Blood is the life-maintaining fluid that circulates through the body's heart, arteries, veins, and capillaries. It carries away waste matter and carbon dioxide, and brings nourishment, electrolytes, hormones, vitamins, antibodies, heat, and oxygen to the tissues.

Because the functions of blood are many and complex, there are many disorders that require clinical care by a physician or other healthcare professional. These conditions include anemias, bleeding disorders, as well as cancers of the blood.

Hematology is the scientific study of blood and blood-forming tissues. A hematologist is a physician who specializes in the functions and disorders of the blood.