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. The kidneys in mammals are usually bean-shaped or externally lobulated. They are located behind the peritoneum on the back 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 or multilobar, unipapillary, 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 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 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.
Structure
Gross anatomy
Location and shape
In mammals, the kidneys are usually bean-shaped; the shape is unique to mammals. Some species have externally lobulated kidneys, as in marine mammals, bovines and bears. The lobulated kidneys of cetacians and pinnipeds have elongated oval shape. 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.The kidneys are located retroperitoneally on the back wall of the body of mammals. In the body, the kidney is surrounded or partially surrounded by a layer of fatty tissue, which separates the kidney from the perirenal fascia.
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 called the cortex, and the inner layer is called the medulla. The medulla consists of 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 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 the functional part of the kidneys, is visually divided into cortex and medulla. The cortex itself is similar to the typical kidneys of less developed vertebrates, which cannot produce concentrated urine, while the medulla is the main site of urine concentration. 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 it is only a fifth part. Increased renal medulla thickness is associated with greater urine concentrating ability in desert mammals.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. 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.
Based on the location of the renal corpuscle in the cortex, nephrons are classified into 3 types: superficial, midcortical and juxtamedullary nephrons. 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. 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. 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.
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. Cortex corticis differs from the rest of the cortical labyrinth in that it doesn't contain glomeruli. Some mammals have nephrons whose loops of Henle do not reach the medulla; such nephrons are called cortical nephrons. Cortical nephrons have a very short thin segment of the loop of Henle, and this segment may even be absent. 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 and inner stripes. 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.The ability to produce more concentrated urine is associated with the length of the 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 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. 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.
Variations
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. Larger animals have multilobar kidneys, such as those of bovines. 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 calyces 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. Such kidneys are called crest kidneys and are also considered unipapillary kidneys. The crest kidneys usually appear in species larger than the rabbit.
The kidneys of the marine mammals, otters and bears are reniculate. The reniculate kidneys consist of small reniculi, each of which is comparable by its structure to a simple unipapillary kidney. The kidneys of marine mammals can have hundreds or thousands of reniculi, each with its own cortex, medulla, and calyx. For example, each whale kidney consist of about renculi which join a common collective system. Although the kidneys of manatees are actually multilobar because their cortex is continuous rather than discrete.
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 long, weighing 400 mg, with 16,000 nephrons, while in the killer whale, the kidney length exceeds, the mass is approximately, with the number of nephrons of the order of 10,000,000. At the same time, the killer whale kidneys are reniculate, with each renicule about 430 mg in weight and in length, making renicule of the killer whale similar to the kidney of a mouse not only in structure, but also in size and mass.