Class 11 Biology Notes Chapter 19 (Chapter 19) – Examplar Problems (English) Book

Detailed Notes with MCQs of Chapter 19: Excretory Products and their Elimination. This is a crucial chapter for understanding physiological processes, and questions frequently appear in various government exams. Pay close attention to the structures, functions, and regulatory mechanisms.

Chapter 19: Excretory Products and their Elimination - Detailed Notes

1. Introduction to Excretion:

• Excretion: The process of elimination of metabolic waste products from the animal body to regulate the composition of body fluids and tissues. It's essential for maintaining homeostasis.
• Metabolic Wastes: Primarily nitrogenous wastes like ammonia, urea, and uric acid. Others include excess salts, water, CO2, and certain drugs.
• Nitrogenous Wastes:
  • Ammonia: Highly toxic, requires large amounts of water for elimination. Organisms excreting ammonia are ammonotelic (e.g., most aquatic invertebrates, bony fishes, aquatic amphibians).
  • Urea: Less toxic than ammonia, requires moderate water. Formed in the liver (Ornithine cycle). Organisms excreting urea are ureotelic (e.g., mammals, terrestrial amphibians, marine fishes).
  • Uric Acid: Least toxic, requires very little water (excreted as a paste or pellet). Organisms excreting uric acid are uricotelic (e.g., reptiles, birds, land snails, insects).

2. Human Excretory System:

• Consists of: A pair of kidneys, a pair of ureters, a urinary bladder, and a urethra.
• Kidneys:
  • Location: Bean-shaped, reddish-brown, located in the abdominal cavity on either side of the vertebral column (between T12 and L3 vertebrae). The right kidney is slightly lower than the left.
  • Structure:
    • Hilum: Notch on the inner concave surface through which ureter, blood vessels (renal artery enters, renal vein leaves), and nerves enter/exit.
    • Renal Capsule: Tough fibrous outer covering.
    • Internal Structure: Divided into two zones:
      • Cortex: Outer zone, granular appearance. Contains Malpighian bodies (glomerulus + Bowman's capsule), PCT, DCT.
      • Medulla: Inner zone, arranged into conical masses called medullary pyramids (projecting into calyces). Contains Loops of Henle and collecting ducts.
      • Renal Columns (Columns of Bertini): Extensions of the cortex between medullary pyramids.
      • Renal Pelvis: Funnel-shaped space inner to the hilum, continuous with the ureter. Calyces (singular: calyx) open into the pelvis.
• Ureters: Thin muscular tubes emerging from the hilum of each kidney, carrying urine to the bladder via peristalsis.
• Urinary Bladder: Muscular sac in the pelvic cavity for temporary storage of urine. Lined by transitional epithelium (urothelium).
• Urethra: Tube arising from the bladder, expelling urine outside the body. Longer in males (carries urine and semen) than in females (carries only urine). Guarded by sphincters (internal - involuntary; external - voluntary).

3. The Nephron (Functional Unit of the Kidney):

• Each kidney has nearly one million nephrons.
• Parts of a Nephron:
  • Malpighian Body (Renal Corpuscle):
    • Glomerulus: A tuft of capillaries formed by the afferent arteriole (branch of renal artery) and drained by the efferent arteriole.
    • Bowman's Capsule: Double-walled cup-shaped structure enclosing the glomerulus.
  • Renal Tubule:
    • Proximal Convoluted Tubule (PCT): Highly coiled region near Bowman's capsule. Lined by simple cuboidal brush border epithelium (microvilli increase surface area for reabsorption).
    • Loop of Henle: Hairpin-shaped loop extending into the medulla. Has a descending limb and an ascending limb. The ascending limb has a thin segment and a thick segment.
    • Distal Convoluted Tubule (DCT): Coiled tubule distant from Bowman's capsule. Opens into the collecting duct.
    • Collecting Duct: Straight tube receiving filtrate from many nephrons. Converge and open into the renal pelvis through medullary pyramids (via Ducts of Bellini).
• Types of Nephrons:
  • Cortical Nephrons (85%): Malpighian corpuscle in the outer cortex, short Loop of Henle extending only slightly into the medulla.
  • Juxtamedullary Nephrons (15%): Malpighian corpuscle close to the medulla, long Loop of Henle extending deep into the medulla. Crucial for concentrating urine.
• Vasa Recta: A fine capillary network running parallel to the Loop of Henle, arising from the efferent arteriole of juxtamedullary nephrons. Plays a significant role in the counter-current mechanism.

