Class 11 Biology Notes Chapter 19 (Excretory products and their elimination) – Biology Book

Alright class, let's begin our detailed study of Chapter 19: Excretory Products and their Elimination. This chapter is crucial for understanding how organisms, particularly humans, maintain internal balance by removing metabolic wastes. Pay close attention, as these concepts frequently appear in government examinations.

Chapter 19: Excretory Products and their Elimination - Detailed Notes

1. Introduction to Excretion

• Definition: The process of elimination of metabolic waste products from the animal body to regulate the composition of body fluids and tissues (maintaining homeostasis, specifically osmoregulation and removal of nitrogenous wastes).
• Metabolic Wastes: Primarily nitrogenous wastes like ammonia, urea, and uric acid. Other wastes include excess salts, water, CO2, ions (Na+, K+, Cl-), creatinine, etc.

2. Major Nitrogenous Wastes & Modes of Excretion

Based on the primary nitrogenous waste product, animals are categorized as:

• a) Ammonotelism (Excrete Ammonia):

  • Waste Product: Ammonia (NH3).
  • Toxicity: Highly toxic.
  • Solubility: Highly soluble in water.
  • Water Requirement: Requires a large amount of water for elimination.
  • Process: Ammonia readily diffuses across body surfaces or gill surfaces (in fish) as ammonium ions (NH4+). Kidneys have a minimal role in its excretion.
  • Examples: Many bony fishes (teleosts), aquatic amphibians (tadpoles), aquatic insects. Organisms exhibiting ammonotelism are called ammonotelic.

• b) Ureotelism (Excrete Urea):

  • Waste Product: Urea (CO(NH2)2).
  • Toxicity: Less toxic than ammonia.
  • Solubility: Soluble in water.
  • Water Requirement: Requires moderate amount of water for elimination (conserves water compared to ammonotelism).
  • Process: Ammonia produced by metabolism is converted into urea in the liver (Urea Cycle/Ornithine Cycle) and released into the blood. Kidneys filter and excrete urea. Some urea is retained in the kidney matrix of some animals to maintain desired osmolarity.
  • Examples: Mammals (including humans), many terrestrial amphibians (frogs), marine fishes, cartilaginous fishes (sharks). Organisms exhibiting ureotelism are called ureotelic.

• c) Uricotelism (Excrete Uric Acid):

  • Waste Product: Uric acid.
  • Toxicity: Least toxic.
  • Solubility: Nearly insoluble in water.
  • Water Requirement: Requires very little water for elimination (maximum water conservation).
  • Process: Excreted as a paste or pellet (semi-solid).
  • Examples: Reptiles, birds, land snails, insects. Organisms exhibiting uricotelism are called uricotelic.

3. Survey of Excretory Structures in the Animal Kingdom

• Protonephridia (Flame Cells): Platyhelminthes (e.g., Planaria), Rotifers, some Annelids, Cephalochordates (e.g., Amphioxus). Primarily concerned with osmoregulation (ionic and fluid volume regulation).
• Nephridia: Earthworms and other annelids. Tubular structures involved in both excretion and osmoregulation.
• Malpighian Tubules: Most insects (e.g., Cockroach). Tubular structures opening into the gut; involved in excretion and osmoregulation by removing wastes from hemolymph. Uricotelic excretion.
• Antennal Glands (Green Glands): Crustaceans (e.g., Prawns). Perform excretory functions.
• Kidneys: Vertebrates. The primary excretory and osmoregulatory organs.

4. Human Excretory System

Consists of:

• A pair of Kidneys

• A pair of Ureters

• A Urinary Bladder

• A Urethra

• a) Kidneys:

  • Location: Retroperitoneal (behind the peritoneum) in the abdominal cavity, between levels of the last thoracic (T12) and third lumbar (L3) vertebra. Right kidney is slightly lower than the left.
  • Shape & Size: Bean-shaped; 10-12 cm length, 5-7 cm width, 2-3 cm thickness.
  • Hilum: A notch on the inner concave surface through which the ureter, blood vessels (renal artery enters, renal vein leaves), and nerves enter/exit.
  • Internal Structure:
    • Renal Capsule: Tough, fibrous outer covering.
    • Cortex: Outer zone, granular appearance. Contains Malpighian corpuscles, PCT, DCT.
    • Medulla: Inner zone, divided into conical masses called Medullary Pyramids projecting into the calyces (singular: calyx). Contains Loops of Henle and Collecting Ducts. The cortex extends between pyramids as Columns of Bertini.
    • Renal Pelvis: Funnel-shaped space inner to the hilum, continuous with the ureter. Calyces open into the pelvis.

