Excretory Products and their Elimination
Animals must remove nitrogenous wastes, CO2, water and excess ions. Ammonia, urea and uric acid are the major nitrogenous wastes.
Modes of excretion
- Ammonotelism (excreting ammonia): most toxic, needs much water; bony fishes, aquatic amphibians, aquatic insects. Excreted by diffusion across body/gill surfaces; kidneys play no significant role.
- Ureotelism (excreting urea): mammals, many terrestrial amphibians, marine fishes. Ammonia is converted to urea in the liver, then filtered and excreted by kidneys.
- Uricotelism (excreting uric acid): least toxic, minimum water loss, excreted as pellet/paste; reptiles, birds, land snails, insects.
Excretory structures across animals
Protonephridia (flame cells) - Planaria; nephridia - earthworm; Malpighian tubules - insects (cockroach); antennal (green) glands - prawns (crustaceans).Human excretory system
A pair of kidneys, a pair of ureters, a urinary bladder and a urethra. Each kidney: 10-12 cm long, 120-170 g. Has an outer cortex and inner medulla (medullary pyramids), hilum, renal pelvis with calyces, and Columns of Bertini. About one million nephrons (functional units).Nephron
Two parts: glomerulus (capillary tuft from afferent arteriole; blood leaves via efferent arteriole) and renal tubule (Bowman's capsule - PCT - Henle's loop - DCT - collecting duct). Glomerulus + Bowman's capsule = malpighian/renal corpuscle. Cortical vs juxtamedullary nephrons (long loop). Vasa recta runs parallel to Henle's loop.Urine formation (3 steps)
- Glomerular filtration (ultrafiltration): ~1100-1200 ml blood/min filtered; GFR ~125 ml/min (180 L/day). JGA regulates GFR.
- Reabsorption: ~99% of filtrate reabsorbed; glucose, amino acids, Na+ active; water passive.
- Secretion: H+, K+, NH3 secreted to maintain ionic and acid-base balance.