mammalian kidney mammalian kidney the mammalian kidneys are the paired organ of the urinary system of mammals, which is a type of metanephric kidney. the kidney in mammals is usually bean-shaped, located retroperitoneally on the back (dorsal) wall of the body. each kidney consists of a renal capsule, peripheral cortex, internal medulla, calices, and renal pelvis, although the calices or renal pelvis may be absent in some species. urine is excreted from the kidney through the ureter. the structure of the kidney may differ between species depending on the environment, in particular on its aridity. the cortex is responsible for filtering the blood, this part of the kidney is similar to the typical kidneys of less developed vertebrates. nitrogen-containing waste products are excreted by the kidneys in mammals mainly in the form of urea. mammalian kidneyunipapillary, multilobar, smooth, bean-shaped camel kidney, in which the renal papillae are completely fused into the renal crest.detailsprecursorureteric bud, metanephrogenic blastemasystemurinary system and endocrine systemarteryrenal arteryveinrenal veinnerverenal plexuslymphcollecting lymphatic vesselsanatomical terminology depending on the species, the kidneys can be unilobar (single lobe) or multilobar, unipapillary (single papilla), with several papillae or multipapillary, may be smooth-surfaced or lobulated, also the kidneys may be reniculate, which are found mainly in marine mammals. the simplest type of kidney in mammals is the unipapillary kidney with single lobe. the human kidney is an example of the mammalian kidney. the cortex and medulla of the kidney contain nephrons. in mammals, the nephron consists of the renal glomerulus inside bowman's capsule, the proximal convoluted tubule, the proximal straight tubule, the loop of henle, and the distal convoluted tubule. the nephrons can be classified as nephrons with a short loop and a long loop of henle. blood that enters the kidneys is filtered through the renal glomeruli into bowman's capsule to form primary urine. then it enters the tubules, which concentrate it. 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. 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. structure didactic model of the mammalian kidney: 1 — fibrous capsule, 2 — cortex, 3 — renal pyramid of the medulla, 4 — renal column of the cortex, 5 — nephron, 6 — renal papilla, 7 — minor renal calyx, 8 — major renal calyx, 9 — renal pelvis, 10 — ureter, 11 — renal artery, 12 — renal vein, 13 — interlobar artery, 14 — renal lobe, 15 — arcuate artery, 16 — interlobular artery. location and form in mammals, kidneys are usually bean-shaped; the shape is unique to mammals (fish, for example, has elongated kidneys). kidneys are located retroperitoneally on the posterior (dorsal) wall of the body. one of the key factors that determine the shape and morphology of the kidneys in mammals is their mass. the concave part of the bean-shaped kidneys is called the renal hilum, through which the renal artery and nerves enter the kidney. the renal vein, collecting lymphatic vessels and ureter exit the kidney through the renal hilum. in the body, the kidney is surrounded by a mass of adipose tissue. general structure the outer layer of each kidney is made up of a fibrous sheath called a renal capsule. the peripheral layer of the kidney is represented by the cortex, and the inner layer is represented by the medulla. the medulla consists of pyramids (also called malpighian pyramids), ascending with their base to the cortex and forming together with it the renal lobe. the pyramids are separated from each other by renal columns (bertin's columns) formed by cortical tissue. the tips of the pyramids end with the renal papillae, from which urine is excreted into the calyces, pelvis, ureter, and, in most species, directly into the bladder, after which it is excreted through the urethra. parenchyma the parenchyma, being a functional part of the kidneys, is visually divided into cortex and medulla. the cortex and medulla are based on nephrons together with an extensive network of blood vessels and capillaries, as well as collecting ducts, into which nephrons empty, and renal interstitium. there is a blood-filtering part of the nephron in the cortex — the renal corpuscle, from which the renal tubule extends to the medulla into the loop of henle, then the tubule returns back to the cortex and with its distal end flows into its collecting duct that is common to several nephrons. the collecting ducts descend again into the medulla and fuse to wider collecting ducts which pass through the inner medulla. the ratio of cortex to medulla varies between species, in domesticated animals the cortex usually occupies a third or fourth part of the parenchyma, while in desert animals with long loops of henle it is only a fifth part. cortex structurally, the cortex consists of cortical labyrinth and medullary rays. the cortical labyrinth contains interlobular arteries, vascular networks formed by afferent and efferent arterioles, renal corpuscles, proximal convoluted tubules, macula densa, distal convoluted tubules, connecting tubules and the initial parts of the collecting ducts. the proximal convoluted tubules predominate in the cortical labyrinth. the continuous layer of the cortex lying above the medullary rays is called the cortex corticis. some mammals have nephrons whose loops of henle do not reach the medulla; such nephrons are called cortical nephrons. the medullary rays of the cortex contain the proximal straight tubules, the cortical part of the thick ascending limb of the loops of henle, and the cortical part of the collecting ducts. the cortex is divided into lobules, each of which is a medullary ray in conjunction with connected to it nephrons, and interlobular arteries that pass between the lobules. medulla the medulla in mammals is divided into outer and inner regions. the outer region consists of short loops of henle and collecting ducts, while the inner region consists