Mammalian kidney

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Paired organ in the urinary system of mammals

Mammalian kidney
Unipapillary, multilobar, smooth, bean-shaped camel kidney, in which the renal papillae are completely fused into the renal crest.[1]
Details
Precursor	Ureteric bud, metanephrogenic blastema
System	Urinary system and endocrine system
Artery	Renal artery
Vein	Renal vein
Nerve	Renal plexus
Lymph	Collecting lymphatic vessels
Anatomical terminology [edit on Wikidata]

The mammalian kidneys are a pair of excretory organs of the

dorsal) wall of the body.^[7] The typical mammalian kidney consists of a renal capsule, a peripheral cortex, an internal medulla, one or more renal calyces, and a renal pelvis.^[7] Although the calyces or renal pelvis may be absent in some species.^[7] The medulla is made up of one or more renal pyramids,^[8] forming papillae with their innermost parts.^[9] 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.^[7][10] Nitrogen-containing waste products are excreted by the kidneys in mammals mainly in the form of urea.^[11]

The structure of the kidney differs between species.[12] The kidneys can be unilobar (a single lobe represented by a single renal pyramid) or multilobar,^[13][14] unipapillary (a single or a common papilla), with several papillae or multipapillary,^[14][15] may be smooth-surfaced or lobulated.^[1][13] The multilobar kidneys can also be reniculate, which are found mainly in marine mammals.^[16] 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.^[17][6][18] Differences in kidney structure are the result of adaptations during evolution to variations in body mass and habitats (in particular, aridity) between species.^[19][20][12]

The cortex and medulla of the kidney contain nephrons,^[21] each of which consists of a glomerulus and a complex tubular system.^[22] The cortex contains glomeruli and is responsible for filtering the blood.^[7] The medulla is responsible for urine concentration^[23] and contains tubules with short and long loops of Henle.^[24] The loops of Henle are essential for urine concentration.^[25] Amongst the vertebrates, only mammals and birds have kidneys that can produce urine more concentrated (hypertonic) than the blood plasma,^[7] but only in mammals do all nephrons have the loop of Henle.^[26]

The kidneys of mammals are vital organs

tubular secretion occur in the kidneys, and urine formation is a result of these processes.^[8] The kidneys produce renin^[32] and erythropoietin^[33] hormones, and are involved in the conversion of vitamin D to its active form.^[34] 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.^[35] The function of the kidneys is regulated by the autonomic nervous system and hormones.^[36]

The potential for

ischemic and toxic injuries.^[47] Permanent damage can lead to chronic kidney disease.^[48][49] Ageing of the kidneys also causes changes in them, and the number of functioning nephrons decreases with age.^[50]

Structure

Gross anatomy

Interlobular artery

.

Location and shape

In mammals, the kidneys are usually bean-shaped;[4] the shape is unique to mammals (fish, for example, have elongated kidneys).^[52] Some species have externally lobulated kidneys, as in marine mammals, bovines and bears.^[53][13] The lobulated kidneys of cetacians and pinnipeds have elongated oval shape.^[54] 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.^[6][55]

The kidneys are located

adipose capsule),^[56][57][58] which separates the kidney from the perirenal fascia.^[58]

General structure

The outer layer of each kidney is made up of a fibrous sheath called a

bladder,^[59][10] after which it is excreted through the urethra.^[61]

Parenchyma

Podocytes

  Proximal convoluted tubule

  Proximal straight tubule

  Intermediate tubule (loop of Henle)

  Distal straight tubule

  Macula densa

  Distal convoluted tubule

  Connecting tubule

  Collecting duct

The

desert animals it is only a fifth part.^[13] Increased renal medulla thickness is associated with greater urine concentrating ability in desert mammals.^[65]

