Concept
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. The human kidney is an example of the mammalian kidney. The cortex and medulla of the kidney contain nephrons. In mammals, the nephron consists of the renal glomerulus inside Bowman's capsule, the proximal convoluted tubule, the proximal straight tubule, the loop of Henle, and the distal convoluted tubule. The nephrons can be classified as nephrons with a short loop and a long loop of Henle. Blood that enters the kidneys is filtered through the renal glomeruli into Bowman's capsule to form primary urine. Then it enters the tubules, which concentrate it. 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 and participate in bone formation.