• b) Nephron:

  • Definition: Structural and functional unit of the kidney. Each kidney has nearly one million nephrons.
  • Parts:
    • Glomerulus: A tuft of capillaries formed by the afferent arteriole (a fine branch of the renal artery). Blood leaves via the efferent arteriole.
    • Bowman's Capsule: A double-walled cup-shaped structure enclosing the glomerulus.
    • Malpighian Body / Renal Corpuscle: Glomerulus + Bowman's Capsule.
    • Renal Tubule: Continues from Bowman's capsule. Consists of:
      • Proximal Convoluted Tubule (PCT): Highly coiled region in the cortex. Lined by simple cuboidal brush border epithelium (microvilli increase surface area for reabsorption).
      • Loop of Henle (Henle's Loop): Hairpin-shaped loop extending into the medulla. Has a descending limb and an ascending limb. The ascending limb has thin and thick segments.
      • Distal Convoluted Tubule (DCT): Another coiled region, mostly in the cortex. Opens into the collecting duct.
      • Collecting Duct: Straight tube receiving filtrate from many nephrons. Runs through the medulla (pyramids), converges and opens into the renal pelvis through medullary pyramids in the calyces (at the papillae).
  • Types of Nephrons:
    • Cortical Nephrons (approx. 85%): Loop of Henle is short and extends only very little into the medulla. Primarily in the cortex.
    • Juxtamedullary Nephrons (approx. 15%): Loop of Henle is very long and runs deep into the medulla. Crucial for concentrating urine.
  • Vasa Recta: A fine capillary network running parallel to the Henle's loop, arising from the efferent arteriole of juxtamedullary nephrons. Absent or highly reduced in cortical nephrons. Plays a significant role in the counter-current mechanism.

• c) Ureters: Tubes emerging from the renal pelvis of each kidney, carrying urine to the bladder via peristalsis.

• d) Urinary Bladder: Muscular sac storing urine temporarily. Lined by transitional epithelium (urothelium).

• e) Urethra: Tube arising from the bladder, expelling urine. Longer in males (passes through the penis) than females. Opening guarded by urethral sphincters (internal - involuntary; external - voluntary).

5. Urine Formation

Involves three main processes occurring in different parts of the nephron:

• a) Glomerular Filtration (Ultrafiltration):

  • Location: Malpighian Corpuscle (Glomerulus & Bowman's Capsule).
  • Process: Filtration of blood under pressure from the glomerulus into Bowman's capsule. It's non-selective except for large proteins and cells.
  • Filtration Membrane: Three layers:
    1. Endothelium of glomerular blood vessels (fenestrated).
    2. Basement membrane between endothelium and epithelium of Bowman's capsule.
    3. Epithelium of Bowman's capsule (Podocytes with filtration slits or slit pores).
  • Glomerular Filtration Rate (GFR): Volume of filtrate formed per minute by both kidneys. Normal GFR ≈ 125 mL/min or 180 Liters/day.
  • Effective Filtration Pressure (EFP): Glomerular Blood Hydrostatic Pressure (GBHP) - [Capsular Hydrostatic Pressure (CHP) + Blood Colloid Osmotic Pressure (BCOP)].

• b) Tubular Reabsorption:

  • Process: Selective movement of substances from the filtrate back into the blood of the peritubular capillaries/vasa recta. About 99% of the filtrate is reabsorbed.
  • Mechanisms: Active transport (e.g., glucose, amino acids, Na+) and passive transport (e.g., water via osmosis, some urea, Cl-).
  • Sites & Substances:
    • PCT: Major site (70-80% electrolytes & water reabsorbed). All essential nutrients (glucose, amino acids), most Na+, K+, Cl-, HCO3-, water. Secretes H+, NH3.
    • Loop of Henle (Descending Limb): Permeable to water, almost impermeable to electrolytes. Filtrate becomes hypertonic.
    • Loop of Henle (Ascending Limb): Impermeable to water, permeable to electrolytes (passive in thin segment, active transport of NaCl in thick segment). Filtrate becomes hypotonic.
    • DCT: Conditional reabsorption of Na+ and water (under hormonal control - Aldosterone, ADH). Reabsorption of HCO3-. Selective secretion of H+, K+, NH3 to maintain pH and ionic balance.
    • Collecting Duct: Extends from cortex to inner medulla. Large amount of water reabsorption (under ADH influence) to produce concentrated urine. Allows passage of small amounts of urea into medullary interstitium to maintain osmolarity. Also secretes H+ and K+.

