In William Harvey’s heart-ligature experiments, what did tying blood vessels reveal about circulation?

William Harvey’s heart-ligature experiments revealed that blood flows one way through the circulatory system.

In the seventeenth century, Harvey studied hearts and blood vessels in living animals and examined how temporary ligatures changed the size of veins and arteries. A ligature placed on a limb vein caused the vein to swell on the side away from the heart, showing the direction in which blood returned. Pressure on arteries produced different effects because arterial blood moved away from the heart.

Harvey also observed the valves of the heart and veins, which act like one-way gates. In 1628 he published these findings in *De Motu Cordis*, arguing that the heart pumps blood around a closed circuit.

Harvey could not see capillaries with the microscopes available to him. Marcello Malpighi later observed capillaries in 1661, providing the missing connection between arteries and veins. Harvey’s work replaced older models in which blood was continually created and consumed.