and form urine |
| Ureters (pair) | Carry urine from kidneys to urinary bladder |
| Urinary bladder | Stores urine temporarily |
| Urethra | Carries urine out of the body |
| Skin (sweat glands) | Excretes water, salts, and urea (as sweat) |
| Lungs | Excrete CO₂ and water vapour |
Kidney Structure (External)
- Shape: Bean-shaped.
- Location: Retroperitoneal (behind the abdominal cavity).
- Size: ~10−12 cm long.
Internal Structure (from outer to inner)
| Region | Description |
|---|---|
| Renal capsule | Outer protective layer |
| Cortex | Outer part — contains Bowman's capsules, PCT, DCT |
| Medulla | Inner part — contains loops of Henle, collecting ducts |
| Renal pelvis | Funnel-shaped cavity collecting urine |
| Renal artery | Brings blood to the kidney |
| Renal vein | Carries filtered blood away |
The Nephron (Functional Unit)
Each kidney contains about 1 million nephrons.
Parts of a Nephron
| Part | Function |
|---|---|
| Bowman's capsule | Cup-shaped structure containing the glomerulus |
| Glomerulus | Network of capillaries — site of filtration |
| Proximal convoluted tubule (PCT) | Reabsorption of glucose, amino acids, salts, water |
| Loop of Henle | Creates concentration gradient in medulla (water reabsorption) |
| Distal convoluted tubule (DCT) | Selective reabsorption and secretion |
| Collecting duct | Final water reabsorption; carries urine to renal pelvis |
Urine Formation (Three Steps)
1. Glomerular Filtration
Blood enters the glomerulus under high pressure. Water, urea, glucose, amino acids, and salts are filtered into Bowman's capsule. Filtrate: ~125 mL/min (180 L/day).
2. Tubular Reabsorption
Useful substances are reabsorbed back into the blood:
- Glucose and amino acids: 100% reabsorbed (PCT).
- Water: ~99% reabsorbed (PCT, loop of Henle, collecting duct).
- Salts: Selective reabsorption.
3. Tubular Secretion
Excess H⁺ and K⁺ ions and certain drugs are actively secreted into the tubule. Helps maintain pH balance (acid-base balance).
Final urine: ~1−1.5 L/day (the rest is reabsorbed).
Osmoregulation
The process by which the body maintains a constant water and salt balance.
Role of ADH (Antidiuretic Hormone):
- When the body is dehydrated: ADH increases → more water reabsorbed → concentrated urine.
- When the body is overhydrated: ADH decreases → less water reabsorbed → dilute urine.
Common Mistakes and Fixes
| Mistake | Fix |
|---|---|
| Confusing Bowman's capsule with the glomerulus | Bowman's capsule = cup; Glomerulus = blood capillary network inside it |
| Thinking all filtrate becomes urine | Only ~1% of filtrate (180 L → 1.5 L urine) |
| Calling the collecting duct part of the nephron | The collecting duct receives urine from multiple nephrons |
| Forgetting that glucose is normally absent in urine | Glucose is 100% reabsorbed — its presence indicates diabetes |
ICSE Exam Focus
This chapter carries 4–6 marks. Key topics: kidney structure (labelled diagram), nephron structure and function, urine formation steps, osmoregulation, ADH role.
Marks Blueprint: Kidney/Nephron structure — 2 marks, Urine formation — 2 marks, Osmoregulation — 2 marks.
Self-Test Questions
-
Draw a labelled diagram of the human kidney showing its internal structure.
-
Draw and label the structure of a nephron.
-
Describe the three stages of urine formation.
-
What is the role of ADH in osmoregulation?
-
Why is glucose not normally present in urine?
-
What is the function of the loop of Henle?
