If 18th-century physiology was such a mess, you can imagine how early medicine must have played out. On the one hand, access to dissection subjects drove great advances in anatomy and physiology as far back as 300 B.C.E. On the other hand, every correct conclusion seemed counterweighted by superstition and social prejudice.
Greek physician Praxagoras (fourth-century B.C.E.) differentiated veins from arteries, but thought arteries carried air (likely because corpse arteries are often empty). In the second century, Galen carried on this tradition, but added that blood was made in the liver, which he said imbued it with "natural spirit," and swirled around the body in veins. It did not pump so much as it sloshed. Once it mixed with "vital spirit" from the lungs, blood was consumed by organs, which "attracted" it the way lodestone attracts iron. Blood also reached the brain via hollow nerves, he said, where it absorbed "animal spirit" [sources: Aird; Galen; West].
These notions held on until William Harvey published his game-changing "On the Motion of the Heart and Blood in Animals" in 1628. Others, such as Arab scholar Ibn an-Nafis, who died in 1288, had earlier made several corrections, but the Western world remained unaware of his work. Another predecessor, Spanish physician Miguel Serveto described circulation correctly in the 16th century, but wrapped his findings in a religious screed which, like Serveto himself, ended up burned on a pyre [sources: Aird; Cambridge Modern History; West].