1. Excretion
Section 27.4

2. What toxic wastes build up in animals?
Ammonia! Caused by the breakdown of proteins by cells
Ammonia is poisonous
Animals either get rid of the ammonia quickly, or they convert it into other nitrogenous compounds that are less toxic
Elimination of metabolic wastes such as ammonia is excretion
Some small aquatic animals simply allow ammonia to diffuse outwards
Most larger animals or those in dry environments have specialized systems for processing and eliminating ammonia from the body

3. How animals process and store nitrogenous waste?
If an animal cannot immediately dispose of ammonia, it must be stored
Ammonia is too dangerous to store as ammonia
Birds, reptiles and insects convert ammonia to uric arid
Mammals and some amphibians convert ammonia to urea
Both are less toxic than ammonia
Urea is soluble in water, uric acid isn’t

4. Why is water important in disposing of nitrogenous waste?
Getting rid of waste involves water
Kidneys play a pivotal role in separating waste and excess water from the blood steam
Waste and water combined to form urine
Kidneys work in a special way
Impossible to pump water across a membrane
Kidneys instead pump ions across membrane to create concentration gradient
Water follows by osmosis
Kidneys cannot excrete excess salt

5. How does excretion work in aquatic animals?
Because they are surrounded by water, ammonia can simply diffuse out of their bodies into surrounding water – diluting the ammonia and carrying it away.
Freshwater and Saltwater environments can result in additional challenges
In Freshwater ammonia often diffuses across the skin of an animal – often gills utilized for this process
Concentration gradient of water can cause issues
As a result they produce very dilute urine, and pump salts inward through gills
In Saltwater, the risk is that all of the water will diffuse out of fish due to concentration gradient
Salt excreted across gills , very concentrated urine produced to conserve water

6. How do land animals remove waster whilst preserving water?
Water preservation is often key for land animals – especially in dry environments
Some terrestrial invertebrates (including annelids and mollusks) produce urine in Nephridia
Nephridia are tube like excretory structures that filter body fluids
Body fluids enter through nephrostomes
Becomes more concentrated as passes through
Urine leaves through excretory pores
Others, such as insects and arachnids will convert ammonia into uric acid
Absorbed form body fluids using Malpighian tubules- concentrates wastes and add them to digestive waste travelling through the gut
Water is absorbed from the waste, resulting in crystals that form a thick paste

7. And what about terrestrial vertebrates?
Excretion is carried out mostly by the kidneys
Mammals and land amphibians convert ammonia into urea, which is excreted as urine
In reptiles and birds it is converted into uric acid
Uric acid passed through ducts into a cavity that also receives digestive waste from the gut
Walls absorb water from the waste
Uric acid separates as white crystals

8. Are there some limitations to how kidneys function?
Most vertebrate kidneys cannot excrete concentrated salt
This is why you can’t drink seawater
Salt would overwhelm kidneys and you would die of dehydration
Some marine reptiles and birds have special glands in head that excrete concentrated salt solutions
Other adaptions exist – eg. Kangaroo rats
Kidneys produce urine 25 times more concentrated than blood
Intestines absorb so much water that their feces are almost dry

9. Key points questions What problem does ammonia pose to animals?
What are the different ways that animals can dispose of ammonia?
How does a kidney work?
How do freshwater and saltwater animals differ in their excretion of nitrogenous waste?
Would an animal that needs to conserve water be more likely to produce urnine or uric acid?