[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.

[8] The kidneys produce renin[32] and erythropoietin[33] hormones, and are involved in the conversion of vitamin D to its active form.

Finally, the renal tubule flows with its distal end into its collecting duct, which is common to several nephrons.

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.

[73] Bovine kidneys also lack renal pelvis, urine from the major calyces is excreted directly into the ureter.

[93] By microanatomic structure, the kidney can be divided into several main elements: interstitium, renal corpuscles, tubules, and vasculature.

[105] Despite their small size, the kidneys of mammals account for a significant part of the minute volume of blood circulation.

[113] The vascular glomeruli of nephrons receive blood from afferent arterioles, which originate in the interlobular arteries with intermediate formation of prearterioles.

In nephrons with a long loop of Henle, the efferent arterioles branch, forming straight vessels called vasa recta, which descend into the medulla.

[119] Most studies fail to detect lymphatic vessels in the renal medulla of animals, in particular, they are not found in sheep and rats.

[128] Normal physiological stimulation of the efferent sympathetic nerves of the kidney is involved in maintaining the balance of water and sodium in the body.

Activation of the efferent sympathetic nerves of the kidney reduces its blood flow, and respectively, filtration and excretion of sodium in the urine, and also increases the rate of renin secretion.

[135] The key role in maintaining a constant level of osmolarity is managed by the control of the ratio of sodium and water in the blood.

To remove excess water but keep salt in the blood, the kidneys produce more dilute urine than the plasma.

[135] In addition to the kidneys, the hypothalamus and neurohypophysis are involved in the regulation of water balance through a feedback system.

The osmoreceptors of the hypothalamus respond to an increase in the osmolarity of the blood plasma, as a result of which the secretion of vasopressin by the posterior pituitary gland is stimulated, and thirst also arises.

[32] The production of erythropoietin by the kidneys is responsible for the differentiation of erythroid progenitor cells in the bone marrow into erythrocytes and is induced by hypoxia.

[145] Maintaining acid-base balance is vital because changes in pH levels affect virtually every biological process in the body.

[158] It can be assumed that the development of a water reabsorption mechanism could be part of the evolution of warm-bloodedness, rather than a direct adaptation to aridity.

[161] The longer loops of Henley in the Australian hopping mouse make it possible to produce very concentrated urine[138] and survive in conditions of water scarcity.

[179] In the postnatal period, the mass of the tubules is not large enough, so the ability to reabsorb fluids is reduced compared to the kidneys of adult mammals.

[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.

[185] Kidney infections in small mammals are usually caused by aerobic bacteria, including Escherichia coli, staphylococci, enterococci, and streptococci.

[193] 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.

[50] Unlike more primitive vertebrates such as fish, in mammals nephrogenesis ends before or some time after birth,[39] caused by the loss of the condensed mesenchyme of the metanephrogenic blastema.

[200] In addition, the repair of nephrons occurs in the course of normal physiological activity throughout the life due to the shedding of tubular epithelial cells.

[204] If minor damage to the nephron tubules occurs, the lost cells are replaced by new ones, and the epithelium regenerates, restoring its structure and function.

In moderate to severe injuries with large cell loss, the chances of regeneration of the tubular epithelium are reduced.

[49] Fibrosis is the second line of body defences,[205] which was supposed to reduce possible hemorrhage and fight possible infection during the evolution of mammals.