The mammalian kidneys are the paired organ of the urinary system of mammals, which is a type of metanephric kidney. The kidney in mammals is usually bean-shaped, located retroperitoneally on the back (dorsal) wall of the body. Each kidney consists of a renal capsule, peripheral cortex, internal medulla, calices, and renal pelvis, although the calices or renal pelvis may be absent in some species. Urine is excreted from the kidney through the ureter. The structure of the kidney may differ between species depending on the environment, in particular on its aridity. The cortex is responsible for filtering the blood, this part of the kidney is similar to the typical kidneys of less developed vertebrates. Nitrogen-containing waste products are excreted by the kidneys in mammals mainly in the form of urea.
Depending on the species, the kidneys can be unilobar (single lobe) or multilobar, unipapillary (single papilla), with several papillae or multipapillary, may be smooth-surfaced or lobulated, also the kidneys may be reniculate, which are found mainly in marine mammals. The simplest type of kidney in mammals is the unipapillary kidney with single lobe. Differences in kidney structure are the result of adaptations during evolution to varying between species body masses and habitats. The human kidney is an example of the mammalian kidney.
The cortex and medulla of the kidney contain nephrons. In mammals, the nephrons can be classified into the nephrons with a short loop and a long loop of Henle, and the loop of Henle is responsible for urine concentration. Amongst the vertebrates, only mammals and birds have kidneys that can produce urine more concentrated (hypertonic) 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, participate in bone formation and in 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 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. 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. 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
Location and form
In mammals, kidneys are usually bean-shaped; the shape is unique to mammals (fish, for example, has elongated kidneys). Kidneys are located retroperitoneally on the posterior (dorsal) wall of the body. One of the key factors that determine the shape and morphology of the kidneys in mammals is their mass. 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. In the body, the kidney is surrounded by a mass of adipose tissue.
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 represented by the cortex, and the inner layer is represented by the medulla. The medulla consists of pyramids (also called malpighian 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 (Bertin's 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.