The cortex and medulla are based on

renal interstitium.^[67] The nephron is composed of a renal corpuscle and a renal tubule. The renal corpuscle is a blood-filtering part of the nephron and is located in the cortex. The renal tubule extends from the renal corpuscle to the medulla into the loop of Henle and then returns back to the cortex. Finally, the renal tubule flows with its distal end into its collecting duct, which 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.^[68][69]

Based on the location of the renal corpuscle in the cortex, nephrons are classified into 3 types: superficial (closer to the renal capsule), midcortical (in the middle part of the cortex) and juxtamedullary (closer to the medulla) nephrons.[24] Generally, they differ in the length of the loop of Henle. Superficial and midcortical nephrons typically have loops of Henle that are shorter than those of juxtamedullary nephrons.^[70] According to the length of the loop of Henle, nephrons are classified into nephrons with a long loop and with a short loop of Henle.^[24] Although those two classifications do not coincide. Usually, juxtamedullary nephrons have long loops of Henle, but there are more long-looped nephrons than juxtamedullary nephrons in the kidneys.^[71]

Cortex

Structurally, the cortex consists of cortical labyrinth and

proximal convoluted tubules, macula densa, distal convoluted tubules, connecting tubules and the initial parts of the collecting ducts.^[72] The proximal convoluted tubules predominate in the cortical labyrinth.^[73] The continuous layer of the cortex lying above the medullary rays is called the cortex corticis.^[9] Cortex corticis differs from the rest of the cortical labyrinth in that it doesn't contain glomeruli.^[74][75] Some mammals (for example, pig) have nephrons whose loops of Henle do not reach the medulla; such nephrons are called cortical nephrons.^[68][76] Cortical nephrons have a very short thin segment of the loop of Henle, and this segment may even be absent.^[76] 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.^[72] 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.^[77]

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.[78] The outer region is also subdivided into outer^[79] (lying directly under the cortex)^[80] and inner stripes.^[79] The stripes differ in that the outer stripe contains proximal straight tubules, while the inner stripe contains thin descending limbs of the loop of Henle (a section of the nephron following the proximal straight tubule).^[80]

The ability to produce more concentrated urine is associated with the length of the inner medulla (with its long loops of Henle).^[81] 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 kidneys and their ability to concentrate urine is low. Dogs and cats, on the other hand, have only long-loop nephrons with an average ability to concentrate urine. The ratio of nephrons with short loops of Henle to those with long loops also varies between species.^[82] Previously, it was mistakenly believed that species with the highest urine concentration ability have only long-looped nephrons. But the kidney of species with high ability to concentrate urine have more short-looped nephrons than long-looped nephrons, so the highest concentration ability requires both types of nephrons.^[83]

Variations

Bovine kidneys with lobulated cortex and fused medulla

(C) Canine

bean-shaped kidneys.

Kidneys can be unipapillary,[14] as in rats and mice,^[88] with few renal papillae, as in spider monkeys, or with many, as in pigs and humans.^[14] Most animals have single renal papilla.^[14] 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.^[15] Such kidneys are called crest kidneys and are also considered unipapillary kidneys (an enlarged modification).^[89][86][19] The crest kidneys usually appear in species larger than the rabbit (for example, in monkeys and camels).^[90][19]

The kidneys of the marine mammals, otters and bears are reniculate.^[16] The reniculate kidneys consist of small reniculi,^[16] each of which is comparable by its structure to a simple unipapillary kidney.^[9] The kidneys of marine mammals can have hundreds^[16] or thousands^[49] of reniculi, each with its own cortex, medulla, and calyx.^[16] For example, each whale kidney consist of about 7000 renculi which join a common collective system.^[49] Although the kidneys of manatees are actually multilobar because their cortex is continuous rather than discrete.^[16]

