The kidneys are a pair of organs of the excretory system in vertebrates, which maintain the balance of water and electrolytes in the body (osmoregulation), filter the blood, remove metabolic waste products, and, in many vertebrates, also produce hormones (in particular, renin) and maintain blood pressure. [1] [2] [3] [4] In healthy vertebrates, the kidneys maintain homeostasis of extracellular fluid in the body. [5] When the blood is being filtered, the kidneys form urine, which consists of water and excess or unnecessary substances, the urine is then excreted from the body through other organs, which in vertebrates, depending on the species, may include the ureter, urinary bladder, cloaca, and urethra. [6]
All vertebrates have kidneys. The kidneys are the main organ that allows species to adapt to different environments, including fresh and salt water, terrestrial life and desert climate. [7] Depending on the environment in which animals have evolved, the functions and structure of the kidneys may differ. [8] Also, between classes of animals, the kidneys differ in shape and anatomical location. [9] [10] In mammals, they are usually bean-shaped. [11] Evolutionarily, the kidneys first appeared in fish as a result of the independent evolution of the renal glomeruli and tubules, which eventually united into a single functional unit. [12] In some invertebrates, the nephridia are analogous to the kidneys but nephridia are not kidneys. [13] The metanephridia, together with the vascular filtration site and coelom, are functionally identical to the ancestral primitive kidneys of vertebrates. [14]
The main structural and functional element of the kidney is the nephron. [15] Between animals, the kidneys can differ in the number of nephrons and in their organisation. [16] According to the complexity of the organisation of the nephron, the kidneys are divided into pronephros, mesonephros and metanephros. [17] The nephron by itself is similar to pronephros as a whole organ. [18] The simplest nephrons are found in the pronephros, which is the final functional organ in primitive fish. [19] The nephrons of the mesonephros, the functional organ in most anamniotes called opisthonephros, [20] are slightly more complex than those of the pronephros. [19] The main difference between the pronephros and the mesonephros is that the pronephros consists of non-integrated nephrons with external glomeruli. [7] The most complex nephrons are found in the metanephros of birds and mammals. [19] [21] [22] The kidneys of birds and mammals have nephrons with loop of Henle. [23]
All three types of kidneys are developed from the intermediate mesoderm of the embryo. [24] It is believed that the development of embryonic kidneys reflects the evolution of vertebrate kidneys from an early primitive kidney, the archinephros. [6] In some vertebrate species, the pronephros and mesonephros are functional organs, while in others they are only intermediate stages in the development of the final kidney, and each next kidney replaces the previous one. [7] The pronephros is a functioning kidney of the embryo in bony fish and amphibian larvae, [7] but in mammals it is most often considered rudimentary and not functional. [18] In some lungfish and bony fishes, the pronephros can remain functional in adults, including often simultaneously with the mesonephros. [7] The mesonephros is the final kidney in amphibians and most fish. [25]
Evolutionary pressure and the need to regulate body fluid homeostasis have led to pre-adaptation of the vertebrate kidneys to different environment conditions and to development of three kidney forms: the pronephros, mesonephros and metanephros. [26] [27] The kidneys of amniotes are unique compared to other internal organs, since three different kidneys are sequentially developed during embryogenesis, replacing each other and reflecting the evolution of the kidneys in vertebrates. [28]
At the very beginning of vertebrates, when they evolved from marine chordates, their evolution probably took place in fresh or slightly saline water. There is a hypothesis according to which marine fish received their kidneys after a previous adaptation of the kidneys to fresh water. As a result, early vertebrates developed renal glomeruli capable of filtering blood and perhaps tubules that reabsorbed ions. [29] Excretion of excess water from the body is the main characteristic of the pronephros in the case of species in which it develops into a functional excretory organ. In some species, the pronephros is functional during the embryonic stage of development, representing the first stage of kidney development, after which the mesonephros develops. The mesonephros probably appeared in the course of evolution in response to the increase in body mass of vertebrates, which also led to an increase in blood pressure. [28]
The evolution of the kidneys, along with the evolution of the lungs, allowed vertebrates called amniotes to live and reproduce in terrestrial environment. [30] [28] Metanephros, the permanent kidney of amniotes, has the unique ability to efficiently retain water in the body. [28] In addition to water conservation, terrestrial life also required maintenance of salt levels in the body along with the excretion of waste products. [30] The first class of animals to become fully terrestrial without a larval stage were the reptiles, which were the first amniotes. [21] The kidney takes a key role in maintenance of the constant internal environment. The relative ionic composition of the extracellular fluid is similar between marine fish and all subsequent species. Therefore, it can be said that the kidneys made it possible to preserve approximately the same composition of extracellular fluid in vertebrates as it was in the primordial ocean. [5]
