Notes Class 11 Chapter 15 Biology Maharashtra Board

Excretion and Osmoregulation

Introduction

Excretion is the process of eliminating metabolic waste products from an organism’s body, which are produced during metabolic processes (catabolic and anabolic). These wastes include fluids, gases, organic, and inorganic compounds, and their elimination is essential to maintain homeostasis. Osmoregulation is the process of controlling solute concentrations and water balance in the body to ensure a stable internal environment.

15.1 Excretion and Excretory Products

What is Excretion?

Definition: Excretion is the elimination of metabolic waste products from the body.
Difference from Digestive Wastes: Unlike digestive wastes (unabsorbed/undigested substances), metabolic wastes are produced inside body cells during metabolism.

Types of Excretory Products

Fluids: Water.
Gaseous Wastes: Carbon dioxide (CO₂).
Nitrogenous Wastes:
- Ammonia (highly toxic, requires large amounts of water for dilution).
- Urea (less toxic, requires less water).
- Uric acid (least toxic, requires minimal water).
Other Wastes:
- Creatinine (from muscle metabolism).
- Mineral salts (sodium, potassium, calcium, etc.).
- Pigments:
  - Bilirubin (from hemoglobin breakdown, excreted in feces).
  - Urochrome (gives urine its yellow color, excreted in urine).
- Pigments from food (e.g., beetroot).
- Excess vitamins, hormones, and drugs.
- Volatile substances from spices (excreted via lungs).

Factors Affecting Waste Production

Environment and Evolution: The type of waste produced depends on the organism’s habitat, not its phylogenetic relationship. For example:
- Tadpoles (aquatic) excrete ammonia, while adult frogs (terrestrial) excrete urea.
- Some turtles excrete uric acid, others excrete urea or ammonia based on habitat.
Thermoregulation: Endotherms consume more food, producing more nitrogenous wastes.
Food Habits: Carnivorous diets (high protein) produce more nitrogenous wastes than herbivorous diets.

Nitrogenous Waste Elimination

Deamination: Excess amino acids are broken down in the liver, producing ammonia.
Toxicity of Ammonia: Ammonia is highly toxic and disrupts pH, enzyme activity, and plasma membrane stability. It is either excreted directly or converted to less toxic forms (urea or uric acid).

Classification Based on Nitrogenous Wastes

Ammonotelism:
- Definition: Excretion of nitrogenous wastes as ammonia.
- Characteristics:
  - Highly toxic, requires 300-500 ml water per gram for dilution.
  - Energy-efficient as no conversion is needed.
- Organisms: Aquatic invertebrates, bony fishes, larval amphibians, protozoa (without excretory organs).
- Excretory Organs: Skin, gills, kidneys.
Ureotelism:
- Definition: Excretion of nitrogenous wastes as urea.
- Characteristics:
  - Less toxic, requires ~50 ml water per gram.
  - Formed in the liver via the ornithine/urea cycle (uses 3 ATP per urea molecule).
- Organisms: Mammals, cartilaginous fishes (e.g., sharks), aquatic reptiles, most adult amphibians.
- Special Case: Sharks retain urea in blood to maintain isotonicity with seawater, preventing water loss.
Uricotelism:
- Definition: Excretion of nitrogenous wastes as uric acid.
- Characteristics:
  - Least toxic, requires 5-10 ml water per gram.
  - Formed via the inosinic acid pathway in the liver.
- Organisms: Birds, some insects, reptiles, land snails (water conservation is critical).
Guanotelism:
- Definition: Excretion of guanine (less common).
- Organisms: Spiders, scorpions, penguins.

15.2 Excretory System in Humans

Overview

The human excretory system removes nitrogenous wastes, excess water, and toxic substances, maintaining homeostasis through osmoregulation and pH regulation. It includes kidneys, ureters, urinary bladder, and urethra.

