Chapter in a nutshell: Excretion removes toxic nitrogenous wastes (mainly urea) from the body. The kidneys are the chief excretory organs; each holds ~1 million nephrons (kidney tubules). Urine is formed in three steps — ultrafiltration at the glomerulus, selective reabsorption of useful substances, and tubular secretion — keeping the blood's composition steady (homeostasis).
1. Excretion
Excretion is the removal of toxic metabolic wastes (chiefly nitrogenous) from the body; it maintains homeostasis (a steady internal state).- Not the same as defaecation (egesting undigested food) or secretion (useful products like enzymes).
- The organs forming, storing and eliminating urine = the excretory (urinary) system.
2. Substances to be Eliminated & Where
| Waste | Source | Removed by |
|---|---|---|
| CO₂ + water | cellular respiration | lungs |
| Urea, uric acid, ammonia (nitrogenous) | breakdown of excess amino acids in the liver | kidneys |
| Excess salts (NaCl), vitamins B & C | diet | kidneys |
| Excess water | diet | kidneys (also skin) |
| Bile pigments (bilirubin) | breakdown of old RBCs | liver → bile → faeces/urine |
3. Excretory Organs
| Organ | Excretes |
|---|---|
| Kidneys (primary) | urea, salts, excess water → urine |
| Lungs | CO₂ and some water vapour |
| Skin / sweat glands | small amounts of salts/urea (mainly for cooling — incidental) |
| Liver | bile pigments; also makes urea |
4. The Urinary System — Parts
- Two kidneys — bean-shaped, ~10 cm long, on either side of the backbone (right slightly lower), protected by the last ribs.
- Two ureters — carry urine from each kidney (from the pelvis at the hilum) to the bladder.
- Urinary bladder — stores urine.
- Urethra — passes urine out; guarded by a sphincter (relaxes during urination/micturition).
5. Internal Structure of the Kidney
A longitudinal section shows:- Cortex — outer dark region.
- Medulla — inner lighter region, with cone-shaped renal pyramids whose tips (papillae) open into the pelvis.
- The kidney contains ~1 million nephrons (uriniferous tubules) — its structural and functional units.
6. Structure of a Nephron (Kidney Tubule)
| Part | Location | Note |
|---|---|---|
| Bowman's capsule | cortex | cup enclosing the glomerulus; capsule + glomerulus = Malpighian (renal) capsule |
| Glomerulus | cortex | knot of capillaries inside Bowman's capsule |
| Proximal convoluted tubule (PCT) | cortex | first coiled region |
| Loop of Henle | medulla | U-shaped (hair-pin) part |
| Distal convoluted tubule (DCT) | cortex | last coiled region → collecting duct |
| Collecting duct | medulla | gathers urine from many nephrons → pelvis |
7. Blood Supply
Renal artery → afferent arteriole (wider) → glomerulus → efferent arteriole (narrower) → peritubular capillaries around the tubule → renal vein → posterior vena cava.- The efferent arteriole being narrower than the afferent keeps the glomerulus under high pressure — essential for ultrafiltration.
8. Formation of Urine — Three Steps
(I) Ultrafiltration: the high glomerular pressure forces the liquid part of blood (water, urea, glucose, amino acids, salts) out into Bowman's capsule as the glomerular filtrate. Blood cells and large proteins are too big to filter and stay behind. (II) Selective reabsorption: as the filtrate passes along the tubule, useful substances are reabsorbed into the surrounding capillaries — all glucose, most water, and required salts/amino acids (in the PCT, loop and DCT). The huge tubule length (~60 km total) gives a vast surface for this. (III) Tubular secretion: extra wastes/ions (e.g. excess H⁺, K⁺, some drugs) are actively added from the blood into the tubule. The remaining fluid is urine.- Glomerular filtrate ≈ 160 L/day → urine ≈ 1.2–1.5 L/day after reabsorption.
9. Composition of Urine
Urine is ~95% water with urea (main waste), uric acid, creatinine, salts (NaCl), and urochrome (yellow colour). Glucose and proteins are absent in normal urine (their presence indicates diabetes/kidney damage).10. Dialysis & Osmoregulation
- Kidney failure lets wastes build up; dialysis (artificial kidney) filters the blood across a selectively permeable membrane against dialysing fluid. A kidney transplant is a permanent cure.
- Osmoregulation: kidneys regulate the water and salt balance of the blood (controlled by the hormone ADH), maintaining homeostasis.
11. Worked / Structured Examples (ICSE pattern)
Q1. Name the structural and functional unit of the kidney. Solution: The nephron (uriniferous/kidney tubule). Q2. Why is glucose absent in the urine of a healthy person? Solution: It is completely reabsorbed during selective reabsorption. Q3. Why is the glomerulus under high pressure? Solution: The efferent arteriole is narrower than the afferent arteriole. Q4. Arrange in sequence: DCT, Bowman's capsule, Loop of Henle, PCT. Solution: Bowman's capsule → PCT → Loop of Henle → DCT. Q5. Name the two main regions seen in a kidney section. Solution: Cortex (outer) and medulla (inner).12. Key Terms — Quick Glossary
| Term | One-line definition |
|---|---|
| Excretion | removal of toxic (nitrogenous) metabolic wastes. |
| Nephron | structural & functional unit of the kidney. |
| Glomerulus | capillary knot in Bowman's capsule. |
| Malpighian capsule | Bowman's capsule + glomerulus. |
| Ultrafiltration | pressure filtration of plasma into the tubule. |
| Selective reabsorption | reuptake of useful substances from the filtrate. |
| Tubular secretion | active addition of extra wastes into the tubule. |
| Ureter / urethra | kidney→bladder tube / bladder→outside tube. |
| Osmoregulation | regulation of body water/salt balance. |
| Dialysis | artificial filtering of blood in kidney failure. |
13. Common Mistakes to Avoid
- Confusing excretion with defaecation (egestion of undigested food).
