Excretion Notes

Excretion

• Excretion: Removal of metabolic waste.
• Elimination: Removal of undigested food (faeces).
• Kidneys filter about 50 gallons of blood daily, excreting approximately 1.5 liters of urine.

Nitrogenous Waste

• Produced from the breakdown of proteins, mainly amino acids.
• Type of waste depends on the animal's evolutionary group and habitat.
• Aquatic Animals (Fish): Excrete ammonia ($NH_3$), which is toxic but highly water-soluble.
• Mammals: Excrete urea ($CO(NH<em>2)</em>2$), less toxic, requires energy to convert from ammonia.
• Insects, Birds, Reptiles: Excrete uric acid ($C<em>5H</em>4N<em>4O</em>3$), which is almost insoluble in water (conserves water), but costs more energy to produce.

Excretory Systems

• Sponges: Rely on diffusion and active transport.
• Flatworms: Use protonephridia with flame cells to expel water and waste.
• Insects: Malpighian tubules remove waste from hemolymph, conserving water.
• Segmented Worms & Molluscs: Nephridia filter extracellular fluid, reabsorbing useful substances.

Kidneys (Vertebrates)

• Excrete metabolic waste (urea).
• Regulate ionic concentration and pH of blood plasma.
• Maintain blood volume and adjust water content.
• Retain essential nutrients.
• Secrete hormones (e.g., erythropoietin).

Human Excretory System

• Kidneys: Filter blood in nephrons; includes glomerulus, Bowman's capsule, proximal tubule, loop of Henle, and collecting duct.
• Ureters: Transport urine from kidneys to bladder.
• Bladder: Stores urine until expelled through the urethra.

Maintenance of Homeostasis

• ADH: Increases water reabsorption in kidneys.
• Renin-Angiotensin System: Regulates blood pressure.
• Kidneys secrete erythropoietin to stimulate red blood cell production in response to low oxygen levels.

Adaptations to Diverse Environments

• Desert Species (e.g., Kangaroos): Long loops of Henle to concentrate urine (14x more concentrated than blood).
• Freshwater Amphibians: Short loops of Henle (urine is 2x concentration of blood).
• Humans: Intermediate loop length (urine is 4x concentration of blood).

Adaptations to Aquatic Environments

• Freshwater Fish: Gain water through osmosis, excrete large volumes of dilute urine, and actively uptake salts through gills.
• Saltwater Fish: Lose water through osmosis, drink seawater, and actively pump out excess salt through gills; produce small amounts of concentrated urine.
• Sharks & Rays: Retain urea to maintain osmolarity, preventing water loss.