It is believed that the ancient primitive form of the kidney was the archinephros, which had series of segmental tubules through the entire length of the trunk part of the body, [13] and each body segment had a pair of tubules. [31] All tubules were opened medially (closer to the midline of the body) into the body cavity known as coelom and united laterally into the two common archinephric ducts which were located in opposite sides of the body. [13] [31] And the archinephric ducts were opened into the cloaca. [13] As an organ, the archnephros is still preserved in the larvae of hagfishes and some caecilians, and is also found in the embryos of some more developed vertebrates. [32]
In lower vertebrates, the pronephros is sometimes called the head kidney due to its anterior position behind the head. [33] In embryogenesis it is usually a transitional structure and is subsequently replaced by the mesonephros in most vertebrates. [7] In mammalian embryogenesis, the pronephros is usually considered to be rudimentary and non-functional. A functional pronephros develops in vertebrates that have a free-swimming larval stage in their development. [7]
Pronephros functions in amphibians in the larval stage, in the adults of some bony fishes, and in the adults of some other fish species. [7] The pronephros is a vital organ in animals that go through the aquatic larval stage. If in larvae the pronephros becomes non-functional, then they rapidly die from edema. [34]
The pronephros is a relatively large organ that has a primitive structure and usually consists of a single pair of bilateral nephrons with an external glomerulus or glomus. [34] [15] The typical pronephric nephron is non-integrated, and the wastes are filtered through the glomerulus or glomus directly to the coelom, in the more advanced pronephros they are filtered into the nephrocoel, which is a cavity adjacent to the coelom. The coelom is connected to the pronephric duct through the ciliated nephrostomes, which drain coelom fluid into the cloaca. [7]
Because of its small size and simple structure, the pronephros of fish and amphibian larvae has become an important experimental model for studying kidney development. [35]
Mesonephros develops after the pronephros, replacing it. The mesonephros is the final kidney in amphibians and most fish. In more advanced vertebrates (amniotes), mesonephros develops during embryogenesis and is then replaced by the metanephros. [36] In reptiles and marsupials, it remains functional for some time after birth along with the metanephros. [37] [38] When mesonephros degenerates in male mammals, its remains are involved in the formation of the reproductive system. [39] Sometimes the anamniote mesonephros is called opistonephros to distinguish it from the stage of development in amniotes. [40] In anamniotes, opisthonephros develops from a region of the nephric ridge, which is derived from intermediate mesoderm, from which both the mesonephros and metanephros are developed in the embryo of amniotes. [41] [42]
Unlike the pronephros, the mesonephros consists of a set of nephrons, the glomeruli of which are enclosed in Bowman's capsules, but in some marine fish glomeruli may be absent. [36] In fish, mesonephric kidneys has no division into cortex and medulla. [43] Usually the mesonephros consists of 10–50 nephrons. The mesonephric tubules may have a connection to the coelom, however, the glomeruli of mesonephric nephrons still remain integrated. Nephrostomes are typically absent in the embryonic mesonephros of birds and mammals. [44] Mesonephros in fish has the ability to add new nephrons while body mass increases. [45]
In amniotes, which include reptiles, birds, and mammals, the pronephros and mesonephros are usually intermediate stages in the formation of metanephros during embryonic development, and metanephros is the final kidney. [28] Genes that are involved in the formation of one form of kidney are reused in the formation of the next one. [28] Metanephros differs from pronephros and mesonephros in development, position in the body, shape, number of nephrons, organization and drainage. [46] [44] Unlike mesonephros, after the end of its development process, metanephros has no longer the ability to add new nephrons through nephrogenesis, [5] although many reptiles show ongoing nephron formation in adults. [47]
Metanephros is the most complex form of kidney. [44] Each metanephric kidney is characterized by a large number of nephrons and a highly branched system of collecting tubules and ducts, [28] that open into the ureter. [48] Such branching in the metanephros is unique in relation to the pronephros and mesonephros. [44] Depending on classes and species urine from the ureters can be excreted directly into the cloaca, or collected in the urinary bladder and then excreted into the cloaca, or collected in the urinary bladder and then excreted outside through the urethra. [46]