• c) Tubular Secretion:

  • Process: Selective movement of substances (like H+, K+, ammonia, creatinine, certain drugs) from the blood of peritubular capillaries into the filtrate.
  • Importance: Maintains ionic balance, acid-base balance (pH) of body fluids, and eliminates waste products/drugs not filtered effectively.
  • Sites: PCT, DCT, Collecting Duct.

6. Mechanism of Concentration of Filtrate (Counter-Current Mechanism)

• Goal: To produce urine significantly more concentrated than the initial filtrate (and blood plasma). Essential for water conservation in mammals.
• Key Players: Loop of Henle and Vasa Recta.
• Principle: Counter-current flow (fluid flowing in opposite directions) in the two limbs of the Loop of Henle and the Vasa Recta, coupled with selective permeability and transport, creates and maintains an osmotic gradient in the medullary interstitium.
• Gradient: Osmolarity increases from cortex (≈300 mOsmol/L) towards the inner medulla (≈1200 mOsmol/L). This gradient is primarily due to NaCl and Urea.
• Process:
  1. NaCl Transport: Actively transported out of the ascending limb of Henle's loop into the interstitium.
  2. Urea: Small amounts diffuse out of the collecting duct into the inner medullary interstitium.
  3. Water Movement: The high osmolarity in the interstitium draws water out of the descending limb of Henle's loop and the collecting duct (facilitated by ADH).
  4. Vasa Recta Role: The counter-current flow in vasa recta allows blood to supply nutrients to the medulla without washing away the osmotic gradient. It picks up NaCl and urea in its descending limb and releases them back in its ascending limb, while gaining water in the ascending limb.
• Result: Allows significant water reabsorption from the collecting duct, leading to concentrated urine.

7. Regulation of Kidney Function

• a) Hormonal Control:

  • Antidiuretic Hormone (ADH) / Vasopressin:
    • Source: Synthesized by Hypothalamus, released by Posterior Pituitary.
    • Stimulus: Increased blood osmolarity, decreased blood volume/pressure (detected by osmoreceptors in the hypothalamus).
    • Action: Increases permeability of DCT and Collecting Duct to water, leading to increased water reabsorption, decreased urine output (antidiuresis), and concentrated urine. Also causes vasoconstriction (hence 'vasopressin').
    • Inhibition: Decreased osmolarity, high fluid intake suppresses ADH release, leading to dilute urine.
  • Renin-Angiotensin-Aldosterone System (RAAS):
    • Stimulus: Fall in Glomerular Blood Flow / Glomerular Blood Pressure / GFR.
    • Mechanism: Juxtaglomerular Apparatus (JGA) - a special sensitive region formed by cellular modifications in the DCT and the afferent arteriole at their contact location - releases Renin.
    • Renin converts Angiotensinogen (from liver) in blood to Angiotensin I.
    • Angiotensin I is converted to Angiotensin II by Angiotensin Converting Enzyme (ACE), mainly in the lungs.
    • Angiotensin II Actions:
      • Potent vasoconstrictor (increases Glomerular Blood Pressure and GFR).
      • Stimulates Adrenal Cortex to release Aldosterone.
    • Aldosterone Action: Acts on DCT and Collecting Duct to increase reabsorption of Na+ and water (water follows Na+ osmotically), leading to increased blood volume and pressure. Also increases K+ secretion.
  • Atrial Natriuretic Factor (ANF):
    • Source: Walls of the Atria of the heart.
    • Stimulus: Increase in blood volume and pressure to the atria.
    • Action: Causes vasodilation (decreases blood pressure). Inhibits Renin release (checks RAAS mechanism). Inhibits Na+ reabsorption from collecting duct. Decreases Aldosterone and ADH release. Overall effect: increases Na+ excretion and urine output, lowers blood volume and pressure.

• b) Neural Control: Primarily via sympathetic nerves affecting renal blood flow and renin secretion.

8. Micturition (Urination)

• Process: Expulsion of urine from the urinary bladder.
• Mechanism:
  1. Bladder fills with urine, stretch receptors in its walls send signals to the Central Nervous System (CNS).
  2. CNS sends motor messages initiating contraction of smooth muscles of the bladder and simultaneous relaxation of the urethral sphincter.
  3. Relaxation of the external urethral sphincter (voluntary control) leads to release of urine.
• Neural Control: Micturition reflex (involuntary) coordinated by the spinal cord, with voluntary control from the brain.
• Average Urine Output: 1 - 1.5 Liters per day.
• Urine Characteristics: Pale yellow colour (due to urochrome pigment), slightly acidic pH (around 6.0), characteristic odour. Presence of glucose (Glycosuria) or ketone bodies (Ketonuria) indicates conditions like Diabetes Mellitus.

