Kidney (vertebrates): Difference between revisions

In

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 and opisthonephros

Mesonephros develops after the pronephros, replacing it. The mesonephros is the final kidney in

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]

Metanephros

In

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

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,

In mammals, the kidneys are usually bean-shaped

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]

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.

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]

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