Reptiles were the first class of animals that had no larval stage and that were fully terrestrial animals. [21] The mesonephros in reptiles functions for some time after birth simultaneously with the metanephros, while later the metanephric kidneys become permanent and the mesonephros degenerates. [38]
The kidneys in reptiles are located mainly in the caudal part (away from the head) of the abdominal cavity [49] [50] or retroperitoneally (behind the peritoneum) in the pelvic cavity in the case of lizards. [49] Reptile kidneys are commonly elongated [51] with color ranging from light to dark brown. [52] The shape of the kidneys varies between reptiles due to variations of their body form. [8] The kidneys of snakes are elongated, cylindrical [53] [50] and lobulated. [52] Turtles and some lizards have urinary bladder [50] that opens into the cloaca [54] but snakes and crocodiles do not have it. [50]
Compared with the metanephros of birds and mammals, the metanephros of reptiles is simpler in structure. [21] Unlike mammals, the kidneys of reptiles do not have a clear distinction between cortex and medulla. [43] The kidneys lack the loop of Henle, have fewer nephrons (from about 3,000 to 30,000), and cannot produce hypertonic urine. [3] [21] Nitrogenous waste products excreted by the kidneys may include uric acid, urea and ammonia. [55] Aquatic reptiles excrete predominantly urea, while terrestrial reptiles excrete uric acid, which allows them to conserve water. [21]
Since the reptile kidneys are unable to produce concentrated urine due to the absence of the loop of Henle, glomerular filtration rate is decreased if water loss needs to be reduced. [56] The glomeruli in reptiles have also decreased in size compared to amphibians. [52] In addition to the renal artery blood supply, reptiles also have a renal portal system, which can redirect blood to the kidneys during periods of water deprivation, bypassing the glomeruli, to prevent ischemic necrosis of tubular cells. [21] [57]
In mammals, the kidneys are usually bean-shaped [58] and located retroperitoneally [59] on the dorsal (posterior) wall of the body. [60] The outer layer of each kidney is made up of a fibrous sheath called the renal capsule. The peripheral layer of the kidney is called the cortex and the inner part is called the medulla. The medulla consists of one or more pyramids, the bases of which start from corticomedullary border. Medulla pyramid with overlying cortex comprises the renal lobe. [28] In multilobar kidneys, the pyramids are separated from each other by dipped into the kidney areas of cortical tissue known as the renal columns. [61] Blood enters the kidney through the renal artery, which in the multilobar kidney then branches in the region of the renal pelvis into large interlobar arteries that pass through the renal columns. [62] [63] The pyramids consist mainly of tubules that transport urine from the cortex, that produces it by blood filtration, to the tips of the pyramids, that form the renal papillae. Urine is excreted through the renal papillae into the calyces and then into the pelvis, ureter, and bladder. [62] [28] Then it is excreted outside through the urethra. [64] In monotremes, the ureters open into the urogenital sinus, which is connected to the urinary bladder and cloaca, [65] and urine is excreted into the cloaca instead of the urethra. [66] [65]
Structurally, kidneys vary between mammals. [67] Which structural type a particular species will have depends mainly on the body mass of the species. [68] Small mammals have simple, unilobar kidneys with a compact structure and a single renal papilla, while large animals have more complex multilobar kidneys, such as those of bovines. [67] [69] Kidneys can also be with a single renal papilla (the unipapillary kidneys), [69] as in mice and rats, [70] with several, as in spider monkeys, or with a large number, as in pigs or humans. [69] Most animals have a single renal papilla. [69] In some animals, such as horses, the apices of the renal pyramids fuse with each other to form a common renal papilla, called the renal crest. [71] The renal crest usually appears in animals larger than rabbits. [68] The kidneys of bovines are multilobar with external lobation. [72] Marine mammals, bears and otters have reniculate kidneys which are made of large amount of lobes called reniculi. [73] Each renculus can be compared to a simple unipapillary kidney as a whole. [74]
Nitrogenous waste products are excreted by the kidneys of mammals primarily in the form of urea, [75] which is highly soluble in water. [76] Each nephron is located in both the cortex and the medulla. The most proximal part of the nephron is glomerulus, which is located in the cortex. [28] The nephrons of the mammalian kidneys have loops of Henle, which are the most efficient way to reabsorb water and produce concentrated urine to conserve water in the body. [12] The mammalian kidneys combine both nephrons with a short and nephrons with a long loop of Henle. [77] The medulla is divided into outer and inner regions. The outer region consists of short loops of Henle and collecting ducts, and the inner region consists of long loops of Henle and collecting ducts. [28] After passing through the loop of Henle, the fluid becomes hypertonic relative to the blood plasma. [78] The renal portal system is absent in mammals. [56]
In birds, the kidneys are typically elongated [79] and located dorsally in the abdominal cavity in the pelvic skeletal depressions. [80] [81]