Structure of the Excretory System

Kidneys:
- Location: A pair of bean-shaped organs located on either side of the vertebral column (12th thoracic to 3rd lumbar vertebra), retroperitoneal (behind the peritoneum).
- Dimensions: 10 cm (length) × 5 cm (width) × 4 cm (thickness).
- Weight: ~150 g (males), ~135 g (females).
- Surface: Outer surface is convex, inner is concave with a notch (hilus) where the renal artery enters, and renal vein and ureter exit.
- Coverings:
  - Renal fascia: Outermost fibrous connective tissue anchoring kidneys to the abdominal wall.
  - Adipose capsule: Middle fatty layer for shock absorption.
  - Renal capsule: Innermost fibrous membrane preventing infections.
- Internal Structure:
  - Renal Cortex: Outer, red, granular region containing Malpighian bodies, convoluted tubules, and blood vessels.
  - Renal Medulla: Inner, pale, striated region with loops of Henle and collecting ducts arranged in renal pyramids.
  - Renal Columns: Cortex extends into medulla as columns of Bertini between pyramids.
  - Renal Papilla: Narrow tip of pyramids opening into minor calyces.
  - Renal Pelvis: Funnel-shaped area in the medulla continuing as the ureter.
- Functional Units: ~1 million nephrons per kidney.
Ureters:
- Structure: Paired muscular tubes (25-30 cm long) arising from the renal pelvis.
- Function: Transport urine to the urinary bladder.
- Mechanism: Ureters pass obliquely through the bladder wall, preventing backflow of urine when the bladder is full.
Urinary Bladder:
- Structure: Median, hollow, muscular sac in the pelvic cavity, posterior to the pubic symphysis.
- Layers:
  - Outer peritoneum.
  - Muscular layer (detrusor muscles: longitudinal-circular-longitudinal).
  - Innermost transitional epithelium (stretchable).
- Trigone: Inverted triangular area at the base with ureter openings (at the base) and urethral opening (at the apex).
- Function: Temporary storage of urine (capacity ~700 ml).
Urethra:
- Structure: Tube arising from the bladder, opening externally.
- Sphincters:
  - Internal Sphincter: Smooth muscle, involuntary.
  - External Sphincter: Skeletal muscle, voluntary.
- Function: Discharges urine; in males, it also serves as a urinogenital passage.

Nephron: Structural and Functional Unit

Definition: Nephrons are microscopic units responsible for filtration, reabsorption, secretion, and osmoregulation.
Components:
1. Renal Corpuscle (Malpighian Body):
  - Bowman’s Capsule: Double-walled, cup-like structure with:
    - Parietal Layer: Simple squamous epithelium.
    - Visceral Layer: Podocytes with filtration slits.
    - Capsular Space: Space between layers where filtrate collects.
  - Glomerulus: Network of fenestrated capillaries formed from the afferent arteriole, reuniting into the efferent arteriole.
  - Function: Ultrafiltration of blood plasma.
2. Renal Tubule:
  - Proximal Convoluted Tubule (PCT):
    - Lined with cuboidal cells with microvilli (brush border).
    - Site of selective reabsorption (glucose, amino acids, ions, water).
  - Loop of Henle (LoH):
    - U-shaped with descending (thin, permeable to water) and ascending (thick, impermeable to water) limbs.
    - Operates countercurrent mechanism for osmoregulation.
  - Distal Convoluted Tubule (DCT):
    - Lined with cuboidal epithelium.
    - Performs tubular secretion and water reabsorption.
  - Collecting Tubule (CT):
    - Reabsorbs water, secretes protons, adjusts urine pH.
    - Opens into the collecting duct.
Types of Nephrons:
- Cortical Nephrons: Shorter loops of Henle, mostly in cortex (most common).
- Juxtamedullary Nephrons: Longer loops extending into medulla, critical for urine concentration.
Blood Supply:
- Renal artery → Afferent arteriole → Glomerulus → Efferent arteriole → Peritubular capillaries (around PCT, DCT) or Vasa recta (around LoH) → Renal vein.
- ~25% of cardiac output goes to kidneys.

Juxtaglomerular Apparatus (JGA)

Structure:
- Juxtaglomerular (JG) Cells: Modified smooth muscle cells in the afferent arteriole with granular sarcoplasm.
- Macula Densa: Densely packed cells in the DCT where it contacts the afferent arteriole.
Function: Regulates blood pressure by releasing renin, which activates the renin-angiotensin-aldosterone system (RAAS).

