Excretion — Class 10 Biological Science
"Your kidneys filter 180 litres of blood every day. They return 99% of it to your body. The remaining 1% — about 1.5 litres — becomes urine. Your kidneys are the most efficient recycling plant ever evolved."
1. Why Excretion?
Metabolism produces WASTE. Nitrogenous waste (urea — from protein breakdown). CO₂ (from respiration). Excess water and salts. These are TOXIC if accumulated. They MUST be removed.
2. The Human Excretory System
| Organ | Function |
|---|---|
| Kidneys (2) | FILTER blood. Produce urine. |
| Ureters (2 tubes) | Carry urine from kidneys to bladder. |
| Urinary Bladder | STORES urine (up to ~500 mL). |
| Urethra | EXPELS urine from body. |
3. The Nephron — The Functional Unit
Each kidney contains ~1 MILLION nephrons. A nephron is a microscopic TUBULE with associated BLOOD VESSELS.
Structure of a Nephron
Bowman's Capsule (cup-shaped — surrounds GLOMERULUS, a knot of capillaries). Proximal Convoluted Tubule (PCT). Loop of Henle (U-shaped). Distal Convoluted Tubule (DCT). Collecting Duct (collects urine from many nephrons → renal pelvis → ureter).
4. Urine Formation — Three Steps
Step 1 — Glomerular Filtration
Blood enters the GLOMERULUS under HIGH PRESSURE (the afferent arteriole is wider than the efferent arteriole → creates back-pressure). Water, glucose, amino acids, urea, salts are FORCED through the capillary walls into Bowman's Capsule. This fluid is called GLOMERULAR FILTRATE (essentially blood MINUS cells and large proteins). ~180 litres filtered per day.
Step 2 — Tubular Reabsorption
As the filtrate passes through the PCT, Loop of Henle, and DCT: USEFUL substances are selectively REABSORBED back into the blood. Water (~99%). ALL glucose. ALL amino acids. Most salts. 'If glucose appears in urine → DIABETES — the kidney couldn't reabsorb it all.'
Step 3 — Tubular Secretion
Additional waste products (H⁺, K⁺, creatinine, drugs, toxins) are ACTIVELY SECRETED from the blood into the filtrate. 'This is the body's last chance to REMOVE toxins that escaped filtration.'
Final Urine: Water + Urea + Excess salts + Other waste → Collecting duct → Renal pelvis → Ureter → Bladder → Urethra → EXPELLED.
5. Other Excretory Organs
Lungs: Exhale CO₂ and water vapour. Skin: Sweat glands excrete water, salts, and trace urea. Liver: Converts toxic AMMONIA (from protein breakdown) → less toxic UREA (which kidneys then filter).
6. Excretion in Plants
Plants produce LESS nitrogenous waste than animals. Gaseous waste (CO₂, O₂) → through STOMATA. Excess water → TRANSPIRATION. Some waste stored in: OLD LEAVES (which fall — autumn leaves). BARK. VACUOLES (as crystals — calcium oxalate). Resins, gums, latex (protective — seal wounds).
7. Common Mistakes
- 'Filtration happens in the tubule' — Filtration happens in the GLOMERULUS + Bowman's Capsule. The tubule does REABSORPTION and SECRETION.
- 'Kidneys only filter waste' — Kidneys also REGULATE water balance, salt balance, blood pH, and blood pressure (via renin). They produce ERYTHROPOIETIN (stimulates RBC production).
- 'Dialysis is a permanent cure' — Dialysis is a LIFE-SUSTAINING treatment — not a cure. The only cure for kidney failure is a TRANSPLANT.
8. AP SSC Exam Focus
| Topic | Marks |
|---|---|
| Nephron structure (diagram) | 4-5 |
| Urine formation steps | 3-4 |
| Excretory organs | 2-3 |
| Excretion in plants | 2-3 |
Key Diagram: Draw and label a NEPHRON.
9. Nephron Structure — Detailed View
'Every AP Board exam asks a diagram of the nephron. You MUST be able to draw and label ALL parts.'
