Excretion Notes

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.

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 ( $CO(NH2)2$ ), less toxic, requires energy to convert from ammonia.
Insects, Birds, Reptiles: Excrete uric acid ( $C5H4N4O3$ ), which is almost insoluble in water (conserves water), but costs more energy to produce.

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: 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.

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.

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).

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.