15.3 Urine Formation

Urine formation occurs in three steps:

Ultrafiltration (Glomerular Filtration):
- Mechanism:
  - Blood enters the glomerulus via the afferent arteriole (larger diameter) and exits via the efferent arteriole (smaller diameter), creating high glomerular hydrostatic pressure (GHP, ~55 mm Hg).
  - Opposing forces: Osmotic pressure of blood (~30 mm Hg) and capsular hydrostatic pressure (~15 mm Hg).
  - Effective filtration pressure (EFP) = GHP – (Osmotic + Capsular) = 55 – (30 + 15) = 10 mm Hg.
- Result: Plasma (except proteins and blood cells) filters through glomerular capillaries into Bowman’s capsule, forming glomerular filtrate (~125 ml/min or 180 L/day).
- Composition: Alkaline, contains urea, amino acids, glucose, ions, and pigments.
Selective Reabsorption:
- Site: Primarily in PCT, also in DCT and collecting tubule.
- Mechanism:
  - PCT: Cuboidal cells with microvilli reabsorb ~99% of filtrate.
    - Active Reabsorption: Glucose, amino acids, Na⁺, K⁺, Ca²⁺, Cl⁻ (ATP-mediated).
    - Passive Reabsorption: Water, sulfates, nitrates (via diffusion).
  - DCT and Collecting Tubule: Reabsorb water and ions under hormonal control.
- Result: Reduces filtrate volume, returns essential substances to blood.
Tubular Secretion (Augmentation):
- Site: DCT and collecting tubule.
- Mechanism: Cells actively secrete wastes (e.g., creatinine, K⁺, H⁺) from peritubular capillaries into the filtrate.
- Function:
  - Augments urine concentration.
  - Regulates blood pH by secreting H⁺.
- Special Case: In marine bony fishes and desert amphibians, tubular secretion is the primary mode of excretion.

15.4 Concentration of Urine

Purpose: To conserve water, humans can produce urine up to 4 times more concentrated than blood (1200 mOsm/L vs. 300 mOsm/L).
Mechanism: Countercurrent mechanism in juxtamedullary nephrons and vasa recta.

Countercurrent Mechanism

Loop of Henle:
- Descending Limb: Thin, permeable to water. Water diffuses out due to high osmolarity of medullary tissue fluid, concentrating the filtrate.
- Ascending Limb: Thick, impermeable to water. Actively reabsorbs Na⁺ and Cl⁻, reducing filtrate osmolarity and increasing medullary tissue fluid osmolarity.
- Result: Creates an osmotic gradient (300 mOsm/L in cortex to 1200 mOsm/L in deep medulla).
Vasa Recta:
- Blood flows in opposite directions (descending → ascending).
- Maintains medullary osmotic gradient by exchanging Na⁺, Cl⁻, and water with tissue fluid without disrupting the gradient.
Role of Urea:
- Urea diffuses from collecting ducts into medullary tissue fluid and back into the thin ascending limb (urea recycling).
- Enhances water reabsorption, concentrating urine.
Role of ADH:
- Antidiuretic hormone (ADH) increases collecting duct permeability to water, allowing more water reabsorption.
- Concentrates urine further.

Adaptations

Desert mammals (e.g., camels) have longer loops of Henle, enhancing water reabsorption and producing highly concentrated urine.

15.5 Composition of Urine

Appearance: Pale yellow, transparent (due to urochrome).
Composition (varies with diet/fluid intake):
- Water: ~95%.
- Solutes: Urea, creatinine, uric acid, Na⁺, K⁺, Ca²⁺, Cl⁻, phosphates, sulfates.
- Pigments: Urochrome, bilirubin.
- Absent in Normal Urine: Albumin, sugar, ketone bodies, bile salts/pigments, occult blood, casts.
Specific Gravity: ~1.02 (increases with ADH).
pH: Acidic (due to H⁺ secretion in DCT/CT).

Regulation of Urine Volume

ADH (Antidiuretic Hormone):
- Released by posterior pituitary when osmoreceptors in the hypothalamus detect high blood osmolarity (e.g., after dehydration or salty food).
- Increases water reabsorption in DCT and collecting ducts, reducing urine volume.
- Absence of ADH (e.g., in diabetes insipidus) causes dilute urine and excessive urination.
RAAS (Renin-Angiotensin-Aldosterone System):
- Triggered by low blood pressure/volume.
- JGA releases renin, converting angiotensinogen to angiotensin I, then to angiotensin II.
- Angiotensin II:
  - Constricts kidney arterioles, raising blood pressure.
  - Stimulates Na⁺, Cl⁻, and water reabsorption in PCT.
  - Triggers aldosterone release from the adrenal cortex.
- Aldosterone: Enhances Na⁺ and water reabsorption in DCT/CT, increasing blood volume.
Atrial Natriuretic Peptide (ANP):
- Released by atrial walls when blood volume/pressure is high.
- Inhibits Na⁺ reabsorption, renin, aldosterone, and ADH release, promoting natriuresis (Na⁺ excretion) and diuresis (water excretion).