of long loops and collecting ducts. the outer region is also subdivided into outer (lying directly under the cortex) and inner stripes. the stripes differ in that the outer stripe contain proximal straight tubules, while the inner strite contain thin descending limbs of the loop of henle (a section of the nephron following the proximal straight tubule). the ratio of the outer and inner medulla most mammalian species have nephrons with both short and long loops of henle, while some species may have only one type. for example, mountain beavers have only nephrons with a short loop, and, accordingly, there is no inner medulla in the kidney. dogs and cats, on the other hand, have only long-loop nephrons. the ratio of nephrons with short loops of henle to those with long loops also varies between species. structural differences between species buffalo kidneys structurally, kidneys vary between mammals. what structural type a particular species will have depends generally on the body mass of the animals. small mammals have unilobar kidneys with a compact structure and a single renal papilla, while larger animals have multilobar kidneys, such as those of bovines, but bovine kidneys are also externally lobulated (visually divided into lobes). by itself, the lobe is equivalent to a simple unipapillary kidney, as in rats or mice. bovine kidneys also lack renal pelvis, urine from the major calices is excreted directly into the ureter. kidneys can be unipapillary, as in rats and mice, with few renal papillae, as in spider monkeys, or with many, as in pigs and humans. most animals have single renal papilla. in some animals, such as horses, the tips of the renal pyramids fuse with each other to form a common renal papilla, called the renal crest. the renal crest usually appears in animals larger than the rabbit. horse kidneys in marine mammals, otters and bears, the kidneys are reniculate, consisting of small reniculi, each of which is comparable to a simple unilobar kidney. marine mammal kidneys can have hundreds of reniculi, each with its own cortex, medulla, and calyx. in manatees, which are also marine mammals, the kidneys are actually multilobar, since the cortex is continuous. the size of the kidneys increases with the mass of mammals, and the number of nephrons in the kidneys between mammals increases allometrically. in mice, the kidneys are approximately 1 cm (0.4 in) long, weighing 400 mg, with 16,000 nephrons, while in the killer whale, the kidney length exceeds 25 cm (10 in), the mass is approximately 4.5 kg (10 lb), with the number of nephrons of the order of 10,000,000. at the same time, killer whale kidneys are reniculate, and each reniculus is comparable to the kidney of mice. reniculate kidneys probably increase the number of nephrons by the addition of individual reniculi without the need to increase the length of the tubules. an alternative adaptation mechanism is an increase in the diameter of the renal glomeruli in large mammals (and, accordingly, an increase in the length of the tubules), like in elephants, in which the glomerulus diameter can be 2 times larger than that of killer whales. microanatomy by microanatomic structure, the kidney can be divided into several main elements: interstitium, renal corpuscles, tubules, and vasculature. the interstitium is the cells and extracellular matrix in the space between the glomeruli, vessels, tubules, and collecting ducts. due to the lack of a basement membrane, the lymphatic capillaries are also considered part of the interstitium. the nephron, together with the collecting duct that continues it, is called the uriniferous tubule. approximately 18–26 different cell types have been described in mammalian kidneys, with a large variation in the range due to a lack of consensus on what counts as a particular cell type, and likely to species differences. at least 16 different cell types make up the renal tubules. the tubules themselves are divided into at least 14 segments, which differ in cell types and functions. the normal functioning of the kidneys is provided by the complex of epithelial, endothelial, interstitial and immune cells. blood supply equine kidney blood supply blood enters the kidney through the renal artery, which in the multilobar kidney branches in the area of the renal pelvis into large interlobar arteries that pass through the renal columns. the interlobar arteries branch at the base of the pyramid, giving rise to arcuate arteries, from which the interlobular arteries extend into the cortex. the interlobar arteries supply the pyramids and the adjacent cortex with an extensive network of blood vessels. the cortex itself is heavily permeated with arteries, while there are no arteries in the medulla. the venous flow of blood runs back parallel to the arteries. in some species, in the cortex there are veins isolated from the arteries under the capsule, which in humans are called stellate veins. these veins flow into the interlobular veins. the renal portal system is absent in mammals, with the exception of monotremes. mammals are the only class of vertebrates (with exception of some species) that does not have a renal portal system. the vascular glomeruli of nephrons receive blood from afferent arterioles, which originate in the interlobular arteries with intermediate formation of prearterioles. each afferent arteriole divides into several renal glomeruli. then these glomeruli join into the efferent arteriole, into which filtered blood goes from the nephrons. in nephrons with a long loop of henle, the efferent arterioles branch, forming straight vessels called vasa recta, descending into the medulla. the descending vasa recta, ascending vasa recta vessels, and the loop of henle together form the countercurrent system of the kidney. in the afferent arteriole, blood is supplied at high pressure, which promotes filtration, and in the efferent arteriole, it is at low pressure, which promotes reabsorption. despite their small size, the kidneys of mammals account