Kidney (vertebrates)

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.^[1][2][3][4] In healthy vertebrates, the kidneys maintain homeostasis of extracellular fluid in the body.^[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.^[6]

All vertebrates have kidneys. The kidneys are the main organ that allows species to adapt to different environments, including fresh and salt water, terrestrial life and desert climate.^[7] Depending on the environment in which animals have evolved, the functions and structure of the kidneys may differ.^[8] Also, between classes of animals, the kidneys differ in shape and anatomical location.^[9][10] In mammals, they are usually bean-shaped.^[11] Evolutionarily, the kidneys first appeared in fish as a result of the independent evolution of the renal glomeruli and tubules, which eventually united into a single functional unit.^[12] In some invertebrates, the nephridia are analogous to the kidneys but nephridia are not kidneys.^[13] The metanephridia, together with the vascular filtration site and coelom, are functionally identical to the ancestral primitive kidneys of vertebrates.^[14]

The main structural and functional element of the kidney is the nephron.^[15] Between animals, the kidneys can differ in the number of nephrons and in their organisation.^[16] According to the complexity of the organisation of the nephron, the kidneys are divided into pronephros, mesonephros and metanephros.^[17] The nephron by itself is similar to pronephros as a whole organ.^[18] The simplest nephrons are found in the pronephros, which is the final functional organ in primitive fish.^[19] The nephrons of the mesonephros, the functional organ in most anamniotes called opisthonephros,^[20] are slightly more complex than those of the pronephros.^[19] The main difference between the pronephros and the mesonephros is that the pronephros consists of non-integrated nephrons with external glomeruli.^[7] The most complex nephrons are found in the metanephros of birds and mammals.^[19][21][22] The kidneys of birds and mammals have nephrons with loop of Henle.^[23]

All three types of kidneys are developed from the intermediate mesoderm of the embryo.^[24] It is believed that the development of embryonic kidneys reflects the evolution of vertebrate kidneys from an early primitive kidney, the archinephros.^[6] In some vertebrate species, the pronephros and mesonephros are functional organs, while in others they are only intermediate stages in the development of the final kidney, and each next kidney replaces the previous one.^[7] The pronephros is a functioning kidney of the embryo in bony fish and amphibian larvae,^[7] but in mammals it is most often considered rudimentary and not functional.^[18] In some lungfish and bony fishes, the pronephros can remain functional in adults, including often simultaneously with the mesonephros.^[7] The mesonephros is the final kidney in amphibians and most fish.^[25]