While the importance of blood to life was understood by primitive peoples, they did not know that blood carries oxygen and food throughout the body, moves waste products, and fights disease. Early societies often ascribed mystical qualities to blood. The Greeks considered blood to be both one of the four essential humors and a carrier of the vital humor blood. For centuries, medical practice based on the findings of the Greek physician Galen advocated bloodletting, or bleeding, because the condition and amount of blood in the body was believed to profoundly affect health.

Nevertheless, the structure and function of blood remained unknown, although it had long been observed that blood standing in a container outside the body settled out into several components: a thick, dark red mass and a pale yellowish fluid (plasma) separated by a thin white layer. William Harvey's revolutionary description of the circulatory system in...