Excretory Products & Their Elimination
NCERT Line-by-Line Breakdown for NEET 2026
Unit 5: Human PhysiologyAnimals accumulate ammonia, urea, uric acid, carbon dioxide, water, and ions like Na+, K+, Cl-, etc., due to metabolic activities. The process of removing these nitrogenous wastes is called Excretion. [cite_start]In this chapter, we explore how the Kidney acts as the master chemist of the body[cite: 341].
1. Modes of Excretion
[cite_start]Depending on the nature of the primary nitrogenous waste, animals are grouped into three categories[cite: 341].
| Mode | Waste Product | Toxicity & Water Need | Examples |
|---|---|---|---|
| Ammonotelism | Ammonia | Highest Toxicity. Needs massive water (Diffusion). | Bony fishes, Aquatic Amphibians, Aquatic Insects. |
| Ureotelism | Urea | Moderate Toxicity. Needs moderate water. | Mammals, Terrestrial Amphibians, Marine fishes. |
| Uricotelism | Uric Acid | Least Toxicity. Minimum water (Paste/Pellet). | Reptiles, Birds, Land Snails, Insects. |
2. Human Excretory System
[cite_start]Consists of a pair of kidneys, pair of ureters, a urinary bladder, and a urethra[cite: 341].
A. Structure of Kidney
- Location: Last thoracic and 3rd lumbar vertebra (Retroperitoneal).
- Hilum: Notch where ureter, blood vessels, and nerves enter.
- Zones: Outer Cortex and Inner Medulla (divided into medullary pyramids).
- Columns of Bertini: Cortex extending between medullary pyramids.
B. The Nephron (Functional Unit)
Each kidney has ~1 million nephrons. Two main parts: Glomerulus and Renal Tubule.
[Image of nephron structure]- Cortical Nephrons (85%): Short Loop of Henle. Vasa recta absent/reduced.
- Juxtamedullary Nephrons (15%): Long Loop of Henle (runs deep into medulla). Essential for concentrating urine.
3. Urine Formation
[cite_start]Involves three main processes taking place in different parts of the nephron[cite: 341].
Step 1: Glomerular Filtration
Non-selective filtration of blood.
GFR (Glomerular Filtration Rate): ~125 ml/min or 180 Liters/day.
Step 2: Reabsorption
99% of filtrate is reabsorbed. (Urine output is only 1.5 L/day).
- Active: Glucose, Amino acids, Na+.
- Passive: Nitrogenous wastes, Water (in initial segments).
Step 3: Secretion
Tubular cells secrete H+, K+, and Ammonia into the filtrate to maintain Ionic and Acid-Base balance.
Function of Tubules (Summary)
| Part | Function |
|---|---|
| PCT (Proximal Convoluted Tubule) | 70-80% Electrolytes & Water reabsorbed. Lined by simple cuboidal brush border epithelium. |
| Henle’s Loop | Descending: Permeable to water. Ascending: Impermeable to water, permeable to electrolytes. |
| DCT (Distal Convoluted Tubule) | Conditional reabsorption of Na+ and Water (under hormone control). |
| Collecting Duct | Large amount of water reabsorbed. Allows passage of small amount of Urea into medulla to maintain osmolarity. |
4. Regulation of Kidney Function
[cite_start]Regulated by hormonal feedback mechanisms involving the Hypothalamus, JGA, and Heart[cite: 342].
Released by Neurohypophysis when osmoreceptors are activated (body fluid loss). Facilitates water reabsorption from DCT/Collecting duct.
Deficiency leads to Diabetes Insipidus (Diuresis).
Fall in GFR activates JG cells to release Renin. Renin converts Angiotensinogen → Angiotensin I → II.
Angiotensin II: Powerful vasoconstrictor (Increases BP) and stimulates Adrenal Cortex to release Aldosterone (Reabsorption of Na+).
Released by Heart Atria in response to high BP. Causes Vasodilation. Antagonistic to RAAS.
5. Disorders & Dialysis
- Uremia: Accumulation of Urea in blood. [cite_start]Leads to kidney failure[cite: 343]. [cite_start]
- Renal Calculi: Stone or insoluble mass of crystallized salts (Oxalates)[cite: 343]. [cite_start]
- Glomerulonephritis: Inflammation of glomeruli[cite: 343].
🏥 Haemodialysis (Artificial Kidney)
Used for Uremic patients. Blood pumped from artery into a dialysing unit containing coiled cellophane tube surrounded by dialysing fluid (same composition as plasma except nitrogenous wastes).
Mechanism: Wastes move out by concentration gradient. Cleared blood pumped back to vein with Anti-heparin.
📝 Rapid Fire MCQs
Q1. Which part of the nephron is impermeable to water but permeable to electrolytes?
- A) Descending limb of Henle’s loop
- B) Ascending limb of Henle’s loop
- C) PCT
- D) DCT
Click to check Answer
Answer: B) Ascending limb of Henle’s loop.
Q2. Presence of Glucose (Glycosuria) and Ketone bodies (Ketonuria) in urine are indicative of:
- A) Diabetes insipidus
- B) Diabetes mellitus
- C) Renal failure
- D) Glomerulonephritis
Click to check Answer
Answer: B) Diabetes mellitus.