Definition:
One of the paired organs of the urinary system containing extensive vascularity and millions of nephrons within the renal cortex and renal medulla; filters the blood and regulates the volume and composition of body fluids during the formation of urine.

Description:
Located retroperitoneally in the abdominal cavity from the level of L₃ to T¹². Anteroposteriorly compressed with concave medial surface called the hilum through which the renal artery enters and the renal vein and ureter exits. Each kidney is surrounded by a fibrous renal capsule and is supported by the adipose capsule. The kidney is divided into an outer reddish brown renal cortex and a lighter colored renal medulla. The medulla is composed of renal pyramids separated by renal columns. The renal papillae empty urine, which is formed in the nephrons, into the calyces, which drain into the renal pelvis; from there the urine flows through the ureter. Blood supply. Renal artery a branch of the aorta; renal vein drains into the inferior vena cava. Innervation. Renal sympathetic plexus.

Functions: The kidneys maintain the constancy of the body's internal environment by regulating the composition and volume (influences blood pressure) of the extracellular fluid. To accomplish this the kidneys perform the following:

1. Excretion of inorganic compounds (Na⁺, K⁺, Ca⁺⁺, Mg⁺⁺, H⁺, HC₃^-, etc.)
2. Excretion of organic wastes (urea, uric acid, creatinine, bilirubin, etc.)
3. Excretion of foreign substances (drugs, pesticides, food additives, etc.)
4. Blood pressure regulation (release renin--major component of the renin-angiotensin-aldosterone mechanism)
5. Erythrocyte volume regulation (release of renal erythropoietic factor)
6. Vitamin D activation (converts this vitamin to its most biologically active form)

Clinical Information

Calculi (kidney stones). Precipitation of organic and inorganic substances into a large calculus (stone) that may cause intense pain when it passes through the urinary tract.
Nephritis. Inflammation of the kidney tissue.
Pyelitis. Inflammation of the renal pelvis and its calyces.
Renal ptosis. Kidneys dropping in position due to loss of support fat in the adipose capsule; generally occurs in elderly persons who are extremely thin, also affects victims of anorexia nervosa.