Human Excretory System

Introduction

• Body constantly at work, even during sleep (e.g., inhaling O2, exhaling CO2)
• Various waste products generated: ammonia, urea, uric acid, water, excess ions (phosphate, sulfate, sodium, potassium, chloride)
• These wastes are partly or fully removed through excretion

Nitrogenous Wastes and Excretion Mechanisms

• Ammonia: Most toxic, requires lots of water for elimination
  • Excreted by: Bony fishes, aquatic amphibians, aquatic insects
  • Process: Ammonotelism
  • Animals: Ammonotelic
• Urea: Less toxic, helps conserve water
  • Excreted by: Mammals (incl. humans), many marine fish, terrestrial amphibians
  • Process: Ureotelism
  • Animals: Ureotelic
  • Conversion: Liver converts ammonia to urea (ornithine cycle) -> filtered by kidneys
• Uric Acid: Least toxic, helps conserve water, more energy required
  • Excreted by: Reptiles, insects, land snails, birds
  • Process: Uricotelism
  • Animals: Uricotelic
  • Note: Formed by degradation of nucleic acids in mammals

Evolution of Excretory Structures

• Flame Cells (Protonephridia): Invertebrates (Platyhelminthes, Rotifers, Cephalochordates, some Annelids)
  • Function: Excretion, Osmoregulation
• Nephridia: Annelids (e.g., earthworms)
  • Function: Removal of nitrogenous wastes, fluid, and ionic balance
• Malpighian Tubules: Arthropods (insects, arachnids)
  • Function: Excretion, Osmoregulation
• Antennal Gland/Green Gland: Crustaceans (crabs, crayfish)
  • Function: Excretion, Osmoregulation
• Kidneys: Vertebrates (incl. mammals)
  • Allied structures: Skin, liver, lungs also involved in excretion
  • Note: Kidney primarily responsible for excretion in humans

Human Excretory System

Organs Involved

• Kidneys:
  • Location: Lower back, either side of spinal cord
  • Structure: Bean-shaped, reddish-brown
    • Hilum: Indentation for blood vessels, nerves, ureter
    • Regions: Outer cortex, inner medulla
    • Medullary Pyramids: Cone-shaped, project into calyces
    • Nephrons: Basic functional units (1 million per kidney)
      • Parts: Glomerulus, Renal Tubule (Bowman’s Capsule, PCT, Henle’s Loop, DCT)
      • Types: Cortical nephrons (short Henle’s Loop), Juxtamedullary nephrons (long Henle’s Loop)
• Ureters: Tube carrying urine from kidneys to bladder
• Urinary Bladder: Muscular organ temporarily storing urine
• Urethra: Tube for urine discharge, regulated by the urethral sphincter
• Other Structures: Skin (sebaceous and sweat glands), liver (converts ammonia to urea, metabolizes drugs, secretes bile pigments), lungs (removal of CO2 and water), saliva

Process of Urine Formation

1. Glomerular Filtration
   • Filtration in glomerulus (endothelium, basement membrane, epithelium of Bowman’s capsule)
   • GFR (Glomerular Filtration Rate): 125 mL/min or 180 L/day
2. Reabsorption
   • Mainly in PCT, Henle’s Loop, DCT, Collecting Duct
   • Substances: Sodium, glucose, amino acids (active transport), water (passive transport), urea (passive transport)
3. Secretion
   • Includes potassium, hydrogen ions, and ammonia into filtrate

Mechanisms Involved

• Counter-Current Mechanism
  • Henle’s loop & Vasa Recta create counter-current for maintaining medulla osmolarity
• Hormonal Regulation
  • ADH (Vasopressin): From hypothalamus/pituitary, regulates water reabsorption
  • Renin-Angiotensin Mechanism: Regulates GFR, blood pressure via JGA
    • Renin -> Angiotensinogen -> Angiotensin I -> Angiotensin II -> Aldosterone (sodium/water reabsorption)
  • ANF (Atrial Natriuretic Factor): Heart muscle hormone, regulates blood pressure and sodium levels

Micturition

• Process: Release of urine from bladder via urethra
• Mechanism: Micturition Reflex
  • Stimulus: Bladder stretch receptors
  • Response: CNS sends signals to contract bladder, relax urethral sphincter
• Output: ~1-1.5 L/day

Disorders and Conditions

• Glycosuria: Presence of glucose in urine (indicative of diabetes mellitus)
• Ketonuria: Presence of ketone bodies (common in diabetes)
• Uremia: Accumulation of urea (can lead to kidney failure; treated by hemodialysis)
• Kidney Stones (Renal Calculi): Crystallized salts
• Glomerulonephritis: Inflammation of glomeruli
• Hemodialysis: Artificial removal of wastes (for uremic patients)
• Kidney Transplantation: In severe cases, donor kidney replaces failed kidney