4. Urine Formation:

• Involves three main processes: Glomerular Filtration, Reabsorption, and Secretion.
• a) Glomerular Filtration:
  • Blood is filtered from the glomerulus into Bowman's capsule through a filtration membrane (endothelium of glomerular capillaries, basement membrane, epithelium of Bowman's capsule - podocytes with filtration slits).
  • Driven by Glomerular Hydrostatic Pressure (GHP), opposed by Blood Colloidal Osmotic Pressure (BCOP) and Capsular Hydrostatic Pressure (CHP).
  • Net Filtration Pressure (NFP) = GHP - (BCOP + CHP).
  • Glomerular Filtration Rate (GFR): The amount of filtrate formed by the kidneys per minute. Normal GFR ≈ 125 ml/min or 180 Litres/day.
  • The filtrate (glomerular filtrate or ultrafiltrate) is blood plasma minus proteins.
• b) Reabsorption:
  • Selective process where useful substances from the filtrate are transported back into the blood (peritubular capillaries/vasa recta). Occurs along the entire renal tubule.
  • PCT: Maximum reabsorption (nearly all essential nutrients like glucose, amino acids; ~70-80% electrolytes and water). Also secretes H+, NH4+. Maintains pH and ionic balance.
  • Loop of Henle:
    • Descending Limb: Permeable to water, almost impermeable to electrolytes. Water moves out, concentrating the filtrate.
    • Ascending Limb: Impermeable to water, permeable to electrolytes (actively and passively transported out). Dilutes the filtrate.
  • 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 Na+-K+ balance.
  • Collecting Duct: Extends from cortex to inner medulla. Large amounts of water reabsorbed (under ADH influence) to produce concentrated urine. Allows passage of small amounts of urea into medullary interstitium to maintain osmolarity. Also plays a role in pH and ionic balance via secretion of H+ and K+.
• c) Secretion:
  • Tubular cells secrete substances like H+, K+, ammonia, creatinine, certain drugs from the blood into the filtrate.
  • Important for maintaining ionic and acid-base balance and eliminating waste products not filtered at the glomerulus. Occurs mainly in PCT and DCT.

5. Mechanism of Concentration of the Filtrate (Counter-Current Mechanism):

• Ability to produce concentrated urine is crucial for water conservation in mammals.
• Involves the Loop of Henle and Vasa Recta acting as counter-current systems.
• Counter-current Multiplier (Loop of Henle): Flow of filtrate in opposite directions in the two limbs creates and maintains an increasing osmolarity gradient in the medullary interstitium (from 300 mOsmol/L in the cortex to ~1200 mOsmol/L in the inner medulla). NaCl and Urea contribute to this gradient.
• Counter-current Exchanger (Vasa Recta): Blood flows in opposite directions in its limbs, minimizing solute loss from the interstitium while removing reabsorbed water. Helps maintain the medullary gradient established by the Loop of Henle.
• Role of Collecting Duct: As filtrate passes through the collecting duct in the hypertonic medullary interstitium, water moves out (facilitated by ADH), concentrating the urine.

6. Regulation of Kidney Function:

• a) Hormonal Control:
  • Antidiuretic Hormone (ADH) / Vasopressin: Released from the posterior pituitary in response to increased blood osmolarity or decreased blood volume/pressure. Increases water permeability of DCT and collecting duct, promoting water reabsorption, reducing urine output (antidiuresis), and concentrating urine.
  • Renin-Angiotensin-Aldosterone System (RAAS): Activated by a fall in GFR/glomerular blood pressure.
    • Juxtaglomerular Apparatus (JGA): Specialized structure formed by DCT and afferent arteriole. Its JG cells release Renin.
    • Renin converts Angiotensinogen (from liver) to Angiotensin I.
    • Angiotensin I is converted to Angiotensin II by Angiotensin Converting Enzyme (ACE).
    • Angiotensin II: Potent vasoconstrictor (increases GFR/blood pressure), stimulates adrenal cortex to release Aldosterone.
    • Aldosterone: Acts on DCT and collecting duct, promoting Na+ and water reabsorption, increasing blood pressure and GFR.
  • Atrial Natriuretic Factor (ANF): Released by atrial walls in response to high blood pressure/volume. Causes vasodilation, inhibits Renin release, inhibits Na+ reabsorption in collecting duct, thus decreasing blood pressure (acts as a check on RAAS).
• b) Neural Control: Primarily via sympathetic nerves regulating blood flow to the kidneys.

