Lesson Overview Lesson OverviewExcretion Lesson Overview 27.4 Excretion

Lesson OverviewLesson Overview ExcretionExcretion

Lesson OverviewLesson Overview27.4 Excretion27.4 Excretion

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THINK ABOUT IT In addition to carbon dioxide and indigestible material, animals generate other wastes that are released into body fluids and that must be eliminated from the body.

What are these wastes and how do animals get rid of them?


The Ammonia ProblemThe breakdown of proteins by cells releases ammonia.

Ammonia is poisonous & kill most cells.

The elimination of ammonia is called excretion.

Some small animals that live in water rid their bodies of ammonia across their skin.

Most larger animals have excretory systems that process ammonia and eliminate it from the body.




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Storing Nitrogenous Wastes Animals that cannot dispose of ammonia continuously, so they evolved ways to store nitrogenous wastes

Ammonia cannot be stored in body fluids = too toxic.

Insects, reptiles, & birds (egg layers) convert ammonia into a sticky white compound -uric acid, less toxic than ammonia and less soluble in water.








Storing Nitrogenous Wastes Mammals and some amphibians convert ammonia to a different nitrogenous compound—urea.

Urea is less toxic than ammonia, but unlike uric acid, it is highly soluble in water.






Maintaining Water Balance Excretory systems are important in maintaining the proper balance of water in blood & body tissues.

Desert animals eliminate things differently that animals in a pond

Many animals use kidneys to separate wastes &excess water from blood to form a fluid called urine.




Maintaining Water Balance Kidneys separate water from waste products.

Kidney cells pump ions from salt to create osmotic gradients.

Water then “follows” those ions passively by osmosis.

Kidneys, however, usually cannot excrete excess salt.


The Ammonia ProblemHow do animals manage toxic nitrogenous waste?


The Ammonia ProblemHow do animals manage toxic nitrogenous waste?

Animals either eliminate ammonia from the body quickly or convert it into other nitrogenous compounds that are less toxic.


Freshwater Animals Many freshwater invertebrates lose ammonia to their environment by diffusion across their skin.

Many freshwater fishes and amphibians eliminate ammonia by diffusion across the gills.


Freshwater Animals Water moves into their bodies across the gills.

Salt diffuses out.

If they didn’t excrete water, they’d look like water balloons with eyes!

Freshwater fish excrete water through kidneys that produce lots of watery urine.

They don't drink, and they actively pump salt in across their gills.


Freshwater Animals Other examples of Excretion of Freshwater animals:

Flame Cells-flatworms have specialized cells

The excess water travels through excretory tubules and leaves through pores in the skin.




Saltwater Animals Marine invertebrates have body

fluids with water concentrations similar to that of the seawater around them.

The bodies of some saltwater fishes, contain a lower concentration of salt than the water

Saltwater fish lose water through osmosis, & salt diffuses in.


Saltwater Animals Saltwater fish conserve water by producing very little concentrated urine.

They drink, and they actively pump salt out across their gills.

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Excretion in Aquatic Animals How do aquatic animals eliminate wastes?


Excretion in Aquatic AnimalsHow do aquatic animals eliminate wastes?

In general, aquatic animals can allow ammonia to diffuse out of their bodies into surrounding water, which dilutes the ammonia and carries it away.


Excretion in Terrestrial AnimalsIn dry environments, land animals can lose large amounts of water from respiratory membranes that must be kept moist.

In addition, they must eliminate nitrogenous wastes in ways that require disposing of water—even though they may not be able to drink water.


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Terrestrial Invertebrates Some terrestrial invertebrates, including annelids and mollusks, produce urine in nephridia.

Nephridia are tubelike excretory structures that filter body fluid.

Body fluid enters the nephridia through openings called nephrostomes and becomes more concentrated as it moves along the tubes.

Urine leaves the body through excretory pores.


Terrestrial Invertebrates Other terrestrial invertebrates, such as insects and arachnids, convert ammonia into uric acid.

Uric acid is absorbed from body fluids by Malpighian tubules.

As water is absorbed from these wastes, they form crystals that form a thick paste, which leaves the body through the anus.

This paste contains little water, so this process minimizes water loss.




Terrestrial Vertebrates In mammals and land amphibians, ammonia is converted into urea, which is excreted in urine by the kidneys.




Terrestrial Vertebrates Reptiles and birds convert ammonia into uric acid, which is passed through ducts into a cavity that also receives digestive wastes from the gut.

The walls of this cavity absorb water from the wastes, causing the uric acid to separate out as a thick, milky-white paste recognized as “bird droppings.”




Adaptations to Extreme Environments Most vertebrate kidneys cannot excrete concentrated salt.

Most vertebrates cannot survive by drinking seawater.

The extra salt would overwhelm the kidneys, & the animal would die of dehydration.

Some marine reptiles & birds have evolved special adaptations to rid themselves of excess salt.

The petrel (bird) has special glands in its nostrils that separate salt from the water it swallows and excrete the salt as a thick, sticky fluid.

Petrel Salt Gland Marine Iguana getting rid of salt






Adaptations to Extreme Environments The kidneys of kangaroo rats, which live in the desert, produce urine that is 25 times more concentrated than their blood!

In addition, their intestines are so good at absorbing water that their feces are almost dry.






Excretion in Terrestrial Animals How do land animals remove wastes while conserving water?



Some terrestrial invertebrates, including annelids and mollusks, produce urine in nephridia.



Other terrestrial invertebrates, such as insects and arachnids, convert ammonia into uric acid.

Mammals and land amphibians convert ammonia into urea, which is excreted in urine. In most reptiles and birds, ammonia is converted into uric acid.

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Lesson Overview Lesson OverviewExcretion Lesson Overview 27.4 Excretion