Excretory Products and Their Elimination - Complete NEET Biology Notes

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.

Introduction to Excretion

Definition: Removal of metabolic waste products from the body.

Types of Nitrogenous Waste

Classification Based on Excretion

NEET Important: Humans are primarily ureotelic but also excrete some ammonia and uric acid.

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Excretory Organs in Different Animals

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Human Excretory System

Components

1. Kidneys (2) - Main excretory organs
2. Ureters (2) - Transport urine to bladder
3. Urinary bladder (1) - Stores urine
4. Urethra (1) - Expels urine from body

Kidney - Structure

Location: Retroperitoneal, between T12-L3 vertebrae

Size: ~11 cm × 6 cm × 3 cm, ~150 g each

External Structure:

• Bean-shaped
• Hilum: Concave side where renal artery, vein, and ureter connect
• Renal capsule: Protective outer covering

Internal Structure:

Nephron - Functional Unit

Number: ~1 million nephrons per kidney

Parts of Nephron:

1. Renal Corpuscle (Malpighian Body)

   • Glomerulus: Tuft of capillaries
   • Bowman's capsule: Cup-shaped structure surrounding glomerulus

2. Renal Tubule

   • Proximal Convoluted Tubule (PCT)
   • Loop of Henle (descending and ascending limbs)
   • Distal Convoluted Tubule (DCT)
   • Collecting Duct (shared by multiple nephrons)

Types of Nephrons

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Urine Formation

Three Processes

1. Glomerular Filtration
2. Tubular Reabsorption
3. Tubular Secretion

1. Glomerular Filtration

Process: Blood filtered through glomerular capillaries into Bowman's capsule

Filtration Membrane (3 layers):

1. Endothelium of glomerular capillaries (fenestrated)
2. Basement membrane
3. Podocytes of Bowman's capsule (filtration slits)

What Gets Filtered:

• Water
• Salts
• Glucose
• Amino acids
• Urea
• Creatinine

What is NOT Filtered:

• Blood cells
• Plasma proteins
• Large molecules

Glomerular Filtration Rate (GFR):

• ~125 mL/min
• ~180 L/day
• Only ~1.5 L excreted as urine (99% reabsorbed)

Net Filtration Pressure (NFP): NFP = Glomerular Hydrostatic Pressure - (Capsular Pressure + Colloidal Osmotic Pressure) NFP = 55 - (15 + 30) = 10 mm Hg

2. Tubular Reabsorption

Location and Substances Reabsorbed:

NEET Important:

• PCT reabsorbs all glucose and amino acids (100%)
• Thick ascending limb is impermeable to water
• DCT and collecting duct are regulated by hormones

3. Tubular Secretion

Process: Active transport of substances from blood into tubular fluid

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Countercurrent Mechanism

Purpose

Creates osmotic gradient in medulla for urine concentration.

How It Works

Loop of Henle:

1. Descending limb: Permeable to water, impermeable to solutes
2. Ascending limb: Impermeable to water, actively pumps out NaCl
3. Creates increasing osmolarity from cortex to medulla (300 → 1200 mOsm/L)

Vasa Recta:

• Hairpin loop of blood vessels
• Maintains gradient by countercurrent exchange
• Prevents "washing out" of medullary gradient

Result: Urine can be concentrated up to 4× plasma concentration

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Regulation of Kidney Function

Hormonal Regulation

1. Antidiuretic Hormone (ADH/Vasopressin)

Source: Hypothalamus (made), Posterior pituitary (released)

2. Aldosterone

Source: Adrenal cortex

3. Atrial Natriuretic Peptide (ANP)

• Released by atria when stretched (high blood volume)
• ↓ Na⁺ reabsorption → ↑ Na⁺ and water excretion
• Decreases blood pressure

Renin-Angiotensin-Aldosterone System (RAAS)

Trigger: Low blood pressure/volume detected by JGA

Pathway:

1. Kidney releases Renin
2. Renin converts Angiotensinogen → Angiotensin I
3. ACE (in lungs) converts Angiotensin I → Angiotensin II
4. Angiotensin II:
   • Vasoconstriction (↑ BP)
   • Stimulates aldosterone release
   • Stimulates ADH release
   • Stimulates thirst

Juxtaglomerular Apparatus (JGA)

Location: Where DCT contacts afferent arteriole

Components:

1. Juxtaglomerular cells: In afferent arteriole, secrete renin
2. Macula densa: In DCT, sense Na⁺ concentration

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Composition of Urine

Normal Urine

Abnormal Urine

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Micturition (Urination)

Neural Control

1. Bladder fills → Stretch receptors activated
2. Signals to micturition center (sacral spinal cord)
3. Parasympathetic stimulation → Detrusor muscle contracts
4. Internal sphincter relaxes (involuntary)
5. External sphincter relaxes (voluntary)
6. Urine expelled

Volume for urge: ~300 mL Maximum capacity: ~700-800 mL

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Disorders of Excretory System

Treatments

• Hemodialysis: Artificial kidney filters blood
• Kidney transplant: Replacement of failed kidney
• Peritoneal dialysis: Uses peritoneum as filter

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Previous Year NEET Questions

Q1 (NEET 2023): GFR (Glomerular Filtration Rate) is approximately:

• (a) 25 mL/min
• (b) 125 mL/min ✓
• (c) 225 mL/min
• (d) 325 mL/min

Q2 (NEET 2022): Which hormone increases water reabsorption in collecting ducts?

• (a) Aldosterone
• (b) ADH ✓
• (c) ANP
• (d) Renin

Q3 (NEET 2021): The ascending limb of loop of Henle is:

• (a) Permeable to water and salts
• (b) Impermeable to water ✓
• (c) Impermeable to salts
• (d) None of the above

Q4 (NEET 2020): Which part of nephron is responsible for concentration of urine?

• (a) PCT
• (b) DCT
• (c) Loop of Henle ✓
• (d) Glomerulus

Q5 (NEET 2019): Uricotelic animals are:

• (a) Fish
• (b) Mammals
• (c) Birds ✓
• (d) Amphibians

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Quick Revision Points

1. Functional unit of kidney: Nephron
2. Number of nephrons: ~1 million per kidney
3. GFR: 125 mL/min or 180 L/day
4. Urine output: ~1.5 L/day
5. Percentage reabsorbed: 99%
6. Glucose reabsorption: 100% in PCT
7. ADH function: ↑ Water reabsorption
8. Aldosterone function: ↑ Na⁺ reabsorption, ↑ K⁺ secretion
9. ANP function: ↑ Na⁺ excretion
10. Countercurrent multiplier: Loop of Henle
11. Countercurrent exchanger: Vasa recta
12. Maximum medullary osmolarity: 1200 mOsm/L
13. Renin source: JG cells

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FAQs

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.