9. Role of Other Organs in Excretion (Accessory Excretory Organs)

• Lungs: Eliminate large amounts of CO2 (approx. 200 mL/minute) and significant quantities of water vapour.
• Liver: Secretes bile containing substances like bilirubin, biliverdin (degraded hemoglobin products), cholesterol, degraded steroid hormones, vitamins, and drugs. Most pass out with feces. Liver also performs the Urea Cycle.
• Skin:
  • Sweat Glands: Produce sweat containing water, NaCl, small amounts of urea, lactic acid, etc. Primarily for cooling, but contributes to excretion.
  • Sebaceous Glands: Secrete sebum containing sterols, hydrocarbons, and waxes. Provides protective oily covering, eliminates some wastes.

10. Disorders of the Excretory System

• Kidney Failure (Renal Failure): Malfunctioning of kidneys leads to accumulation of urea and other wastes in the blood (Uremia). Can be fatal.
  • Treatment: Hemodialysis (Artificial Kidney). Blood drained from an artery, passed through a dialyzing unit containing a cellophane tube surrounded by dialyzing fluid (same composition as plasma except nitrogenous wastes). Wastes diffuse out, purified blood returned to a vein. Kidney Transplantation is the ultimate method.
• Renal Calculi (Kidney Stones): Stones or insoluble masses of crystallized salts (e.g., oxalates) formed within the kidney, usually in the pelvis. Cause severe pain.
• Glomerulonephritis: Inflammation of the glomeruli of the kidney, often due to immune responses or infections. Can lead to reduced filtration and kidney damage.

Multiple Choice Questions (MCQs)

1. Which of the following nitrogenous wastes requires the maximum amount of water for its elimination?
   a) Urea
   b) Uric Acid
   c) Ammonia
   d) Creatinine

2. Malpighian tubules are the excretory structures of:
   a) Platyhelminthes
   b) Annelids
   c) Insects
   d) Crustaceans

3. The functional unit of the human kidney is the:
   a) Neuron
   b) Nephron
   c) Alveolus
   d) Renal Pyramid

4. Glomerular filtration occurs in the:
   a) Loop of Henle
   b) Proximal Convoluted Tubule (PCT)
   c) Distal Convoluted Tubule (DCT)
   d) Malpighian Corpuscle

5. Which part of the nephron is largely impermeable to water but permeable to electrolytes?
   a) Proximal Convoluted Tubule (PCT)
   b) Descending limb of Loop of Henle
   c) Ascending limb of Loop of Henle
   d) Collecting Duct

6. The counter-current mechanism primarily involves the interaction between:
   a) PCT and DCT
   b) Loop of Henle and Vasa Recta
   c) Glomerulus and Bowman's Capsule
   d) Collecting Duct and Renal Pelvis

7. Which hormone is responsible for increasing water reabsorption in the DCT and Collecting Duct?
   a) Aldosterone
   b) ADH (Vasopressin)
   c) ANF (Atrial Natriuretic Factor)
   d) Renin

8. The Juxtaglomerular Apparatus (JGA) releases Renin in response to:
   a) Increased blood pressure in glomerulus
   b) Decreased blood osmolarity
   c) A fall in Glomerular Filtration Rate (GFR)
   d) Increased Na+ concentration in DCT

9. The presence of glucose in urine (Glycosuria) is indicative of:
   a) Renal Calculi
   b) Glomerulonephritis
   c) Uremia
   d) Diabetes Mellitus

10. Which of the following structures is primarily responsible for maintaining the medullary osmotic gradient by allowing passage of small amounts of urea into the interstitium?
    a) Proximal Convoluted Tubule
    b) Loop of Henle
    c) Distal Convoluted Tubule
    d) Collecting Duct

Answer Key for MCQs:

1. c) Ammonia
2. c) Insects
3. b) Nephron
4. d) Malpighian Corpuscle
5. c) Ascending limb of Loop of Henle
6. b) Loop of Henle and Vasa Recta
7. b) ADH (Vasopressin)
8. c) A fall in Glomerular Filtration Rate (GFR)
9. d) Diabetes Mellitus
10. d) Collecting Duct

Revise these notes thoroughly. Understanding the processes, structures, and regulatory mechanisms is key. Good luck with your preparation!