Kidneys are two small bean shaped organs, each about the size of the fist, located on either side of the spine just above the waist at the small of the back. It is a part of the urinary system. Each kidney is about 10cm long, 5-6cm wide, 2.5cm thick and weighs between 115 to 170g in the adult human. The right kidney is situated slightly lower than the left because the liver pushes it down.

The outer portion of the kidney called cortex is composed of blood vessels and urine tubes and is supported by a fibrous matrix. Deep to the cortex lies the medulla which is divided into 10-20 conical regions called renal pyramids. Renal pyramids also known as the malphighian pyramids contains the secreting apparatus and tubules. Each pyramid together with the associated overlying cortex forms a renal lobe.

The tip of each pyramid (called a papilla) empties into a calyx, which empties into the renal pelvis. Pelvis forms a small reservoir for urine produced by the kidney. The whole kidney is covered by a membrane called renal capsule.

The cortex and the medulla are made up of nephrons, which is the most basic functional units of the kidney and each kidney contains about a millions of nephrons. Filtration, re-absorption and secretion of the blood occurs in the nephron. Each nephron is made of a glomerulus and a tubule. The glomerulus is like a miniature filtering or sieving device while the tubule is a tiny tubelike structure attached to the glomerulus.