- Mixing up ureter (kidney→bladder) and urethra (bladder→outside).
- Saying the afferent arteriole is narrower — it is the efferent that is narrower (creates high pressure).
- Forgetting that glucose is fully reabsorbed (absent in normal urine).
- Reversing cortex (outer) and medulla (inner).
- Calling the lungs/skin the primary excretory organ — the kidneys are primary.
14. Likely Exam Questions (with crisp answers)
- Define excretion. → Removal of toxic nitrogenous metabolic wastes from the body.
- Name the chief nitrogenous waste in humans and where it is made. → Urea; made in the liver.
- List the parts of the urinary system. → Two kidneys, two ureters, urinary bladder, urethra.
- Name the two regions of the kidney. → Cortex and medulla.
- What is a nephron? → The microscopic functional unit of the kidney that forms urine.
- Name the parts of a nephron in order. → Bowman's capsule (with glomerulus) → PCT → Loop of Henle → DCT → collecting duct.
- What is ultrafiltration? → Pressure filtration of plasma from the glomerulus into Bowman's capsule.
- Define selective reabsorption. → Reabsorption of useful substances (glucose, water, salts) from the filtrate back into the blood.
- Why does the glomerulus filter under high pressure? → The efferent arteriole is narrower than the afferent arteriole.
- Give the normal daily volume of urine. → About 1.2–1.5 litres.
- Name two substances absent in normal urine. → Glucose and proteins.
- What is dialysis? → Artificial filtering of the blood when the kidneys fail.
- Which hormone controls water reabsorption? → ADH (antidiuretic hormone).
- Other than kidneys, name two excretory organs and what they remove. → Lungs (CO₂); skin (salts/water via sweat).
- What does the term homeostasis mean here? → Maintaining a constant internal environment (blood composition) via excretion/osmoregulation.
15. The Skin as an Excretory Organ
The skin's sweat glands secrete sweat (water + small amounts of salts and urea). Its primary role is thermoregulation (cooling by evaporation), so its excretory role is incidental. Sebaceous glands secrete sebum (oily; contains some waste lipids).16. Nitrogenous Waste & the Liver
Excess amino acids cannot be stored. In the liver they undergo deamination — the amino (–NH₂) group is removed and converted to ammonia, which is quickly turned into the less-toxic urea; the remaining carbon part becomes usable glucose. Urea then travels in the blood to the kidneys for removal.- Ammonotelic animals excrete ammonia (aquatic), ureotelic excrete urea (mammals, humans), uricotelic excrete uric acid (birds/insects).
17. Ureter vs Urethra (don't confuse)
| Ureter | Urethra |
|---|---|
| 2 in number | 1 |
| kidney → urinary bladder | bladder → outside |
| carries urine continuously | passes urine intermittently |
18. Quick Facts (from the text)
- ~2 million tubules total; each 4–5 cm; combined length > 60 km → huge reabsorption surface.
- Blood filtered ≈ 1–2 L/min; glomerular filtrate ≈ 160 L/day → urine ≈ 1.2 L/day.
- The kidney's high reabsorption is why we excrete so little of the large filtrate.
19. More Worked / Structured Examples
Q6. Why does the body convert ammonia into urea? Solution: Ammonia is highly toxic and very soluble; urea is far less toxic and safer to transport/excrete. Q7. A urine test shows glucose. What might this indicate? Solution: Possible diabetes mellitus (glucose not fully reabsorbed). Q8. Name the process and site where most water is reabsorbed. Solution: Selective reabsorption, mainly in the proximal convoluted tubule (and loop of Henle/DCT). Q9. What keeps urine from flowing back when the bladder contracts? Solution: The valve-like openings of the ureters into the bladder.20. More Exam Questions (with crisp answers)
- Define deamination. → Removal of the amino group from excess amino acids in the liver to form ammonia/urea.
- Name the excretory product of (i) humans, (ii) birds. → Urea; uric acid.
- Why is the skin not a true excretory organ? → Its main function is cooling; excretion of wastes via sweat is incidental.
- State two differences between a ureter and a urethra. → Number (2 vs 1); ureter carries urine kidney→bladder, urethra bladder→outside.
- Why does the kidney have such a large total tubule length? → To provide a vast surface area for reabsorption of water and useful solutes.
- Which blood vessel brings blood to the kidney and which takes it away? → Renal artery brings; renal vein takes away.
- What is micturition? → The act of passing urine out of the bladder.
- Name the funnel-shaped cavity of the kidney where urine collects. → The pelvis.