Excretion is the process of removing metabolic waste products from the body. This chapter explores different nitrogenous wastes produced by animals and the specialized excretory organs that have evolved to eliminate them. It focuses on the human urinary system, detailing the structure of the kidneys and the processes of urine formation through filtration, reabsorption, and secretion in the nephrons. The chapter also examines osmoregulation and how the excretory system helps maintain water-salt balance in the body.
Chapter 8: Excretory Products and their Elimination
Introduction to Excretion
Excretion is the removal of metabolic waste products from the body. These waste products include:
- Nitrogenous wastes from protein metabolism
- CO₂ from cellular respiration
- Excess water and salts
- Bile pigments from breakdown of hemoglobin
Modes of Excretion
Types of Nitrogenous Wastes
- Ammonia (NH₃): Highly toxic, requires large amount of water for excretion
- Ammonotelism: Excretion of ammonia (aquatic animals like most fishes)
- Urea (NH₂CONH₂): Less toxic, requires moderate amount of water
- Ureotelism: Excretion of urea (mammals, most amphibians)
- Uric Acid: Least toxic, requires minimal water
- Uricotelism: Excretion of uric acid (birds, reptiles, insects)
Human Excretory System
Components of Excretory System
- Kidneys: Main excretory organs
- Ureters: Transport urine from kidneys to bladder
- Urinary Bladder: Temporary storage of urine
- Urethra: Passage for urine elimination
Kidney Structure
- Location: Posterior abdominal wall, one on each side of vertebral column
- Size: 10-12 cm long, 5-7 cm wide, 2-3 cm thick
- Weight: About 120-170 g each
- External Structure:
- Bean-shaped with concave hilum
- Covered by fibrous capsule
- Blood vessels and ureter enter/exit at hilum
- Internal Structure:
- Cortex: Outer granular layer
- Medulla: Inner striated layer with pyramids
- Pelvis: Funnel-shaped structure that collects urine
- Calyces: Cup-like structures that receive urine from collecting ducts
Nephron – Structural and Functional Unit of Kidney
Each kidney contains about 1-1.3 million nephrons.
Structure of Nephron:
- Malpighian Corpuscle:
- Bowman’s Capsule: Double-walled cup
- Glomerulus: Tuft of capillaries
- Renal Tubule:
- Proximal Convoluted Tubule (PCT): Highly convoluted part near glomerulus
- Loop of Henle: U-shaped with descending and ascending limbs
- Distal Convoluted Tubule (DCT): Convoluted part after loop of Henle
- Collecting Duct: Receives urine from many nephrons
Types of Nephrons:
- Cortical Nephrons (85%): Located in cortex, short loop of Henle
- Juxtamedullary Nephrons (15%): Near medulla, long loop of Henle
Urine Formation
Urine formation involves three main processes:
1. Glomerular Filtration
- Blood pressure forces water and small molecules from blood into Bowman’s capsule
- Filtration Rate: About 125 ml/minute or 180 liters/day
- Glomerular Filtrate: Contains water, glucose, amino acids, salts, urea, uric acid
- Non-filtered: Blood cells, proteins, lipids
2. Tubular Reabsorption
- Selective reabsorption of useful substances from filtrate back into blood
- PCT: Reabsorbs glucose, amino acids, Na⁺, K⁺, HCO₃⁻, water (65-70%)
- Loop of Henle: Reabsorbs Na⁺, Cl⁻, creates concentration gradient
- DCT: Reabsorbs Na⁺, HCO₃⁻, water (under ADH control)
- Collecting Duct: Reabsorbs water (under ADH control)
3. Tubular Secretion
- Active secretion of substances from blood into tubule
- Secreted substances: H⁺, K⁺, NH₄⁺, creatinine, drugs
- Helps maintain pH and electrolyte balance
Concentration of Urine – Countercurrent Mechanism
- Loop of Henle and vasa recta create concentration gradient in medulla
- Ascending limb pumps salt out but is impermeable to water
- Descending limb allows water to exit but not salt
- This creates hyperosmotic medullary interstitium
- Collecting ducts pass through this region, allowing water reabsorption
Regulation of Kidney Function
Role of ADH (Antidiuretic Hormone)
- Released from posterior pituitary in response to dehydration
- Acts on collecting ducts to increase water reabsorption
- Reduces urine volume, increases concentration
Renin-Angiotensin System
- Regulates blood pressure and sodium balance
- Juxtaglomerular apparatus releases renin when blood pressure drops
- Renin converts angiotensinogen to angiotensin I
- Angiotensin I is converted to angiotensin II
- Angiotensin II:
- Constricts blood vessels (increases blood pressure)
- Stimulates aldosterone secretion (increases Na⁺ reabsorption)
Atrial Natriuretic Factor (ANF)
- Released from atria when blood pressure increases
- Increases Na⁺ and water excretion
- Decreases blood pressure
Micturition
- Process of urine elimination from bladder
- Involves both voluntary and involuntary controls
- Stretch receptors in bladder trigger micturition reflex
- Can be voluntarily controlled by external urethral sphincter
Disorders of Excretory System
- Uremia: Accumulation of urea in blood due to kidney malfunction
- Renal Calculi: Kidney stones formed from mineral salts
- Glomerulonephritis: Inflammation of glomeruli
- Renal Failure: Kidneys fail to filter waste products
- Treatment: Dialysis, kidney transplantation
Dialysis
- Artificial method to filter blood when kidneys fail
- Hemodialysis: Blood is filtered through an artificial kidney machine
- Peritoneal Dialysis: Peritoneal cavity acts as dialyzing surface
Complete Chapter-wise Hsslive Plus One Zoology Notes
Our HSSLive Plus One Zoology Notes cover all chapters with key focus areas to help you organize your study effectively:
- Chapter 1 The Living World
- Chapter 2 Animal Kingdom
- Chapter 3 Structural Organisation in Animals
- Chapter 4 Biomolecules
- Chapter 5 Digestion and Absorption
- Chapter 6 Breathing and Exchange of Gases
- Chapter 7 Body Fluids and Circulation
- Chapter 8 Excretory Products and their Elimination
- Chapter 9 Locomotion and Movement
- Chapter 10 Neural Control and Coordination
- Chapter 11 Chemical Coordination and integration