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💧 Excretory Products and their Elimination

1. Modes of Excretion

Animals accumulate ammonia, urea, uric acid, CO₂, water, etc., which must be removed.

  • Ammonotelic: Excrete ammonia. Highly toxic, requires large amounts of water. E.g., bony fishes, aquatic amphibians.
  • Ureotelic: Excrete urea. Less toxic, requires less water. E.g., Mammals, terrestrial amphibians.
  • Uricotelic: Excrete uric acid as a paste/pellet. Minimum water loss. E.g., Birds, reptiles, insects.

2. Human Excretory System

Consists of a pair of kidneys, a pair of ureters, a urinary bladder, and a urethra.

The structural and functional unit of the kidney is the Nephron.

Parts of a Nephron

1. Glomerulus: Tuft of capillaries.

2. Bowman's Capsule: Encloses the glomerulus. (Together called Malpighian body).

3. PCT (Proximal Convoluted Tubule): Site of maximum reabsorption.

4. Henle's Loop: Descending limb (permeable to water) and Ascending limb (impermeable to water). Maintains high osmolarity.

5. DCT (Distal Convoluted Tubule): Conditional reabsorption of Na⁺ and water.

3. Urine Formation

Involves three main processes:

  1. Glomerular Filtration: Ultrafiltration of blood under pressure into the Bowman's capsule.
  2. Reabsorption: Nearly 99% of the filtrate is reabsorbed by the renal tubules actively or passively.
  3. Secretion: Tubular cells secrete substances like H⁺, K⁺, and ammonia into the filtrate to maintain ionic and acid-base balance.

4. Regulation of Kidney Function

Regulated by hormonal feedback mechanisms.

Key Hormones

ADH (Vasopressin): From hypothalamus/posterior pituitary. Increases water reabsorption from latter parts of tubule, preventing diuresis.

RAAS (Renin-Angiotensin-Aldosterone System): Activated by low blood pressure. Aldosterone increases Na⁺ and water reabsorption.

ANF (Atrial Natriuretic Factor): Released by heart atria when blood pressure is high. Causes vasodilation, acting as a check on the RAAS mechanism.

5. Disorders

Uremia (accumulation of urea in blood), Renal calculi (kidney stones, insoluble mass of crystallized salts like oxalates), Glomerulonephritis (inflammation of glomeruli).