7. Micturition:

• The process of release of urine from the urinary bladder.
• Stretch receptors in the bladder wall send signals to the CNS when the bladder fills.
• CNS sends motor signals initiating contraction of bladder smooth muscles and relaxation of the urethral sphincter, causing urine release.
• It's a reflex that can be voluntarily controlled to some extent by adults.
• Average urine output: 1-1.5 Litres/day. Urine is slightly acidic (pH ~6.0). Presence of glucose (Glycosuria) or ketone bodies (Ketonuria) indicates diabetes mellitus.

8. Role of Other Organs in Excretion:

• Lungs: Eliminate large amounts of CO2 (~200 mL/minute) and significant water vapour.
• Liver: Synthesizes urea (Ornithine cycle), detoxifies substances, secretes bile containing bilirubin, biliverdin, cholesterol, degraded steroids, vitamins, drugs (eliminated with feces).
• Skin:
  • Sweat Glands: Produce sweat (water, NaCl, small amounts of urea, lactic acid). Primarily for cooling, but aids excretion.
  • Sebaceous Glands: Secrete sebum (sterols, hydrocarbons, waxes). Provides protective oily covering, eliminates some substances.

9. Disorders of the Excretory System:

• Uremia: Accumulation of urea in the blood due to kidney malfunction. Highly harmful, may lead to kidney failure. Treatment: Hemodialysis.
• Renal Failure (Kidney Failure): Complete or near-complete cessation of kidney function. Requires dialysis or kidney transplantation.
• Hemodialysis (Artificial Kidney): Blood drained from an artery, pumped through a dialyzing unit containing a cellophane tube surrounded by dialyzing fluid (same composition as plasma except nitrogenous wastes). Waste products diffuse out, cleared blood returned to a vein. Heparin is added to prevent clotting.
• Renal Calculi (Kidney Stones): Stones or insoluble masses of crystallized salts (e.g., oxalates) formed within the kidney. Cause severe pain.
• Glomerulonephritis: Inflammation of the glomeruli of the kidney.

Multiple Choice Questions (MCQs):

1. Which of the following is the primary nitrogenous waste product in birds and reptiles?
   a) Ammonia
   b) Urea
   c) Uric acid
   d) Guanine

2. The functional unit of the human kidney is the:
   a) Renal pyramid
   b) Nephron
   c) Calyx
   d) Ureter

3. Glomerular filtration occurs because:
   a) The afferent arteriole has a narrower diameter than the efferent arteriole.
   b) The efferent arteriole has a narrower diameter than the afferent arteriole.
   c) The Bowman's capsule actively pumps fluid from the glomerulus.
   d) The Loop of Henle creates a pressure gradient.

4. Maximum reabsorption of useful substances like glucose and amino acids occurs in which part of the nephron?
   a) Loop of Henle
   b) Distal Convoluted Tubule (DCT)
   c) Proximal Convoluted Tubule (PCT)
   d) Collecting Duct

5. The counter-current mechanism primarily involves the interaction between:
   a) PCT and DCT
   b) Glomerulus and Bowman's capsule
   c) Loop of Henle and Vasa Recta
   d) Afferent and Efferent arterioles

6. Which hormone is responsible for increasing water reabsorption in the DCT and collecting duct, thereby concentrating urine?
   a) Aldosterone
   b) ADH (Vasopressin)
   c) ANF
   d) Renin

7. The Juxtaglomerular Apparatus (JGA) is formed by cellular modifications in the:
   a) PCT and Afferent arteriole
   b) DCT and Efferent arteriole
   c) DCT and Afferent arteriole
   d) Loop of Henle and Collecting duct

8. A fall in Glomerular Filtration Rate (GFR) activates:
   a) JG cells to release Renin
   b) Adrenal cortex to release Aldosterone directly
   c) Atrial walls to release ANF
   d) Posterior pituitary to release ADH

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

10. Which of the following organs also plays a significant role in excretion by eliminating CO2 and water vapour?
    a) Liver
    b) Skin
    c) Lungs
    d) Spleen

Answer Key:

1. c) Uric acid
2. b) Nephron
3. b) The efferent arteriole has a narrower diameter than the afferent arteriole.
4. c) Proximal Convoluted Tubule (PCT)
5. c) Loop of Henle and Vasa Recta
6. b) ADH (Vasopressin)
7. c) DCT and Afferent arteriole
8. a) JG cells to release Renin
9. c) Diabetes mellitus
10. c) Lungs

Make sure you revise these notes thoroughly. Understand the processes like filtration, reabsorption, secretion, and the counter-current mechanism. The regulation part involving hormones (ADH, RAAS, ANF) is very important for competitive exams. Good luck with your preparation!