How did William Harvey explain the active phase of the heartbeat?

Prior to William Harvey’s research, it was incorrectly believed that the active phase of the heartbeat—when the heart muscles contract—involved the heart increasing its internal volume to draw blood into itself.

Through his observations of cold-blooded animals and those near death, which allowed him to study slower heartbeats, Harvey corrected this misconception. He concluded that the active phase of the heartbeat is actually when the heart muscles contract to decrease the heart's internal volume. This action expels blood from the heart with considerable force, establishing the heart's function as an organic pump within the circulatory system.1