The structure of the avian kidneys differs from the structure of the mammalian kidneys. [67] The avian kidney is lobulated and usually consists of three lobes. [80] The lobes are divided into lobules, each of which has a cortex and a medulla. [67] [3] The medulla of the each lobule is shaped like a cone, and, unlike mammals, it is not subdivided into the inner and outer regions, while structurally it is similar to the outer medulla of the mammalian kidney. [67] In the avian kidney, the renal pelvis is absent, [82] and each lobule has a separate branch to the ureter. [3] No birds, except for the ostrich, have a bladder; urine is excreted from the kidneys through the ureters to the cloaca. [83]
Avian kidneys combine so called reptilian-type nephrons, without the loop of Henle, and mammalian-type nephrons, with the loop of Henle. [23] Most nephrons are reptilian-type. [84] The loop of Henle of birds is similar to that of mammals, the main difference is that the nephron of birds has only a short loop of Henle. [77] Like mammals, although to a lesser extent, [67] birds are able to produce concentrated urine, thus conserving water in the body. [23] Nitrogenous waste products are excreted mainly in the form of uric acid, which is a white paste that is poorly soluble in water, which also helps to reduce water loss. [85] Additional water reabsorption occurs in the cloaca and distal intestine. Altogether, this allows birds to excrete their wastes without significant loss of water. [5]
In birds, the arterial blood is supplied to the kidneys by the cranial, middle and caudal renal arteries. [86] Like reptiles, birds have a renal portal system, but it does not deliver blood to the loops of Henle, blood is delivered only to the proximal and distal tubules of the nephrons. When birds are in a state of dehydration, nephrons without a loop of Henle stop filtering, while nephrons with a loop continue, but due to the presence of a loop, they can produce concentrated urine. [56]
In humans, the kidneys are two reddish-brown bean-shaped blood-filtering organs that are a multilobar, multipapillary form of mammalian kidneys, usually without signs of external lobulation. They are located on the left and right in the retroperitoneal space, and in adult humans are about 12 centimetres in length. They receive blood from the paired renal arteries; blood exits into the paired renal veins. Each kidney is attached to a ureter, a tube that carries excreted urine to the bladder.
The nephron is the minute or microscopic structural and functional unit of the kidney. It is composed of a renal corpuscle and a renal tubule. The renal corpuscle consists of a tuft of capillaries called a glomerulus and a cup-shaped structure called Bowman's capsule. The renal tubule extends from the capsule. The capsule and tubule are connected and are composed of epithelial cells with a lumen. A healthy adult has 1 to 1.5 million nephrons in each kidney. Blood is filtered as it passes through three layers: the endothelial cells of the capillary wall, its basement membrane, and between the foot processes of the podocytes of the lining of the capsule. The tubule has adjacent peritubular capillaries that run between the descending and ascending portions of the tubule. As the fluid from the capsule flows down into the tubule, it is processed by the epithelial cells lining the tubule: water is reabsorbed and substances are exchanged ; first with the interstitial fluid outside the tubules, and then into the plasma in the adjacent peritubular capillaries through the endothelial cells lining that capillary. This process regulates the volume of body fluid as well as levels of many body substances. At the end of the tubule, the remaining fluid—urine—exits: it is composed of water, metabolic waste, and toxins.
The collecting duct system of the kidney consists of a series of tubules and ducts that physically connect nephrons to a minor calyx or directly to the renal pelvis. The collecting duct participates in electrolyte and fluid balance through reabsorption and excretion, processes regulated by the hormones aldosterone and vasopressin.
Renal physiology is the study of the physiology of the kidney. This encompasses all functions of the kidney, including maintenance of acid-base balance; regulation of fluid balance; regulation of sodium, potassium, and other electrolytes; clearance of toxins; absorption of glucose, amino acids, and other small molecules; regulation of blood pressure; production of various hormones, such as erythropoietin; and activation of vitamin D.
The proximal tubule is the segment of the nephron in kidneys which begins from the renal pole of the Bowman's capsule to the beginning of loop of Henle. At this location, the glomerular parietal epithelial cells (PECs) lining bowman’s capsule abruptly transition to proximal tubule epithelial cells (PTECs). The proximal tubule can be further classified into the proximal convoluted tubule (PCT) and the proximal straight tubule (PST).
In the kidney, the loop of Henle is the portion of a nephron that leads from the proximal convoluted tubule to the distal convoluted tubule. Named after its discoverer, the German anatomist Friedrich Gustav Jakob Henle, the loop of Henle's main function is to create a concentration gradient in the medulla of the kidney.
