Animal Waste Management and Excretion

How do animals manage toxic nitrogenous waste?
Animals either eliminate ammonia quickly or convert it into less toxic compounds like uric acid or urea.
How do aquatic animals eliminate wastes?
Aquatic animals eliminate wastes by allowing ammonia to diffuse out of their bodies into surrounding water. Freshwater animals excrete lots of watery urine, while saltwater animals produce very little concentrated urine.
How do land animals remove wastes while conserving water?
Land animals conserve water by converting ammonia into uric acid (insects, reptiles, and birds) or urea (mammals and some amphibians). They also use structures like nephridia (annelids and mollusks) and Malpighian tubules (insects and arachnids) to filter and concentrate wastes before excretion.

Excretion: The elimination of metabolic wastes, such as ammonia.
Kidney: An organ that separates wastes and excess water from blood, forming urine.
Nephridium: A tubelike excretory structure that filters body fluid, found in annelids and mollusks.
Malpighian tubule: An excretory structure in insects and arachnids that absorbs uric acid from body fluids and adds them to digestive wastes.

The breakdown of proteins by cells releases nitrogenous waste: ammonia, which is poisonous.
Animals either eliminate ammonia quickly or convert it into less toxic compounds.
Excretion: The elimination of metabolic wastes, such as ammonia.
Small animals in wet environments release ammonia by diffusion across their skin.
Larger animals have excretory systems to process and eliminate ammonia.

Animals store nitrogenous wastes until they can be eliminated.
Ammonia is usually converted into less toxic forms for storage.
- Insects, reptiles, and birds convert ammonia into uric acid (a sticky white compound).
  - Uric acid is less toxic and less soluble in water.
- Mammals and some amphibians convert ammonia to urea.
  - Urea is less toxic than ammonia, and highly soluble in water.

Excretory systems maintain the balance of water in blood and body tissues.
Kidneys separate wastes and excess water from blood, forming urine.
Cells pump ions to create osmotic gradients; water follows passively by osmosis.
Kidneys usually cannot excrete excess salt.

Aquatic animals eliminate wastes by allowing ammonia to diffuse out of their bodies into surrounding water.
Aquatic animals need to either eliminate water from their bodies or conserve it, depending on whether they live in fresh or salt water.

Osmosis: Water moves into their bodies.
Diffusion: Salt diffuses out.
They excrete water through kidneys that produce lots of watery urine.
They don't drink and they actively pump salt in across their gills.
Many freshwater invertebrates lose ammonia to their environment by simple diffusion across their skin or gill membranes.
Some freshwater invertebrates and most freshwater fishes excrete excess water in very dilute urine.
Flatworms have flame cells to remove excess water from body fluids.
Freshwater fishes pump salt actively inward across their gills.

Osmosis: They lose water.
Diffusion: Salt diffuses in.
They conserve water by producing very little concentrated urine.
They drink and actively pump salt out across their gills.
Marine invertebrates and vertebrates release ammonia by diffusion across their body surfaces or gill membranes.
Marine fishes excrete salt across their gills, and their kidneys produce small quantities of very concentrated urine to conserve water.

Annelids and mollusks produce urine in nephridia (tubelike excretory structures that filter body fluid).
- Fluid enters through nephrostomes and becomes more concentrated as it moves along the tubes.
- Urine leaves through excretory pores.
Insects and arachnids convert ammonia into uric acid using Malpighian tubules.
- Malpighian tubules absorb uric acid from body fluids and add them to digestive wastes.
- Water is absorbed from these wastes, forming a thick paste that leaves the body through the anus, minimizing water loss.

Terrestrial vertebrates excrete wastes mostly via the kidneys.
Mammals and land amphibians convert ammonia into urea, which is excreted in urine.
Reptiles and birds convert ammonia into uric acid.
- Uric acid passes through ducts into a cavity that also receives digestive wastes from the gut.
- The walls of this cavity absorb most of the water from the wastes, causing the uric acid to separate out as white crystals.
- This results in a thick, milky-white paste.

Most vertebrate kidneys cannot excrete concentrated salt.
Some marine reptiles and birds have specialized glands in their heads that excrete very concentrated salt solutions.
Kangaroo rats produce urine that is 25 times more concentrated than their blood.
Their intestines are very efficient at absorbing water, resulting in almost dry feces.