The kidneys are a pair of organs of the excretory system in vertebrates, which maintain the balance of water and electrolytes in the body (osmoregulation), filter the blood, remove metabolic waste products, and, in many vertebrates, also produce hormones (in particular, renin) and maintain blood pressure.
[5] When the blood is being filtered, the kidneys form urine, which consists of water and excess or unnecessary substances, the urine is then excreted from the body through other organs, which in vertebrates, depending on the species, may include the ureter, urinary bladder, cloaca, and urethra.
The kidneys are the main organ that allows species to adapt to different environments, including fresh and salt water, terrestrial life and desert climate.
[13] The metanephridia, together with the vascular filtration site and coelom, are functionally identical to the ancestral primitive kidneys of vertebrates.
[19] The nephrons of the mesonephros, the functional organ in most anamniotes called opisthonephros,[20] are slightly more complex than those of the pronephros.
[18] In some lungfish and bony fishes, the pronephros can remain functional in adults, including often simultaneously with the mesonephros.
[28] At the very beginning of vertebrates, when they evolved from marine chordates, their evolution probably took place in fresh or slightly saline water.
As a result, early vertebrates developed renal glomeruli capable of filtering blood and perhaps tubules that reabsorbed ions.
[29] 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.
[30][28] Metanephros, the permanent kidney of amniotes, has the unique ability to efficiently retain water in the body.
[28] In addition to water conservation, terrestrial life also required maintenance of salt levels in the body along with the excretion of waste products.
The relative ionic composition of the extracellular fluid is similar between marine fish and all subsequent species.
Therefore, it can be said that the kidneys made it possible to preserve approximately the same composition of extracellular fluid in vertebrates as it was in the primordial ocean.
[13] As an organ, the archnephros is still preserved in the larvae of hagfishes and some caecilians, and is also found in the embryos of some more developed vertebrates.
[34] The pronephros is a relatively large organ that has a primitive structure and usually consists of a single pair of bilateral nephrons with an external glomerulus or glomus.
[7] Because of its small size and simple structure, the pronephros of fish and amphibian larvae has become an important experimental model for studying kidney development.
[28] Metanephros differs from pronephros and mesonephros in development, position in the body, shape, number of nephrons, organization and drainage.
[44] Each metanephric kidney is characterized by a large number of nephrons and a highly branched system of collecting tubules and ducts,[28] that open into the ureter.
[49] Reptile kidneys are commonly elongated[51] with color ranging from light to dark brown.
[3][21] Nitrogenous waste products excreted by the kidneys may include uric acid, urea and ammonia.
[21] Since the reptile kidneys are unable to produce concentrated urine due to the absence of the loop of Henle, glomerular filtration rate is decreased if water loss needs to be reduced.
[52] In addition to the renal artery blood supply, reptiles also have a renal portal system, which can redirect blood to the kidneys during periods of water deprivation, bypassing the glomeruli, to prevent ischemic necrosis of tubular cells.
[21][57] In mammals, the kidneys are usually bean-shaped[58] and located retroperitoneally[59] on the dorsal (posterior) wall of the body.
[28] In multilobar kidneys, the pyramids are separated from each other by areas of cortical tissue that dip into the medulla, known as renal columns.
[72] Marine mammals, bears and otters have reniculate kidneys which are made of large amount of lobes called reniculi.
[74] Nitrogenous waste products are excreted by the kidneys of mammals primarily in the form of urea,[75] which is highly soluble in water.
[56] In birds, the kidneys are typically elongated[79] and located dorsally in the abdominal cavity in the pelvic skeletal depressions.
[77] Like mammals, although to a lesser extent,[67] birds are able to produce concentrated urine, thus conserving water in the body.