Blood circulation

Blood circulation is the fact that blood travels a closed circuit: the heart's left side pumps it out through the arteries to the whole body, it returns through the veins to the heart's right side, passes through the lungs, and comes back to start again. The heart is a muscular pump, and the one-way valves in the heart and veins enforce the direction of flow. The total quantity of blood is modest and conserved; it is used and returned, not consumed and remade.

The model it replaced had governed medicine for roughly 1,400 years. Galen taught that the liver continuously manufactured blood from digested food, that this blood ebbed and flowed through the vessels like a tide, was consumed by the tissues, and seeped from one side of the heart to the other through invisible pores in the central wall. Getting the plumbing right is the foundation of physiology: how oxygen and nutrients reach tissue, how drugs and infections travel, why a tourniquet stops bleeding, what blood pressure even means. Bloodletting, transfusion, anesthesia dosing, and modern cardiology all assume the closed loop Harvey proved.

William Harvey, physician to two English kings, attacked the question the way a merchant audits a ledger. He measured the capacity of the heart, multiplied by the pulse rate, and got a volume of blood per hour so enormous — hundreds of pounds — that no source could keep supplying it and no sink could keep absorbing it. The only account that balanced was recirculation. He backed the arithmetic with dissection and with a simple public demonstration: bind an arm, and the veins swell below the band; press their little knots (the venous valves) and blood can be pushed only toward the heart, never away. He could not see how blood crossed from the arteries to the veins — the capillaries are too small — but he insisted the connection must exist. Marcello Malpighi found those capillaries under a microscope in 1661, completing the loop Harvey had deduced.

Circulation turned the body into a system that could be reasoned about quantitatively — the first great triumph of measurement applied to living things. It made physiology a science, reframed the heart and lungs, and eventually underwrote blood transfusion, intravenous medicine, cardiac surgery, and the whole understanding of how disease and treatment move through a person. It was also a landmark of method: a fourteen-century consensus overturned not by a rival authority but by a calculation anyone could redo.