Last Updated: April 2026
Excretory Products and Their Elimination is a high-weightage chapter in NEET Biology, appearing in 3–5 questions every year. In NEET 2025, this chapter contributed 4 direct questions from Class 11 Biology. This chapter is from Unit 8 (Human Physiology) and is a must-master for scoring 340+ in Biology.
Types of Nitrogenous Waste Excretion
| Type | Waste Product | Animals | Why |
|---|---|---|---|
| Ammonotelic | Ammonia (NH₃) | Aquatic invertebrates, bony fish, aquatic amphibians | Highly toxic but very soluble — needs lots of water to dilute |
| Ureotelic | Urea (CO(NH₂)₂) | Mammals, adult amphibians, marine fish | Less toxic, moderate solubility — efficient for semi-terrestrial life |
| Uricotelic | Uric acid | Reptiles, birds, insects, land snails | Least toxic, least soluble — excreted as paste, minimal water loss |
The Human Excretory System
Organs of Excretion
- Kidneys — primary excretory organ (urea, uric acid, creatinine)
- Lungs — excrete CO₂ and water vapour
- Skin — sweat (NaCl, urea, lactic acid) via sweat glands
- Liver — excretes bile pigments (bilirubin, biliverdin) via bile
Structure of the Kidney
Each kidney contains approximately 1 million nephrons — the structural and functional units. The kidney is divided into:
- Cortex — outer layer containing Malpighian corpuscles (glomerulus + Bowman’s capsule) and convoluted tubules
- Medulla — inner layer containing loops of Henle and collecting ducts arranged into medullary pyramids
- Pelvis — funnel-shaped space collecting urine from medullary pyramids → ureter
The Nephron — Detailed Structure
Parts of a Nephron
| Part | Location | Function |
|---|---|---|
| Malpighian capsule (Bowman’s capsule + Glomerulus) | Cortex | Ultrafiltration of blood |
| Proximal Convoluted Tubule (PCT) | Cortex | Reabsorption of glucose, amino acids, 70% NaCl and water; secretion of H⁺ and K⁺ |
| Loop of Henle (descending limb) | Medulla | Permeable to water (water reabsorbed); impermeable to ions |
| Loop of Henle (ascending limb) | Medulla | Impermeable to water; permeable to NaCl (Na⁺ pumped out) |
| Distal Convoluted Tubule (DCT) | Cortex | Conditional reabsorption (Na⁺, water) under hormonal control (ADH, aldosterone); secretion of H⁺, K⁺, NH₃ |
| Collecting Duct | Medulla | Reabsorption of water (under ADH); permeable to urea |
Urine Formation — Three Processes
1. Glomerular Filtration (Ultrafiltration)
- Blood pressure forces fluid from glomerular capillaries into Bowman’s capsule
- Glomerular Filtration Rate (GFR) = 125 mL/min = 180 L/day
- Filtrate contains: water, glucose, amino acids, urea, creatinine, ions — NO proteins or blood cells
- Filtration occurs due to net filtration pressure = Glomerular hydrostatic pressure − (Capsular pressure + Osmotic pressure)
2. Selective Reabsorption
| Substance | Where Reabsorbed | Mechanism |
|---|---|---|
| Glucose | PCT (completely) | Active transport (co-transport with Na⁺) |
| Amino acids | PCT | Active transport |
| Na⁺, Cl⁻ | PCT (70%), ascending LoH, DCT | Active (Na⁺/K⁺ ATPase), passive |
| Water | PCT (obligatory), DCT/collecting duct (facultative) | Osmosis; ADH-dependent in DCT/CD |
| HCO₃⁻ | PCT | Active transport |
3. Tubular Secretion
Tubular secretion removes waste that was not filtered:
- H⁺ ions — secreted in PCT and DCT (maintains blood pH)
- K⁺ ions — secreted in DCT under aldosterone control
- Creatinine — secreted in PCT
- NH₃ — secreted in DCT
Counter-Current Mechanism — Concentration of Urine
The counter-current mechanism between the loop of Henle and vasa recta creates an osmotic gradient in the medullary interstitium (from 300 mOsm/L in cortex to 1200 mOsm/L in deep medulla). This allows the collecting duct to concentrate urine under ADH influence.
