Excretion Section 27.4

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

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

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

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

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

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

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

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?