The size of the kidneys increases with the mass of mammals, and the number of

Blood enters the kidney through the renal artery,^[55] which in the multilobar kidney branches in the area of the renal pelvis into large interlobar arteries that pass through the renal columns.^[10][109] The interlobar arteries branch at the base of the pyramid, giving rise to arcuate arteries, from which the interlobular arteries extend into the cortex.^[109] The interlobar arteries supply the pyramids and the adjacent cortex with an extensive network of blood vessels.^[10] The cortex itself is heavily permeated with arteries, while there are no arteries in the medulla.^[17] The venous flow of blood runs back parallel to the arteries.^[109] In some species, there are veins isolated from the arteries under the capsule in the cortex, which in humans are called stellate veins. These veins flow into the interlobular veins.^[110] The renal portal system is absent in mammals,^[111] with the exception of monotremes.^[112] Mammals are the only class of vertebrates (with exception of some species) that does not have a renal portal system.^[113]

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, which descend 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.^[109]

Lymphatic drainage

The kidney is well supplied with lymphatic vessels,^[114] which remove excess fluid with substances and macromolecules dissolved in it from the interstitium that fills the space between the tubules and blood vessels.^[115][116] The anatomy of the lymphatic system of the kidney is similar between mammals.^[117] Lymphatics basically follow the path of blood vessels.^[118]

The lymphatic system of the kidneys begins in the cortex with the initial blind-end intralobular lymphatic capillaries passing near the tubules and renal corpuscles, but the lymphatic vessels do not go inside the renal corpuscles. The intralobular lymphatic capillaries are connected to the arcuate lymphatics.^[119] The arcuate lymphatics pass into the interlobar lymphatics, which pass near the interlobar arteries.^[119][117] The arcuate and interlobar lymphatics are lymphatic precollectors.^[97] Finally, the interlobar lymphatics join the collecting hilar lymphatics leaving the kidney through renal hilum.^[119] Lymphatic vessels are usually absent in the medulla of the mammalian kidneys, and the role of lymphatic vessels is assumed to be performed by vasa recta.^[120][121]

In some species, there may be differences in the anatomy of the lymphatic system of the kidney. For example, sheep lack lymphatics in the renal capsule, and rabbits lack interlobular lymphatics.^[119] Most studies fail to detect lymphatic vessels in the renal medulla of animals, in particular, they are not found in sheep and rats. But some studies have found lymphatic vessels in the renal medulla of pigs and rabbits.^[121] Depending on the species, there may or may not also be a connection between the lymphatics of the renal capsule and the internal renal lymphatic system.^[122]

Nerve supply

The

In mammals, nitrogenous metabolic products are excreted predominantly in the form of urea,^[11] which is the end by-product of mammalian protein metabolism^[131][132] and is highly soluble in water.^[133] Most of the urea is excreted by the kidneys.^[131] Blood filtration, as in other vertebrates, occurs in the renal glomeruli, where pressurized blood passes through a permeable barrier that filters out blood cells and large protein molecules, forming primary urine. The filtered primary urine is osmotically and ionically the same as blood plasma. In the tubules of the nephron, substances useful for the body, dissolved in the primary urine, are subsequently reabsorbed, as the urine is being concentrated.^[134]

Osmoregulation

The mammalian kidneys maintain an almost constant level of plasma

In addition to excretory, the kidneys also perform an endocrine function, they produce certain hormones. The juxtaglomerular cells of the kidneys produce renin, which is a key regulator of the renin–angiotensin system, which is responsible for blood pressure regulation.^[32]

The production of

The kidneys are involved in the metabolism of vitamin D. In the liver, vitamin D is converted to calcifediol (25OHD), while the kidneys convert calcifediol to calcitriol (1,25(OH)₂D), which is the active form of the vitamin and is essentially a hormone. Vitamin D is involved in the formation of bones and cartilage, and also performs a number of other functions, for example, it is involved in the functioning of the immune system.^[34]

