Quick Summary
Excretion eliminates metabolic waste products (urea, uric acid, CO₂, water, salts). In humans, the kidneys are the primary excretory organs. Each kidney contains ~1 million nephrons — the functional units where urine is formed by filtration, reabsorption, and secretion. Counter-current mechanism concentrates urine. NEET frequently tests nephron structure and urine formation.
Types of Nitrogenous Excretion
| Type | Product | Organisms |
|---|---|---|
| Ammonotelic | Ammonia (NH₃) | Aquatic animals — fish, amphibian larvae, Hydra |
| Ureotelic | Urea | Mammals (including humans), adult frogs, cartilaginous fish |
| Uricotelic | Uric acid | Reptiles, birds, insects, land snails |
Urea is less toxic than ammonia; requires water for excretion. Uric acid is least toxic; excreted dry — conserves water.
Human Excretory System
Organs: Kidneys (main), lungs (CO₂), skin (sweat — small amounts of urea, NaCl), liver (bile pigments)
Kidneys:
- Located in the posterior abdominal cavity (retroperitoneal)
- Right kidney slightly lower than left (due to liver)
- Cortex (outer) + Medulla (inner, contains pyramids) + Pelvis (collects urine)
Nephron — Structure
Two types:
- Cortical nephrons (85%): Short loop of Henle, mostly in cortex
- Juxtamedullary nephrons (15%): Long loop of Henle extending deep into medulla — important for concentration
Parts of a Nephron
| Part | Location | Key Function |
|---|---|---|
| Bowman's capsule | Cortex | Surrounds glomerulus; receives filtrate |
| Glomerulus | Inside Bowman's capsule | High-pressure capillary tuft; filtration |
| Proximal Convoluted Tubule (PCT) | Cortex | Reabsorbs 70% glucose, amino acids, Na⁺, water |
| Loop of Henle | Medulla | Creates medullary concentration gradient; concentrates urine |
| Distal Convoluted Tubule (DCT) | Cortex | Selective reabsorption/secretion; regulated by ADH, aldosterone |
| Collecting duct | Medulla | Final concentration of urine; regulated by ADH |
Urine Formation
Step 1 — Glomerular Filtration (GFR)
- Blood pressure forces plasma (minus large proteins and cells) into Bowman's capsule
- GFR = 125 mL/min = 180 L/day (filtrate); only ~1.5 L/day becomes urine
- Ultrafiltrate contains: glucose, amino acids, urea, Na⁺, K⁺, water (no proteins, RBCs)
Step 2 — Tubular Reabsorption
- PCT: Active reabsorption of glucose, amino acids, Na⁺; passive reabsorption of Cl⁻, water
- Loop of Henle: Descending limb — permeable to water (water moves out); Ascending limb — permeable to Na⁺/Cl⁻ (ions move out)
- DCT: Reabsorption of Na⁺ (aldosterone); Ca²⁺ (PTH); water (ADH)
Step 3 — Tubular Secretion
- H⁺, K⁺, creatinine, ammonia secreted into tubule
- Regulates blood pH and ionic composition
Counter-Current Mechanism
- Purpose: Concentrate urine
- Players: Loop of Henle + Vasa recta (capillaries alongside loop)
- Mechanism: Descending limb loses water; ascending limb loses salt → builds up hyperosmotic medulla
ADH (antidiuretic hormone / vasopressin): Increases water permeability of DCT and collecting duct → more water reabsorbed → concentrated, small volume urine Aldosterone: Increases Na⁺ reabsorption in DCT → water follows → increases blood volume
Kidney Disorders
| Disorder | Description |
|---|---|
| Renal failure | Kidneys fail to filter adequately; treated by dialysis or transplant |
| Renal calculi (kidney stones) | Crystallisation of uric acid or calcium oxalate in renal pelvis |
| Glomerulonephritis | Inflammation of glomeruli; proteins leak into urine (proteinuria) |
| Haemodialysis | Artificial kidney — blood filtered through cellophane membrane |
| Gout | Excess uric acid → deposits in joints |
Important Questions
One Mark
- What is the GFR?
- Name the hormone that regulates water reabsorption in DCT.
- What type of excretion do birds show?
- Where in the nephron does maximum reabsorption occur?
Short Answer (3 marks)
- Distinguish between ammonotelic, ureotelic, and uricotelic organisms with examples.
- Describe tubular reabsorption in the PCT.
- What is the role of ADH and aldosterone in kidney function?
Long Answer (5 marks)
- Describe the structure of a nephron with a labelled diagram. Explain the process of urine formation in three steps.
- Explain the counter-current mechanism in the kidney. What is its significance?
Common Mistakes
- GFR is 125 mL/min (= 180 L/day), not 1.5 L/day — 1.5 L is urine output after reabsorption.
- ADH increases water REABSORPTION (concentrates urine) — NOT water excretion.
- Loop of Henle: descending limb is permeable to water; ascending limb is permeable to salt — students often reverse these.
- Uricotelic organisms excrete uric acid (birds, reptiles) — NOT ureotelic (mammals).
Board Exam Tips
- Nephron diagram with labels (Bowman's capsule, PCT, loop, DCT, collecting duct) is a standard 5-mark question.
- "Explain urine formation in three steps" is a reliable 5-mark question.
- Ammonotelic/ureotelic/uricotelic comparison appears as a 3-mark question.
- NEET: GFR, PCT reabsorption, ADH role, counter-current mechanism contribute 3–4 questions per paper.
Frequently Asked Questions
A nephron has: Bowman's capsule (surrounds glomerulus for filtration), Proximal Convoluted Tubule (reabsorption of glucose, amino acids), Loop of Henle (concentration of urine — descending limb permeable to water, ascending limb permeable to salts), Distal Convoluted Tubule (selective reabsorption), Collecting duct.