For example, they convert a precursor of vitamin D to its active form, calcitriol; and synthesize the hormones erythropoietin and renin.

[8] In humans, the kidneys are located high in the abdominal cavity, one on each side of the spine, and lie in a retroperitoneal position at a slightly oblique angle.

[22] Input from the sympathetic nervous system triggers vasoconstriction in the kidney, thereby reducing renal blood flow.

[22] The kidney also receives input from the parasympathetic nervous system,[23] by way of the renal branches of the vagus nerve; the function of this is yet unclear.

Kidney development, also called nephrogenesis, proceeds through a series of three successive developmental phases: the pronephros, mesonephros, and metanephros.

In addition, passive countercurrent exchange by the vessels carrying the blood supply to the nephron is essential for enabling this function.

The kidney participates in whole-body homeostasis, regulating acid–base balance, electrolyte concentrations, extracellular fluid volume, and blood pressure.

Filtration, which takes place at the renal corpuscle, is the process by which cells and large proteins are retained while materials of smaller molecular weights are[30] filtered from the blood to make an ultrafiltrate that eventually becomes urine.

A plasma glucose level of approximately 160 is sufficient to allow glucosuria, which is an important clinical clue to diabetes mellitus.

[34] Secretion is the reverse of reabsorption: molecules are transported from the peritubular capillary through the interstitial fluid, then through the renal tubular cell and into the ultrafiltrate.

Calcitriol, the activated form of vitamin D, promotes intestinal absorption of calcium and the renal reabsorption of phosphate.

This primarily occurs through maintenance of the extracellular fluid compartment, the size of which depends on the plasma sodium concentration.

Each hormone acts via multiple mechanisms, but both increase the kidney's absorption of sodium chloride, thereby expanding the extracellular fluid compartment and raising blood pressure.

The lungs are the part of respiratory system which helps to maintain acid–base homeostasis by regulating carbon dioxide (CO2) concentration in the blood.

In basic conditions, the respiratory rate will slow down so that the body holds onto more CO2 and increases the H+ concentration and decreases the pH.

Under acidic conditions, the high concentration of CO2 in the blood creates a gradient for CO2 to move into the cell and push the reaction HCO3 + H ↔ H2CO3 ↔ CO2 + H2O to the left.

When the reaction is pushed to the left it also increases the HCO3 concentration in the cell and HCO3 is then able to move out into the blood which additionally raises the pH.

Any significant rise in plasma osmolality is detected by the hypothalamus, which communicates directly with the posterior pituitary gland.

Dialysis removes metabolic waste products as well as excess water and sodium (thereby contributing to regulating blood pressure); and maintains many chemical levels within the body.

Finally, ultra-structural examination is performed with electron microscopy and may reveal the presence of electron-dense deposits or other characteristic abnormalities that may suggest an etiology for the patient's renal disease.

The kidneys of fish and amphibians are typically narrow, elongated organs, occupying a significant portion of the trunk.

[47] In the most primitive vertebrates, the hagfish and lampreys, the kidney is unusually simple: it consists of a row of nephrons, each emptying directly into the archinephric duct.

Reptiles have relatively few nephrons compared with other amniotes of a similar size, possibly because of their lower metabolic rate.

The latter feature is due to the presence of elongated loops of Henle; these are much shorter in birds, and not truly present in other vertebrates (although the nephron often has a short intermediate segment between the convoluted tubules).

[47] Kidneys of various animals show evidence of evolutionary adaptation and have long been studied in ecophysiology and comparative physiology.

Kidney morphology, often indexed as the relative medullary thickness, is associated with habitat aridity among species of mammals[49] and diet (e.g., carnivores have only long loops of Henle).

[36] In ancient Egypt, the kidneys, like the heart, were left inside the mummified bodies, unlike other organs which were removed.

[50] According to studies in modern and ancient Hebrew, various body organs in humans and animals served also an emotional or logical role, today mostly attributed to the brain and the endocrine system.

The kidney is mentioned in several biblical verses in conjunction with the heart, much as the bowels were understood to be the "seat" of emotion – grief, joy and pain.

[61] The urinary tract including the ureters, as well as their function to drain urine from the kidneys, has been described by Galen in the second century AD.