Bowman's Capsule: Cup-shaped structure that surrounds the GLOMERULUS. 'Think of it as the COLLECTION BOWL — the filtrate lands here after being forced out of the blood.'
Glomerulus: A KNOT of capillaries with very high blood pressure. 'The afferent arteriole (bringing blood IN) is WIDER than the efferent arteriole (taking blood OUT). This creates BACK-PRESSURE → forces fluid out of the blood into Bowman's Capsule.'
Proximal Convoluted Tubule (PCT) : The FIRST tubular segment after Bowman's Capsule. REABSORBS: ALL glucose, ALL amino acids, ~70% of water and salts (Na⁺, Cl⁻). 'The PCT is the WORKHORSE of the nephron — it reabsorbs MOST of the useful stuff.'
Loop of Henle: U-shaped tube that extends into the MEDULLA (inner part of kidney). Creates a SALT GRADIENT in the kidney medulla — which allows WATER REABSORPTION later. 'The longer the loop, the more concentrated the urine can be. Desert animals like KANGAROO RATS have VERY LONG loops of Henle → they produce EXTREMELY CONCENTRATED urine → they can survive without drinking water.'
Distal Convoluted Tubule (DCT) : Further REABSORPTION (regulated by hormones). Aldosterone: INCREASES salt reabsorption. ADH: INCREASES water reabsorption. 'The DCT is the FINE-TUNER — it adjusts the final composition of urine based on the body's needs.'
Collecting Duct: Receives urine from MULTIPLE nephrons. Passes through the medulla → water is reabsorbed (if ADH is present) → URINE becomes CONCENTRATED. Leads to the RENAL PELVIS → URETER.
10. Urine Formation — Detailed Steps
'Urine formation is a THREE-STEP process. You MUST be able to describe each step in detail and mention WHERE it occurs in the nephron.'
Step 1 — Glomerular Filtration (in the Glomerulus + Bowman's Capsule)
- Blood pressure forces fluid OUT of the glomerular capillaries.
- The FILTRATE contains: water, glucose, amino acids, urea, salts (Na⁺, K⁺, Ca²⁺, Cl⁻), vitamins.
- FILTRATE does NOT contain: blood cells, platelets, large proteins (albumin).
- ~180 LITRES of filtrate produced per day — nearly 125 mL per minute.
- 'This is PURE PHYSICS — blood pressure forces fluid through a filter. No energy is used. Energy is used LATER for reabsorption.'
Step 2 — Tubular Reabsorption (in PCT, Loop of Henle, DCT)
- USEFUL substances are moved BACK into the blood.
- Glucose: 100% reabsorbed (if blood glucose is normal).
- Amino acids: 100% reabsorbed.
- Water: ~99% reabsorbed (only 1-2 litres become urine).
- Salts: selectively reabsorbed (Na⁺, Cl⁻, K⁺, Ca²⁺).
- 'Glucose in urine = DIABETES MELLITUS. The body cannot reabsorb all the glucose because blood glucose is too high. This is a KEY diagnostic test.'
Step 3 — Tubular Secretion (in DCT and Collecting Duct)
- ADDITIONAL waste products are actively TRANSPORTED from the blood into the filtrate.
- Includes: H⁺ ions (helps maintain blood pH), K⁺ ions, creatinine (from muscle metabolism), drugs and toxins.
- 'This is the KIDNEY'S final pass — removing wastes that escaped filtration or that the body needs to eliminate for pH balance.'
11. Dialysis — Artificial Kidney
'When KIDNEYS FAIL (Chronic Kidney Disease — CKD), waste accumulates in the blood → UREMIA (urea poisoning). Dialysis is a LIFE-SAVING treatment.'
Haemodialysis
Patient's blood is pumped through a DIALYSIS MACHINE. Blood flows through THIN TUBES made of partially permeable membrane. Tubes are surrounded by DIALYSIS FLUID (clean fluid with the CORRECT composition of water, salts, and glucose — but NO urea). By DIFFUSION: Urea and excess salts move from blood into dialysis fluid. By OSMOSIS: Excess water moves from blood into dialysis fluid. CLEAN blood is returned to the patient.
