Kidney: One of a pair of organs in the abdomen. The kidneys remove waste and extra water from the blood (as urine) and help keep chemicals (such as sodium, potassium, and calcium) balanced in the body. The kidneys also make hormones that help control blood pressure and stimulate bone marrow to make red blood cells. Each kidney is located lateral to the spinal column in the lumbar region behind the abdominal cavity. Each is, therefore, retroperitoneal, or in back of the peritoneal cavity. Each is embedded in a mass of fatty tissue which is surrounded by a fibrous covering called the renal fascia or Gerota's fascia. The kidney parenchyma is the glandular part of the organ where the filtration of blood takes place and urine is produced. The outer connective tissue layer is called the cortex; the inner structure is called the medulla. The medulla consists of renal (medullary) pyramids which are separated from one another by extensions of the cortex into the medulla called renal columns. The function of the kidney is to remove metabolic waste products from the blood and to maintain the electrolyte balance. This is accomplished by the production of urine through filtration, reabsorption, secretion and maintenance of acid-base pH. The lymphatics of the kidney drain to retroperitoneal nodes along the abdominal aorta and vena cava (paracaval), primarily to those near the origin of the renal arteries. Perirenal nodes are those at the renal hilus.