The kidneys are responsible for purifying the blood. They are one of the best examples of homeostatic organs, organs responsible for maintaining a balance of elements. They work muck like sanitation workers, filtering and disposing of wastes from the blood.

    The kidneys are protected by floating ribs located in central region of the back. They are small dark red, bean-shaped organs. Due to the placement of the liver, the right kidney is a bit lower than the left.

A transparent, fibrous, renal capsule encloses the kidney. Underneath a mass of fat, known as the adipose capsule encases each kidney, and provides some protection. The kidney can be divided into three regions, the renal cortex, the renal medulla, and the renal pelvis. The renal cortex is the light colored, outer region of the kidney. The renal medulla is the darker, reddish-brown region.

The primary function of the kidneys is to clean the blood, resulting in a large amount of blood being continuously present in the kidneys. Approximately one-quarter of the body’s blood enters the kidneys each minute. The renal artery is the large arterial vessel, which enters the kidneys then branches off to segmented arteries. This vessel brings in oxygenated blood from the heart. The renal vein is the large vessel by which cleansed blood exits the kidney.

The actual point of filtration occurs at the  glomerus of nephrons. Nephrons are the structural and functional units of the kidneys. Within each nephron are two main structures, the renal tube and the glomerus. The glomerus is a collection of capillaries in a "knot".  The purpose of the glomerus is to act as a filter, it contains filtrate which is made of blood plasma, without proteins. A cup shaped enlargement of the glomerus is located at the terminal (end) of the glomerus. This enlargement is known as Bowman’s capsule or the glomerular. The rest of the nephron extends for another 3 cm. Twists and knots of blood vessels characterize this remaining area of the nephron.

    The filter membrane contains wastes, and some useful substances. These substances, such as proteins, may be reabsorbed into the blood stream. The process of useful substances moving from the filtrate membrane, to the blood stream is known as tubular re-absorption. Tubular re-absorption begins when the filtrate enters the convoluted tube.

    Tubular secretion is the converse of tubular re-absorption. Waste products move from the blood, to the tubule cells, to the filtrate. This process provides a way for waste substances to move from the blood stream to the filtrate.