Mammalian kidney

The mammalian kidneys are a pair of excretory organs of the urinary system of mammals, being functioning kidneys in postnatal-to-adult individuals (i. e. metanephric kidneys). The kidneys in mammals are usually bean-shaped or externally lobulated. They are located behind the peritoneum (retroperitoneally) on the back (dorsal) wall of the body. The typical mammalian kidney consists of a renal capsule, a peripheral cortex, an internal medulla, one or more renal calyces, and a renal pelvis. Although the calyces or renal pelvis may be absent in some species. The medulla is made up of one or more renal pyramids, forming papillae with their innermost parts. Generally, urine produced by the cortex and medulla drains from the papillae into the calyces, and then into the renal pelvis, from which urine exits the kidney through the ureter. Nitrogen-containing waste products are excreted by the kidneys in mammals mainly in the form of urea.

The structure of the kidney differs between species. The kidneys can be unilobar (a single lobe represented by a single renal pyramid) or multilobar, unipapillary (a single or a common papilla), with several papillae or multipapillary, may be smooth-surfaced or lobulated. The multilobar kidneys can also be reniculate, which are found mainly in marine mammals. The unipapillary kidney with a single renal pyramid is the simplest type of kidney in mammals, from which the more structurally complex kidneys are believed to have evolved. Differences in kidney structure are the result of adaptations during evolution to variations in body mass and habitats (in particular, aridity) between species.

The cortex and medulla of the kidney contain nephrons, each of which consists of a glomerulus and a complex tubular system. The cortex contains glomeruli and is responsible for filtering the blood. The medulla is responsible for urine concentration and contains tubules with short and long loops of Henle. The loops of Henle are essential for urine concentration. Amongst the vertebrates, only mammals and birds have kidneys that can produce urine more concentrated (hypertonic) than the blood plasma, but only in mammals do all nephrons have the loop of Henle.

The kidneys of mammals are vital organs that maintain water, electrolyte and acid-base balance in the body, excrete nitrogenous waste products, regulate blood pressure, and participate in bone formation and regulation of glucose levels. The processes of blood plasma filtration, tubular reabsorption and tubular secretion occur in the kidneys, and urine formation is a result of these processes. The kidneys produce renin and erythropoietin hormones, and are involved in the conversion of vitamin D to its active form. Mammals are the only class of vertebrates in which only the kidneys are responsible for maintaining the homeostasis of the extracellular fluid in the body. The function of the kidneys is regulated by the autonomic nervous system and hormones.

The potential for regeneration in mature kidneys is limited because new nephrons cannot be formed. But in cases of limited injury, renal function can be restored through compensatory mechanisms. The kidneys can have noninfectious and infectious diseases; in rare cases, congenital and hereditary anomalies occur in the kidneys of mammals. Pyelonephritis is usually caused by bacterial infections. Some diseases may be species specific, and parasitic kidney diseases are common in some species. The structural characteristics of the mammalian kidneys make them vulnerable to ischemic and toxic injuries. Permanent damage can lead to chronic kidney disease. Ageing of the kidneys also causes changes in them, and the number of functioning nephrons decreases with age.