- Descending loop: water exits (osmosis) → tubular fluid becomes hyperosmotic
- Ascending loop: NaCl pumped out actively → tubular fluid becomes hyposmotic
- Net result: medullary gradient maintained → concentrated urine when ADH acts on collecting duct
Hormonal Regulation of Kidney Function
| Hormone | Source | Target | Effect |
|---|---|---|---|
| ADH (Vasopressin) | Posterior pituitary | DCT and collecting duct | Increases water reabsorption → concentrated urine (antidiuresis) |
| Aldosterone | Adrenal cortex | DCT and collecting duct | Na⁺ reabsorption ↑, K⁺ secretion ↑ → water retention ↑ |
| ANF (Atrial Natriuretic Factor) | Cardiac atria | Kidney tubules | Inhibits aldosterone → Na⁺ excretion ↑ → diuresis |
| Renin | JGA cells (kidney) | Angiotensinogen | Activates angiotensin II → constricts efferent arterioles → stimulates aldosterone |
| Parathyroid hormone (PTH) | Parathyroid gland | PCT and DCT | Increases Ca²⁺ reabsorption, decreases phosphate reabsorption |
Micturition (Urination) Reflex
- Urine is stored in the urinary bladder
- When bladder volume reaches ~300–400 mL, stretch receptors send signals to micturition centre in sacral spinal cord
- Parasympathetic impulses → detrusor muscle contracts, internal sphincter relaxes → urine expelled
- Voluntary control via external sphincter (under cerebral cortex control)
Other Excretory Organs
Kidneys in Different Animals
| Animal Group | Excretory Organ | Product |
|---|---|---|
| Amoeba, Paramecium | Contractile vacuole | Ammonia |
| Platyhelminthes (Planaria) | Flame cells / protonephridia | Ammonia |
| Earthworm | Nephridia | Ammonia + urea |
| Insects | Malpighian tubules | Uric acid |
| Crustaceans | Green glands (antennal glands) | Ammonia |
Common Kidney Disorders
- Uraemia — accumulation of urea in blood due to kidney failure
- Renal calculi (kidney stones) — oxalate crystals, urate crystals
- Glomerulonephritis — inflammation of glomerulus
- Dialysis — artificial kidney; blood filtered through cellulose membrane using osmosis principle
Practice MCQs — Excretory System NEET 2027
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Frequently Asked Questions (FAQ)
What is the difference between ultrafiltration and selective reabsorption?
Ultrafiltration is the non-selective, pressure-driven process in the Bowman’s capsule where all small molecules (glucose, amino acids, urea, water, salts) are filtered from blood into the renal tubule. Selective reabsorption is the subsequent active and passive transport of useful substances (glucose, amino acids, water) back into the blood from the filtrate.
Why is the ascending limb of loop of Henle impermeable to water?
The ascending limb of the loop of Henle is impermeable to water because it lacks aquaporin water channels. This allows NaCl to be pumped out actively, reducing the osmolarity of tubular fluid while maintaining the medullary osmotic gradient — essential for the counter-current mechanism and urine concentration.
What is the role of ADH in kidney function?
ADH (Antidiuretic Hormone), also called vasopressin, is released from the posterior pituitary when blood osmolarity rises (e.g., dehydration). It acts on the distal convoluted tubule and collecting duct, increasing their permeability to water by inserting aquaporin channels. This causes more water reabsorption, producing concentrated urine and reducing blood osmolarity.
What is GFR and what is its normal value?
GFR (Glomerular Filtration Rate) is the volume of filtrate formed per minute by all nephrons combined. Normal GFR in humans is approximately 125 mL/min (about 180 L/day). Of this, 99% is reabsorbed — only about 1.5 L is excreted as urine per day.
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