1. 2. 3. 4. Explain How do insects, reptiles, and birds eliminate ammonia and how do mammals eliminate ammonia Apply Concepts How do kidneys help maintain homeostasis while processing nitrogenous wastes Review In what form do (a) annelids and mollusks, (b) insects and arachnids, (c) mammals and land amphibians, and (d) reptiles and birds excrete nitrogenous wastes Relate Cause and Effect Explain how differing water balance needs relate to an animals conversion of ammonia to wither urea or uric acid CH 27 ANIMAL SYSTEMS I 27.4 Excretion The Ammonia Problem Breakdown of proteins by cells releases a nitrogencontaining waste: ammonia Ammonia is poisonous Moderate concentrations of ammonia can kill most cells. Excretion Elimination of metabolic wastes, such as ammonia Small animals in wet environments get rid of ammonia by allowing it to diffuse out of their body fluids across their skin Most larger animals have excretory systems that process ammonia and eliminate it from the body. Animals that cannot dispose of ammonia continuously have evolved ways to store nitrogenous wastes until they can be eliminated. Storing Nitrogenous Wastes Ammonia itself cannot be stored in body fluids because it is too toxic Insects, reptiles, and birds convert ammonia into a sticky white compound called uric acid Much water. less toxic than ammonia and is less soluble in Mammals and some amphibians convert ammonia to urea Urea is less toxic than ammonia Is highly soluble in water. Maintaining Water Balance Excretory systems are extremely important in maintaining the proper balance of water in blood and body tissues May also excrete excess water or have to conserve water. Kidneys Separate wastes and excess water from blood to form urine Pump ions from salt to create osmotic gradients Water then “follows” those ions passively by osmosis Usually cannot excrete excess salt. Freshwater Animals Freshwater invertebrates lose ammonia to their environment by simple diffusion across their skin. Freshwater fishes and amphibians eliminate ammonia by diffusion across the same gill membranes they use for respiration. Bodies of freshwater animals, such as fishes, contain a higher concentration of salt than the water they live in. Water moves into their bodies by osmosis Salt diffuses out. Freshwater fish excrete lots of watery urine Don't drink water Actively pump salt in across their gills. Saltwater Animals Typically release ammonia by diffusion across their body surfaces or gill membranes. Many marine invertebrates have body fluids with water concentrations similar to that of the seawater around them. Many saltwater animals, such as fishes, contain a lower concentration of salt than the water they live in. Lose water through osmosis, and salt diffuses in. Saltwater fish conserve water by producing very little concentrated urine Drink water Actively pump salt out across their gills. Excretion in Terrestrial Animals Land animals can lose large amounts of water from respiratory membranes that must be kept moist Must eliminate nitrogenous wastes in ways that require disposing of water. Terrestrial Invertebrates Produce urine in nephridia Nephridia Tubelike excretory structures that filter body fluid. Terrestrial Invertebrates Body fluid enters the nephridia through nephrostomes and becomes more concentrated as it moves along the tubes Urine leaves the body through excretory pores. Terrestrial Invertebrates Insects and arachnids convert ammonia into uric acid Malpighian tubules Absorb uric acid from body fluids Concentrate the wastes and add them to digestive wastes. Water is absorbed from wastes Crystals form a thick paste which leaves the body through the anus Paste contains little water Minimizes water loss. Terrestrial Vertebrates Mammals and land amphibians convert ammonia into urea Excreted in urine by the kidneys. Reptiles and birds convert ammonia into uric acid Passed through ducts into a cavity that also receives digestive wastes from the gut Walls of cavity absorb water Uric acid separates as thick, milky-white paste recognized as “bird droppings.” Most vertebrate kidneys cannot excrete concentrated salt Most vertebrates cannot survive by drinking seawater All that extra salt would overwhelm the kidneys, and the animal would die of dehydration.