5.3 Excretory Products and Their Elimination

Modes of Excretion

• Ammonotelism: Excretion of ammonia (most toxic); requires lots of water (e.g., bony fishes).
• Ureotelism: Excretion of urea (less toxic); requires moderate water (e.g., mammals).
• Uricotelism: Excretion of uric acid (least toxic); requires very little water (e.g., birds, reptiles).

Human Excretory System

• Organs: Kidneys, Ureters, Urinary Bladder, Urethra.
• Kidney Structure: Outer cortex, inner medulla (with medullary pyramids and calyces).
• Nephron: The functional unit of the kidney.

Structure of a Nephron

• Malpighian Body:
  • Glomerulus: Tuft of capillaries.
  • Bowman's Capsule: Cup-like structure enclosing the glomerulus.
• Renal Tubule:
  • Proximal Convoluted Tubule (PCT)
  • Loop of Henle
  • Distal Convoluted Tubule (DCT)
  • Collecting Duct

Urine Formation

1. Glomerular Filtration (Ultrafiltration): Blood is filtered in the Malpighian body.
2. Selective Reabsorption: Essential substances (glucose, amino acids, water, salts) are reabsorbed in the PCT, Loop of Henle, DCT, and collecting duct.
3. Tubular Secretion: Waste products (H⁺, K⁺, ammonia) are secreted into the filtrate.

Counter-Current Mechanism

• Function: Concentrates the urine.
• Mechanism: The opposing flow of filtrate in the Loop of Henle and blood in the vasa recta creates a concentration gradient in the medulla, allowing for water reabsorption from the collecting duct.

Regulation of Kidney Function

• Antidiuretic Hormone (ADH): Increases water reabsorption.
• Renin-Angiotensin-Aldosterone System (RAAS): Increases blood pressure and GFR by promoting Na⁺ and water reabsorption.
• Atrial Natriuretic Factor (ANF): Decreases blood pressure.

Micturition & Other Excretory Organs

• Micturition: The process of releasing urine from the bladder.
• Other Organs:
  • Lungs: Excrete CO₂ and water vapor.
  • Liver: Excretes bilirubin, biliverdin, cholesterol.
  • Skin: Excretes sweat (water, salts, urea).

Disorders of the Excretory System

• Uraemia: Accumulation of urea in the blood.
• Renal Failure: Kidneys stop functioning.
• Renal Calculi: Kidney stones.
• Nephritis: Inflammation of the kidneys.

Treatments for Renal Failure

• Haemodialysis (Artificial Kidney): Filtering the blood outside the body.
• Kidney Transplant: Replacing a failed kidney with a healthy one from a donor.