1. Explain How do insects, reptiles, and birds eliminate ammonia and how do mammals eliminate ammonia

2. Apply Concepts How do kidneys help maintain homeostasis while processing nitrogenous wastes

3. 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

4. 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 I27.4 Excretion

The Ammonia Problem

Breakdown of proteins by cells releases a nitrogen-containing 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 less toxic than ammonia and is less soluble in

water.

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.