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Only the organs specifically used for the excretion are considered a part of the excretory system. In the narrow sense, the term refers to the urinary system. However, as excretion involves several functions that are only superficially related, it is not usually used in more formal classifications of anatomy or function.
Excretion is elimination of metabolic waste, which is an essential process in all organisms. In vertebrates , this is primarily carried out by the lungs , kidneys , and skin . [ 1 ] This is in contrast with secretion , where the substance may have specific tasks after leaving the cell .
A flame cell is a specialized excretory cell found in simple invertebrates, including flatworms (Platyhelminthes), rotifers and nemerteans; these are the simplest animals to have a dedicated excretory system.
The excretion of urea is called ureotelism. Land animals, mainly amphibians and mammals, convert ammonia into urea, a process which occurs in the liver and kidney. These animals are called ureotelic. [3] Urea is a less toxic compound than ammonia; two nitrogen atoms are eliminated through it and less water is needed for its excretion.
Stylised diagram of the last part of the insect's digestive tract showing malpighian tubule (orthopteran type)The Malpighian tubule system is a type of excretory and osmoregulatory system found in some insects, myriapods, arachnids and tardigrades.
Excretion of excess water from the body is the main characteristic of the pronephros in the case of species in which it develops into a functional excretory organ. In some species, the pronephros is functional during the embryonic stage of development, representing the first stage of kidney development, after which the mesonephros develops.
The nephridium (pl.: nephridia) is an invertebrate organ, found in pairs and performing a function similar to the vertebrate kidneys (which originated from the chordate nephridia).
The nephron uses four mechanisms to convert blood into urine: filtration, reabsorption, secretion, and excretion. [5]: 395–396 These apply to numerous substances. The structure and function of the epithelial cells lining the lumen change during the course of the nephron, and have segments named by their location and which reflects their ...