Blood pressure regulation

Some mammalian

The excreted urine is slightly acidic. The excretion of H⁺ together with urine also occurs through buffer systems, in particular, NH₄⁺ (ammonium).^[152] Only a small amount of NH₄⁺ is filtered through the glomerulus; ^[152] most of the ammonium excreted is the result of H⁺ ion oxidation of NH₃ (ammonia) formed in the cells of the proximal convoluted tubule, which is secreted into the lumen of the tubule either as NH₃ or as NH₄⁺.^[153] The formation of ammonia is also accompanied by the formation of new HCO₃^-, which replenishes the extracellular buffer system.^[153] In the thick ascending tubule of the loop of Henle, on the contrary, NH₄⁺ is absorbed, which causes its accumulation in the interstitium.^[154] The final stage of urine oxidation occurs in the collecting ducts, where H⁺ ions are secreted with the involvement of ATP, and NH₃ is transported from the interstitium and secreted into the urine, where NH₃ is oxidized by H⁺ to form NH₄⁺.^[151] By regulating HCO₃^- reabsorption and H⁺ secretion, the kidneys help maintain blood pH homeostasis.^[149]

Glucose homeostasis

Together with the

Mammalian metanephric kidney

The first mammals are believed to have appeared during the

One of the key factors that determine the shape and morphology of the kidneys in mammals is their mass.^[162] The simplest type of kidney in mammals is the unipapillary kidney, consisting of a cortex, medulla, and renal pelvis.^[163] But the unipapillary kidney is limited by the number of nephrons at which it functions optimally.^[20] It is assumed that unipapillary kidney was the original kidney structure in mammals, from which multilobar kidneys evolved.^[19]

More complex multilobar kidneys likely emerged as an adaptation to the increased body mass of mammals and the corresponding need for an increase in the number of nephrons in the kidneys.

Reniculate kidneys are typical mainly for marine mammals. They are believed to be an adaptation both to the large body mass, allowing the number of nephrons to increase by increasing the number of renculi, and to a diet with large amounts of saline water, as well as an adaptation for long term diving.^[19] Reniculate kidneys probably allow the number of nephrons to be increased by adding renculi without the need to increase tubule length as the organ size increases.^[93] Consumption of excess salt in marine mammals leads to intracellular dehydration, resulting in a need for rapid removal of excess salt from the body, which in the case of reniculate kidneys is facilitated by an increase in the total surface area between the cortex and medulla.^[19] The need to dive for long periods of time requires a reduction in the body's oxygen consumption,^[164] while the kidneys are an energy-consuming organ,^[165] so the glomerular filtration rate decreases during diving.^[164] In contrast, the glomerular filtration rate is very high between dives.^[19]

Development

Stages of kidney development

In mammals, kidney development during

The metanephros develops from the

In some mammals, kidney organogenesis ends before birth, while in others it may continue for some time into the postpartum period^[177] (for example, in rodents it ends about a week after birth).^[178] When the formation of new nephrons (nephrogenesis) ends, the number of nephrons in the kidney becomes final.^[177]

Postnatal maturation

After birth and in the

Kidney diseases or disorders may be congenital, inherited, non-infectious, and infectious.^[41] Diseases vary between mammalian species. Some diseases may be specific only to some species, while the others may be more common in one species and less common in another.^[44] For example, chronic progressive nephropathy is common in mice, rats and naked mole-rats,^[181] but at the same time there is no analogous disease in humans.^[182]

Congenital and inherited anomalies

Congenital anomalies and hereditary disorders of the kidneys among mammals are rare, but can have a significant impact on kidney function,

After maturation, the kidneys slowly begin to undergo ageing processes, which are characterized by changes in anatomy, physiology, function and regenerative capabilities. During the life of mammals, glomerulosclerosis affects glomeruli, the basement membrane thickens, the tubules undergo atrophic changes, and the renal interstitium fibrosis increases. The number of functioning nephrons gradually decreases throughout the life. In terms of function, the glomerular filtration rate decreases and the ability to concentrate urine decreases, too. Age-related changes themselves may not be noticeable and may not lead to kidney failure or disease, but are a risk factor for kidney or urinary tract diseases.^[50]

Repair and regeneration

Unlike more primitive