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Master Excretion for NEET 2026 with comprehensive notes on kidney structure, nephron function, urine formation, and regulation. NCERT-aligned content with diagrams and PYQs.
Remember these points for your NEET preparation
Excretion is an important chapter in Human Physiology, contributing 3-5 questions in NEET annually. This comprehensive guide covers all NCERT concepts for NEET 2026.
Definition: Removal of metabolic waste products from the body.
| Waste Product | Source | Animals | Toxicity | Water Requirement |
|---|---|---|---|---|
| Ammonia | Protein metabolism | Aquatic animals | Very high | Very high |
| Urea | Ammonia conversion in liver | Mammals, amphibians | Moderate | Moderate |
| Uric acid | Purine metabolism | Birds, reptiles, insects | Low | Very low |
| Type | Waste Product | Examples |
|---|---|---|
| Ammonotelic | Ammonia | Fish, aquatic invertebrates |
| Ureotelic | Urea | Mammals, amphibians |
| Uricotelic | Uric acid | Birds, reptiles, insects |
NEET Important: Humans are primarily ureotelic but also excrete some ammonia and uric acid.
| Animal | Excretory Organ |
|---|---|
| Protozoa | Contractile vacuole |
| Flatworms | Flame cells (protonephridia) |
| Earthworm | Nephridia |
| Insects | Malpighian tubules |
| Crustaceans | Green (antennal) glands |
| Vertebrates | Kidneys |
Location: Retroperitoneal, between T12-L3 vertebrae
Size: ~11 cm × 6 cm × 3 cm, ~150 g each
External Structure:
Internal Structure:
| Region | Location | Features |
|---|---|---|
| Cortex | Outer | Contains glomeruli, PCT, DCT |
| Medulla | Inner | Contains loops of Henle, collecting ducts |
| Pyramids | Medulla | Triangular structures with apices pointing inward |
| Pelvis | Central | Funnel-shaped, continuous with ureter |
| Calyces | Around pyramids | Minor (collect urine) and major calyces |
Number: ~1 million nephrons per kidney
Parts of Nephron:
Renal Corpuscle (Malpighian Body)
Renal Tubule
| Feature | Cortical Nephrons | Juxtamedullary Nephrons |
|---|---|---|
| Location | Mostly in cortex | Close to medulla |
| Percentage | 80-85% | 15-20% |
| Loop of Henle | Short | Long (deep into medulla) |
| Vasa recta | Absent/rudimentary | Well-developed |
| Function | General filtration | Urine concentration |
Process: Blood filtered through glomerular capillaries into Bowman's capsule
Filtration Membrane (3 layers):
What Gets Filtered:
What is NOT Filtered:
Glomerular Filtration Rate (GFR):
Net Filtration Pressure (NFP): NFP = Glomerular Hydrostatic Pressure - (Capsular Pressure + Colloidal Osmotic Pressure) NFP = 55 - (15 + 30) = 10 mm Hg
Location and Substances Reabsorbed:
| Segment | Substances Reabsorbed | Mechanism |
|---|---|---|
| PCT | 70-80% water, all glucose, all amino acids, Na⁺, Cl⁻, K⁺, HCO₃⁻ | Active and passive |
| Descending LoH | Water | Osmosis |
| Ascending LoH | Na⁺, Cl⁻, K⁺ | Active (thick), Passive (thin) |
| DCT | Na⁺, Ca²⁺, water (regulated) | Active, hormone-dependent |
| Collecting Duct | Water, urea | ADH-dependent |
NEET Important:
Process: Active transport of substances from blood into tubular fluid
| Substance | Location | Significance |
|---|---|---|
| H⁺ ions | PCT, DCT | Acid-base balance |
| K⁺ ions | DCT, Collecting duct | K⁺ regulation |
| Creatinine | PCT | Waste removal |
| Drugs, toxins | PCT | Detoxification |
Creates osmotic gradient in medulla for urine concentration.