15.6 Role of Other Organs in Excretion

Skin:
- Structure: Contains sweat glands (abundant in palms/face) and sebaceous glands (at hair follicle necks).
- Excretory Role:
  - Sweat Glands: Excrete water, NaCl, lactic acid, and urea (primarily for thermoregulation).
  - Sebaceous Glands: Secrete sebum, which lubricates and protects skin.
- Note: Human skin is thick and impermeable, limiting diffusion of wastes like ammonia.
Lungs:
- Excrete volatile substances like CO₂, water vapor, and compounds from spices/food (e.g., garlic, onion), causing bad breath.

15.7 Disorders and Diseases

Kidney Stones (Renal Calculi):
- Types:
  - Calcium Stones: Calcium oxalate or phosphate.
  - Struvite Stones: Formed due to bacterial infections.
  - Uric Acid Stones: Due to low water intake or high-protein diets.
  - Cystine Stones: Genetic disorder causing excessive cystine excretion.
- Symptoms: Pain in back/sides, hazy/reddish urine, frequent urination, pain during micturition.
- Diagnosis: Blood uric acid levels, urine color, kidney X-ray, sonography.
- Prevention: Avoid oxalate-rich foods (e.g., tomatoes).
Uremia:
- Definition: Blood urea levels >0.05% (normal: 0.01-0.03%).
- Consequence: Can lead to kidney failure.
Nephritis:
- Definition: Inflammation of kidneys causing proteinuria (protein in urine).
- Cause: Increased glomerular membrane permeability due to high blood pressure or toxins.
- Effect: Alters blood osmotic pressure, causing edema.
Renal Failure:
- Acute Renal Failure (ARF):
  - Sudden decline in glomerular filtration (e.g., after severe bleeding).
  - Symptoms: Oliguria (<400 ml/day), elevated serum creatinine.
- Chronic Kidney Disease (CKD):
  - Progressive, irreversible decline in glomerular filtration rate.
  - Causes: Chronic glomerulonephritis.
  - Symptoms: Reduced kidney size, anemia.
Dialysis:
- Purpose: Artificial blood filtration when renal function falls below 5-7%.
- Types:
  - Hemodialysis:
    - Blood is filtered outside the body through a cellophane tube (semipermeable) in a dialysate (isosmotic to plasma).
    - Heparin prevents clotting; anti-heparin is added before returning blood.
    - Slow process due to low flow rate.
  - Peritoneal Dialysis:
    - Dialysate is introduced into the peritoneal cavity, where the peritoneum acts as a semipermeable membrane.
    - Less efficient but can be done at home.
- Limitation: Cannot replicate kidney hormone production (erythropoietin, renin, calcitriol).
Kidney Transplant:
- Types: Cadaveric (deceased donor) or living donor (related/non-related).
- Requirement: Immunosuppressants to prevent rejection.
- Advantage: Restores normal kidney function, improving quality of life.

Key Concepts

Homeostasis: Kidneys maintain constant internal conditions by regulating water, electrolytes, and pH.
Osmoregulation:
- Osmoconformers: Marine organisms with body fluids isotonic to the environment.
- Osmoregulators: Freshwater and terrestrial organisms controlling internal solute concentrations.
- Stenohaline: Tolerate narrow salinity ranges.
- Euryhaline: Tolerate wide salinity changes (e.g., barnacles, clams).
Micturition:
- Reflex triggered when the bladder is ~half full (~350 ml).
- Stretch receptors signal the spinal cord, contracting bladder muscles and relaxing sphincters.
- Infants lack voluntary control due to undeveloped neurons to the external sphincter.

Interesting Facts

Vampire Bats: Excrete dilute urine during feeding to reduce weight for flight and concentrated urine during the day to conserve water.
Marine Birds (e.g., Albatross): Have salt glands near nostrils to excrete excess salts, aiding osmoregulation.
Floating Kidney: A condition where the kidney is not firmly anchored, causing it to move excessively.

Practical Applications

Blood and Urine Tests:
- Creatinine: Indicator of kidney function (normal: 0.6-1.4 mg/dl males, 0.6-1.2 mg/dl females). Elevated levels suggest poor renal function.
- Urine Analysis: Checks for albumin, sugar, ketone bodies, bile salts, casts, etc., to diagnose disorders.
Dietary Restrictions for Kidney Patients: Low protein, low oxalate, adequate hydration.
Treatments for Kidney Stones: Lithotripsy (shock waves to break stones), dietary changes.