Introduction

Excretion is the process of removing metabolic waste products from the body to maintain homeostasis. NEET emphasizes types of excretory products, kidney structure, nephron function, urine formation, and associated formulas.

Understanding this topic is crucial for human physiology, MCQs, and diagram-based questions in NEET.

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1. Excretory Products

A. Ammonia (NH₃):

• Produced during deamination of amino acids

• Highly toxic → Requires large water for excretion

• Excreted by aquatic animals

B. Urea (CO(NH₂)₂):

• Formed in the liver via urea cycle

• Less toxic than ammonia, soluble in water

• Excreted by mammals (humans, dogs)

C. Uric Acid (C₅H₄N₄O₃):

• Formed from purine metabolism

• Low solubility → Excreted as paste or crystals

• Excreted by birds, reptiles, insects

D. Creatinine:

• Produced from muscle metabolism

• Excreted by kidney

NEET Tip:

• Remember ammonotelic → aquatic, ureotelic → mammals, uricotelic → birds/reptiles

2. Human Excretory System

Major Organs:

1. Kidneys → Filter blood, produce urine

2. Ureters → Transport urine to bladder

3. Urinary bladder → Stores urine

4. Urethra → Excretes urine outside

Kidney Structure:

• Outer cortex, inner medulla

• Functional unit → Nephron (~1 million/kidney)

• Types of nephron: Cortical and Juxtamedullary

3. Nephron and Its Parts

1. Bowman’s Capsule: Surrounds glomerulus, site of filtration

2. Proximal Convoluted Tubule (PCT): Reabsorption of glucose, amino acids, Na⁺, water

3. Loop of Henle: Concentrates urine → Descending limb (water reabsorption), Ascending limb (Na⁺, Cl⁻ reabsorption)

4. Distal Convoluted Tubule (DCT): Reabsorption of Na⁺, K⁺, H⁺ under hormonal control

5. Collecting Duct: Final concentration of urine

NEET Tip:

• Flow: Glomerulus → Bowman’s Capsule → PCT → Loop of Henle → DCT → Collecting Duct

4. Urine Formation

Three main processes:

1. Glomerular Filtration:

• Passive, pressure-driven

• Filtrate contains water, salts, glucose, urea

2. Tubular Reabsorption:

• Useful substances reabsorbed into blood

• Example: 100% glucose, 99% water

3. Tubular Secretion:

• Active transport of ions & toxins into tubule

• Examples: H⁺, K⁺, NH₄⁺, creatinine

NEET Formulas / Facts:

• Glomerular Filtration Rate (GFR): ~125 mL/min in humans

• Filtration Fraction (FF) = GFR / Renal Plasma Flow (RPF)

5. Role of Hormones in Excretion

1. ADH (Antidiuretic Hormone):

   • Secreted by posterior pituitary

   • Increases water reabsorption in collecting duct

2. Aldosterone:

   • Secreted by adrenal cortex

   • Increases Na⁺ reabsorption and K⁺ excretion

3. Renin-Angiotensin System:

   • Regulates blood pressure and GFR

6. Osmoregulation

• Definition: Regulation of water and salt balance in body

• Achieved by kidneys through urine concentration

• Aquatic vs Terrestrial adaptations:

  • Marine fish → Secrete small amount of concentrated urine

  • Freshwater fish → Excrete large volume of dilute urine

  • Desert animals → Highly concentrated urine

NEET Tip:

• NEET may ask adaptations of kidney in different habitats

7. Excretory Disorders

1. Kidney Stone (Nephrolithiasis): Formation of crystals → Obstruction

2. Urinary Tract Infection (UTI): Bacterial infection

3. Renal Failure: Loss of kidney function → Dialysis required

4. Glomerulonephritis: Inflammation of glomerulus → Proteinuria

NEET Tip:

• Understand cause → effect → treatment

8. NEET Important Formulas / Memory Aids

1. GFR (Glomerular Filtration Rate) = Filtrate volume / min (~125 mL/min)

2. Filtration Fraction = GFR / Renal Plasma Flow

3. Osmoregulation → Water reabsorption regulated by ADH & Aldosterone

Mnemonic for excretory organs:

• “KUBU” → Kidney, Ureter, Bladder, Urethra

9. NEET Exam Tips

• Focus on types of excretory products and animals

• Memorize nephron structure and urine formation steps

• Understand hormonal regulation of kidney

• Diagram-based questions → Kidney, nephron, urine flow, GFR

• Prepare excretory adaptations and disorders

Conclusion

Excretory Products and Their Elimination is a high-yield NEET topic. Mastery of blood urea, uric acid, ammonia, kidney and nephron structure, urine formation, hormonal control, osmoregulation, and excretory disorders is essential for scoring in MCQs, numerical, and diagram-based questions.

With this StudentBro.in guide, students can efficiently revise formulas, diagrams, and physiology concepts, enhancing speed and accuracy during NEET exams.

Remember: Excretion = Excretory Products + Kidney + Nephron + Urine Formation + Hormonal Control + Osmoregulation + Disorders → Core for human physiology questions.