Loop of Henle:
Vasa Recta:
Result: Urine can be concentrated up to 4× plasma concentration
| Factor | ADH Level | Effect on DCT/CD | Urine Volume | Urine Concentration |
|---|---|---|---|---|
| Dehydration | ↑ | ↑ Water reabsorption | ↓ | ↑ (concentrated) |
| Over-hydration | ↓ | ↓ Water reabsorption | ↑ | ↓ (dilute) |
Source: Hypothalamus (made), Posterior pituitary (released)
| Factor | Aldosterone | Effect | Result |
|---|---|---|---|
| Low Na⁺/High K⁺ | ↑ | ↑ Na⁺ reabsorption, ↑ K⁺ secretion | ↑ Blood volume, ↓ K⁺ |
| High Na⁺/Low K⁺ | ↓ | Opposite | ↓ Blood volume |
Source: Adrenal cortex
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Trigger: Low blood pressure/volume detected by JGA
Pathway:
Location: Where DCT contacts afferent arteriole
Components:
| Component | Concentration |
|---|---|
| Water | 95% |
| Urea | 2% |
| Creatinine | 0.1% |
| Uric acid | 0.03% |
| Na⁺, K⁺, Cl⁻ | Variable |
| pH | 4.5-8.0 (average 6.0) |
| Condition | Cause | Component Found |
|---|---|---|
| Glycosuria | Diabetes mellitus | Glucose |
| Proteinuria | Kidney damage | Protein |
| Ketonuria | Diabetes, starvation | Ketone bodies |
| Hematuria | Infection, injury | Blood |
Volume for urge: ~300 mL Maximum capacity: ~700-800 mL
| Disorder | Cause | Characteristics |
|---|---|---|
| Uremia | Kidney failure | Urea accumulation in blood |
| Renal calculi | Calcium/uric acid stones | Kidney stones |
| Glomerulonephritis | Inflammation of glomeruli | Protein in urine |
| Renal failure | Multiple causes | Inability to filter blood |
| Diabetes insipidus | ADH deficiency | Large volume of dilute urine |
Q1 (NEET 2023): GFR (Glomerular Filtration Rate) is approximately:
Q2 (NEET 2022): Which hormone increases water reabsorption in collecting ducts?
Q3 (NEET 2021): The ascending limb of loop of Henle is:
Q4 (NEET 2020): Which part of nephron is responsible for concentration of urine?
Q5 (NEET 2019): Uricotelic animals are:
Q: Why do we produce more urine in winter than summer? A: In winter, we sweat less due to lower temperatures. To maintain water balance, excess water is excreted through kidneys. Also, cold causes peripheral vasoconstriction, increasing blood flow to kidneys.
Q: How does diabetes mellitus cause glycosuria? A: In diabetes, blood glucose levels exceed the renal threshold (~180 mg/dL). The PCT cannot reabsorb all glucose, so excess glucose appears in urine.
Q: Why is the ascending limb of loop of Henle impermeable to water? A: This is crucial for the countercurrent mechanism. The thick ascending limb actively pumps out NaCl without water following, creating the osmotic gradient in the medulla needed for urine concentration.
Q: What happens if both kidneys fail? A: Complete kidney failure leads to uremia (toxic waste accumulation), electrolyte imbalance, and death without treatment. Hemodialysis or kidney transplant is required for survival.
Q: Why is the loop of Henle longer in juxtamedullary nephrons? A: Longer loops extend deeper into the medulla, creating a steeper osmotic gradient. This allows for greater urine concentration, important for water conservation.
This chapter rewards a diagram-first approach more than most — the steps of urine formation are easiest to hold in memory once you can see them in order. Message us on WhatsApp at +91 88264 44334 with where the confusion is (structure, urine formation, or regulation), and we'll send a diagram-first breakdown built for how NEET tests it.
Related reading: Body Fluids and Circulation Notes · Human Physiology Complete Notes · Class 11 Biology Coaching
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