The importance of blood to human life has been recognized since prehistorical era. Early man knew that if enough blood leaves the body, then life ceases to function. As a result of this blood has acquired mystic and religious significance throughout the ages. In the classical Greek medicine, blood was associated with air, with springtime, and with a merry and gluttonous (sanguine) personality. It was also believed to be produced exclusively by the liver.In Hippocratic medicine, blood was considered to be one of the four humor, the others being phlegm, yellow bile and black bile.Blood is a specialized bodily fluid that delivers necessary substance and nutrients to the body's cells and transports waste products away from those some cells. Blood is circulated around the blood vessels by the pumping action of the heart.Medical term related often begins with hemo- or hemato- from the ancient word MA or (haima) for blood.In histological terms, blood is considered a specialized form of connective tissue, given its origin in the bones and the presence of potential molecular fibers in the form of fibrinogen.