Duration: ~4 hours per session, 3 times per week. 'Dialysis replaces the FILTRATION function of the kidney. But it cannot replace the HORMONAL functions (erythropoietin, renin). That is why dialysis patients often also need hormone injections.'
Kidney Transplant
The ONLY CURE for kidney failure. A HEALTHY kidney from a DONOR is surgically placed in the patient's body. Donor can be LIVING (relative or volunteer) or DECEASED (cadaver donor). The patient must take IMMUNOSUPPRESSIVE drugs for life to prevent REJECTION.
12. Kidney Stones
'Kidney stones are HARD deposits of minerals and salts that form inside the kidneys. They can be EXTREMELY painful.'
Causes: Dehydration (low water intake — urine becomes concentrated → minerals crystallise). Diet (high oxalate foods: spinach, chocolate, nuts. High salt, high protein). Obesity. Family history. Certain medical conditions.
Types: Calcium stones (most common — calcium oxalate or calcium phosphate). Uric acid stones (from high purine diet — red meat, seafood). Struvite stones (from urinary tract infections).
Treatment: Drink MORE WATER (most small stones pass naturally). Medication. Lithotripsy (shock waves break stones into small pieces). Surgery (in severe cases).
13. Excretory Organs in Other Organisms
| Organism | Excretory Organ | Waste Product |
|---|---|---|
| Amoeba (protozoan) | CONTRACTILE VACUOLE | Excess water (osmoregulation) |
| Earthworm | NEPHRIDIA (segmental tubes) | Urea, ammonia |
| Cockroach | MALPIGHIAN TUBULES | Uric acid (saves water — adapted to dry land) |
| Fish | KIDNEYS + GILLS | Ammonia (highly toxic, requires lots of water) |
| Bird | KIDNEYS + CLOACA | Uric acid (paste-like — saves water for flight) |
| Frog | KIDNEYS + SKIN | Urea |
| Plants | STOMATA (gas), VACUOLES (waste crystals), OLD LEAVES (fallen) | CO₂, O₂, calcium oxalate crystals |
'Notice the PATTERN: aquatic organisms excrete AMMONIA (needs lots of water). Terrestrial organisms excrete UREA or URIC ACID (saves water). Birds and reptiles excrete URIC ACID (semi-solid — ideal for egg-layers). This is ADAPTATION to environment.'
14. Common Mistakes (Additional)
- 'The liver only excretes bile' — The liver converts toxic AMMONIA into UREA (the main nitrogenous waste). This is a CRUCIAL excretory function. Bile is a separate secretion.
- 'Dialysis can be done at home with no training' — Haemodialysis requires TRAINED medical personnel and specialised equipment. However, PERITONEAL DIALYSIS can be done at home by the patient after training.
- 'The bladder stores urine indefinitely' — The bladder stores urine up to ~500 mL. Beyond that: the MICTURITION REFLEX triggers URINATION. Holding urine for too long can lead to UTIs.
15. Self-Test Questions
- Draw a labelled diagram of a NEPHRON and explain the function of each part.
- Describe the THREE steps of urine formation. Where does each step occur in the nephron?
- Explain what DIALYSIS is. How does a dialysis machine work?
- Why does the LOOP OF HENLE vary in length across different animals?
- Compare the excretory systems of an AMOEBA, an EARTHWORM, and a BIRD.
- What happens if a person's kidneys FAIL? What are the treatment options?
- What are KIDNEY STONES? How can they be prevented?
- How do plants excrete waste? Give THREE methods.
- Why do desert animals produce VERY CONCENTRATED urine?
- Explain the role of ADH in urine concentration.
Answers to Selected Questions: Q4: Animals in DESERTS have longer loops of Henle → produce more concentrated urine → conserve water. Animals in FRESHWATER have shorter loops → produce dilute urine. Q6: Two options — DIALYSIS (life-sustaining, not a cure) or KIDNEY TRANSPLANT (the only cure). Q9: Desert animals must conserve EVERY drop of water. Their nephrons have VERY LONG loops of Henle → maximum water reabsorption → highly concentrated urine.
