Lecture Notes: Excretion in the Human Body

Introduction to Excretion

• Human body continuously works; never completely at rest.
• Excretion is the elimination of metabolic waste products from the body.
• Waste products include carbon dioxide, ammonia, urea, uric acid, and excess ions.

Importance of Excretion

• Retained waste products can be fatal.
• Essential for maintaining health and homeostasis.

Types of Nitrogenous Wastes

• Ammonia: Highly toxic; excreted by aquatic organisms (e.g., bony fishes, amphibians).
  • Process: Monotelism (excretion of ammonia).
• Urea: Less toxic; excreted by mammals and amphibians.
  • Process: Ureotelism (conversion of ammonia to urea in the liver).
• Uric Acid: Least toxic; excreted by reptiles, birds, and insects.
  • Process: Uricotelism (excretion in paste form to conserve water).

Evolution of Excretory Systems

• Simple organs in primitive animals evolved into complex organs in higher vertebrates.
  • Flame Cells: Excretory cells in lower invertebrates.
  • Nephridia: Found in annelids like earthworms.
  • Malpighian Tubules: Found in arthropods (insects, arachnids).
  • Antennal/Green Glands: Excretory in crustaceans.
• Kidneys: Main excretory organ in mammals.

Excretory Organs and Functions

• Skin: Sebaceous glands secrete sebum; sweat glands remove excess salts and water.
• Liver: Detoxifies ammonia, breaks down drugs, produces bile for waste.
• Lungs: Remove carbon dioxide and excess water.
• Saliva: Removes small amounts of nitrogenous waste.

Structure of the Human Excretory System

• Components: Kidneys, ureters, urinary bladder, urethra.
  • Kidneys: Bean-shaped organs with outer cortex and inner medulla containing nephrons.
  • Ureters: Transport urine from kidneys to bladder.
  • Urinary Bladder: Stores urine.
  • Urethra: Discharges urine from the body.

Nephrons and Urine Formation

• Nephron Structure: Composed of glomerulus and renal tubule.
• Urine Formation Process:
  1. Glomerular Filtration: Blood filtration in glomerulus (ultrafiltration).
  2. Reabsorption: 99% of filtrate (water, nutrients) is reabsorbed in renal tubules.
  3. Secretion: Selective secretion of potassium, hydrogen ions, ammonia.
• Collecting Duct: Involved in urine concentration and osmolarity maintenance.

Regulation of Kidney Functions

• Hormonal Mechanisms:
  • ADH: Increases water reabsorption during dehydration.
  • Renin-Angiotensin Mechanism: Regulates blood pressure and GFR.
  • Atrial Natriuretic Factor (ANF): Released by heart to decrease blood volume.

Micturition Process

• Stretch receptors in the bladder signal CNS for urination.
• Involves smooth muscle contraction and relaxation of urethral sphincter.

Characteristics of Urine

• Average daily output: 1 to 1.5 liters.
• Light yellow, slightly acidic (pH 6).
• Changes in characteristics can indicate metabolic disorders (e.g., glycosuria, uremia).

Kidney Malfunctions and Disorders

• Uremia: Failure to eliminate urea, treated with hemodialysis.
• Kidney Stones: Crystallized salts (calcium oxalate/phosphate).
• Glomerulonephritis: Inflammation of kidney glomeruli.

Conclusion

• Excretion is vital for health; disorders can lead to serious conditions.