Excretion is the mechanism by which excretory structures or organs remove excreta, true cellular “junk” from the organism, such as ammonia (NH3), urea, CO2, salts and H2O.
In this way, the body will maintain the balance of the internal environment, that is, homeostasis.
Animal excretory mechanisms
In less complex animals that live in the aquatic environment, the elimination of cellular waste resulting from metabolism is generally achieved by simple diffusion on the body surface. So in protozoa, sponges and cnidaria, salts, ammonia and CO2 are excreted by the body wall.
We flatworms like the planarian, the protonephrids They are formed by flagellate cells (flame cells) attached to tubules and excretory pores that are distributed longitudinally on both sides of the body.
In annelids, segmental nephrids - complex structures associated with blood capillaries - take care of the expulsion of nitrogenous waste. In arthropods, several structures are related to nitrogen excretion.
Among them, we can mention the green glands of crustaceans, thigh glands of arachnids and Malpighi Tubules, found in both arachnids and insects.
In vertebrates, the main excretory organs are the kidneys. By receiving blood containing different types of substances, useful or not, the kidneys filter, selecting what will be eliminated and returning to the blood what can be reused.
Excreta Types - Nitrogen Compounds
The main function of carbohydrates, such as glucose, is to be a source for the production of ATP by the cells. The fatty acids, originated from the digestion of fats, and some amino acids They are also used by cells to obtain ATP (with the exception of brain neurons). For this, amino acids initially suffer deamination, that is, lose the amino radical. The rest of the molecule can be broken down by the process of cellular respiration in CO.2 and H2The with release of large amount of ATP. Amino acid deamination occurs in the liver, and the amine radical is converted to ammonia.
Invertebrates and many freshwater fish excrete ammonia, highly toxic and soluble substance, which requires a large amount of water to be disposed of. Therefore this type of excreta occurs only in aquatic animals, for which obtaining water is not a problem. Animals whose main excretion product is ammonia are called ammoniotellic.
Land animals turn ammonia into urea or in uric acid. Because these substances are much less toxic than ammonia, they can be temporarily accumulated in the body and excreted in concentrated solutions without major loss of water by the body. Urea is the main excreta of mammals it's from adult amphibians (amphibian larvae excrete ammonia), being eliminated dissolved in water, forming the urine. Animals whose main excretion product is urea are called ureotelics.
O uric acid It is the main excreta of insects, terrestrial snails, birds and some reptiles, being eliminated together with the feces, in the form of a highly concentrated whitish paste. The fact that uric acid can be excreted virtually without water loss is an important adaptation for water saving in the terrestrial environment. Animals whose main excretion product is uric acid are called Uricothelics.