Both of the kidneys are situated in the region of the loins, one on each side of the vertebral column behind the connective tissue of the abdominal, or peritoneal, cavity (retroperitoneal cavity), protected by the lower ribs.

    Each kidney (ren) is 10 - 12 cm (3.9 - 4.7 in) long, 3 - 4 cm (1.2 - 1.6 in) thick, 5 - 6 cm (2 - 2.3 in) wide and weighs 120 - 300 g (4.8 - 12 oz). The right kidney is situated somewhat lower than the left one and is adjacent to the liver, duodenum and the right intestinal bend. The left kidney is contiguous to the  stomach, spleen, pancreas and the left intestinal bend and extends approximately from the 11th thoracic to the 3rd lumbar vertebra.

     The kidneys are bean-shaped whereby the inner concave bend presents an indentation (sinus renalis) in which blood and lymph vessels, nerves and the renal pelvis with the beginning end of the ureter enter. Such an indentation where vessels, nerves and excretory ducts enter and leave, is referred to as hilus or hilum, in this case the hilus renale. Due to the inflow and outflow, this location is also referred to as renal "portal".

    The kidney is enclosed by a tough layer, the renal capsule or capsula fibrosa, which, in turn, is embedded in a fatty capsule, the capsula adiposa. The fatty capsule serves the kidney as a cushion protecting it from concussions and heat loss. Together with the larger vessels, it holds the kidneys in place since these are not firmly attached to the abdominal wall.

     There fore, during respiration and a change of position of the body, the position of the kidneys sometimes shifts by up to 2 cm (0.8 in).

     Such an indentation where vessels, nerves and excretory ducts enter and leave, is referred to as hilus or hilum, in this case the hilus renale. Due to the inflow and outflow, this location is also referred to as renal "portal". The cross section of renal tissues shows distinctly two different layers, the renal cortex and the renal medulla.The renal cortex (cortex renalis) is adjacent to the renal capsule. It is a 6 - 10 mm (0.24 - 0.4 in) thick, grained, red-brown colored region, containing renal corpuscles (corpusculum renale) which are responsible for the filtration of the blood.

    Each renal corpuscle is attached to a renal tubule (tubulus renalis) in which the filtered fluid is prepared for excretion in a complicated metabolic process. Renal corpuscles and renal tubules constitute the smallest functional unit of the kidney, the nephron.

     The renal cortex extends up to the renal pelvis in the form of widely spaced columns, approaching and bordering in the process the pyramids of the renal medulla. The renal medulla (medulla renalis) does not form a continuous layer but consists of  7 - 20 aciculated renal pyramids (pyramides renalis) enveloped by cortical substance.

     The renal tubules are located in the renal medulla. Beginning at the lower part of the pyramids, the renal medulla extends in the form of so-called medullary rays into the cortical tissue, thus forming the aciculae (striped structure).The pointed ends of the pyramids, the renal papillae (papillae renales), extend into the renal chalices (calyces renales). Here, the urine is discharged into the renal pelvis by way of the chalices.

    The interior part of the medulla has a pale-reddish color, the exterior area toward the cortex appears, due to larger blood vessels, is darker, almost blue-red.

    The kidneys control the salt and mineral contents, regulate the fluid balance and thus participate in the acid and base balance of the body.  

    The main function of the kidneys is to maintain the balance of the normal composition of the blood and all other body fluids. They must identify and decide which substances are to be excreted and which substances must remain in the purified blood of the body. The influences and demands on the kidneys change constantly as we change constantly our way of life. The composition and quantity of our food intake varies from meal to meal, times of high physical strain and resting phases alternate, environmental temperatures change, and so on.

     A disorder of the kidney functions results in diseases of the kidneys and, depending on the cause, manifest themselves through different symptoms. Inflammations can present themselves in an acute form but can also become chronic. Kidney stones (mainly due to inadequate nutrition or a hyperfunction of the parathyroid glands) cause, depending on the location of the stone, severe abdominal pain, accompanied by nausea and dark or cloudy urine which may also contain blood.

     In the extreme case, kidney failure can occur. Treatment is possible by dialysis, which takes over the functions of the kidneys, or by transplantation of another person's kidney.

Urinary System