Blood carries oxygen from the lungs to all the other tissues in the body and carries carbon dioxide back to the lungs for release into the air. When oxygen transport fails, a person dies within a few minutes.

Food is processed by the digestive system into smaller components such as proteins, fats, and carbohydrates which are delivered to tissues by blood. These nutrients supply the needs of individual cells for metabolism. Cell waste products are carried by the blood to the kidneys for elimination from the body.

In addition to oxygen and nutrients, blood also transports called hormones, which regulate certain body functions. Hormones allow organs to communicate.

The blood delivers nutrients and information and is also responsible for the immune system, helping fend off infection and fight disease. Blood carries clotting mechanisms which stops itself from leaking out of the body after an injury.

Many diseases are caused by abnormalities in the blood. These diseases are categorized by which component of the blood is affected. Diseases of the red blood cells impact the transport of oxygen, nutrients and waste. Plasma carries the hormones. White blood cell diseases affect the body's defenses while platelets are the clotting mechanism.

Blood related processes