The renal medulla is the innermost part of the kidney. The renal medulla is split up into a number of sections, known as the renal pyramids. Blood enters into the kidney via the renal artery, which then splits up to form the segmental arteries which then branch to form interlobar arteries. The interlobar arteries each in turn branch into arcuate arteries, which in turn branch to form interlobular arteries, and these finally reach the glomeruli. At the glomerulus the blood reaches a highly disfavourable pressure gradient and a large exchange surface area, which forces the serum portion of the blood out of the vessel and into the renal tubules. Flow continues through the renal tubules, including the proximal tubule, the loop of Henle, through the distal tubule and finally leaves the kidney by means of the collecting duct, leading to the renal pelvis, the dilated portion of the ureter.
The glomerulus is a network of small blood vessels (capillaries) known as a tuft, located at the beginning of a nephron in the kidney. Each of the two kidneys contains about one million nephrons. The tuft is structurally supported by the mesangium, composed of intraglomerular mesangial cells. The blood is filtered across the capillary walls of this tuft through the glomerular filtration barrier, which yields its filtrate of water and soluble substances to a cup-like sac known as Bowman's capsule. The filtrate then enters the renal tubule of the nephron.
In biology, a tubule is a general term referring to small tube or similar type of structure. Specifically, tubule can refer to:
The development of the urinary system begins during prenatal development, and relates to the development of the urogenital system – both the organs of the urinary system and the sex organs of the reproductive system. The development continues as a part of sexual differentiation.
The mesonephros is one of three excretory organs that develop in vertebrates. It serves as the main excretory organ of aquatic vertebrates and as a temporary kidney in reptiles, birds, and mammals. The mesonephros is included in the Wolffian body after Caspar Friedrich Wolff who described it in 1759.
Kidney development, or nephrogenesis, describes the embryologic origins of the kidney, a major organ in the urinary system. This article covers a 3 part developmental process that is observed in most reptiles, birds and mammals, including humans. Nephrogenesis is often considered in the broader context of the development of the urinary and reproductive organs.
A countercurrent mechanism system is a mechanism that expends energy to create a concentration gradient.
Pronephros is the most basic of the three excretory organs that develop in vertebrates, corresponding to the first stage of kidney development. It is succeeded by the mesonephros, which in fish and amphibians remains as the adult kidney. In amniotes, the mesonephros is the embryonic kidney and a more complex metanephros acts as the adult kidney. Once a more advanced kidney forms, the previous version typically degenerates by apoptosis or becomes part of the male reproductive system.
Intermediate mesoderm or intermediate mesenchyme is a narrow section of the mesoderm located between the paraxial mesoderm and the lateral plate of the developing embryo. The intermediate mesoderm develops into vital parts of the urogenital system.
Within the nephron of the kidney, the ascending limb of the loop of Henle is a segment of the heterogenous loop of Henle downstream of the descending limb, after the sharp bend of the loop. This part of the renal tubule is divided into a thin and thick ascending limb; the thick portion is also known as the distal straight tubule, in contrast with the distal convoluted tubule downstream.
Osmoregulation is the active regulation of the osmotic pressure of an organism's body fluids, detected by osmoreceptors, to maintain the homeostasis of the organism's water content; that is, it maintains the fluid balance and the concentration of electrolytes to keep the body fluids from becoming too diluted or concentrated. Osmotic pressure is a measure of the tendency of water to move into one solution from another by osmosis. The higher the osmotic pressure of a solution, the more water tends to move into it. Pressure must be exerted on the hypertonic side of a selectively permeable membrane to prevent diffusion of water by osmosis from the side containing pure water.
The common raven, also known as the northern raven, is a large, all-black passerine bird. Found across the Northern Hemisphere, it is the most widely distributed of all corvids. Their Northern range encompasses Arctic and temperate regions of Eurasia and North America, and they reach as far South as Northern Africa and Central America. The common raven is an incredibly versatile passerine to account for this distribution, and their physiology varies with this versatility. This article discusses its physiology, including its homeostasis, respiration, circulatory system, and osmoregulation.
The rock dove, Columbia livia, has a number of special adaptations for regulating water uptake and loss.
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 (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.
{{cite book}}
: CS1 maint: date and year (link){{cite book}}
: CS1 maint: date and year (link){{cite book}}
: CS1 maint: date and year (link){{cite book}}
: CS1 maint: date and year (link){{cite book}}
: CS1 maint: date and year (link)