Galen was educated as a doctor supposedly because his father had a dream in which Asklepios, the god of medicine, appeared to him. After an initial period of training in his native city of Pergamum (now Bergama in Turkey), Galen spent the years 148–57 traveling and studying at Corinth, Smyrna, and Alexandria. He then returned home and took the post of surgeon to the Pergamum gladiators. Eventually Galen left for Rome where he made a considerable reputation for himself serving the emperors Marcus Aurelius and Commodus.

In addition to his imperial duties Galen wrote extensively and more than 130 of his texts have survived. Even though some are undoubtedly spurious, there still remains an impressive opus on virtually every aspect of the medicine of his times.

It is this opus that acquired an unprecedented authority, which persisted, unopposed, for another 1500 years. It was not until William Harvey proposed his new theory of the circulation of the blood (1628) that anatomists were presented with a viable alternative to the traditional system of Galen.

Galen made few advances in pathology believing as he did in Hippocrates' humoral theory – that disease is caused by an imbalance of the four body humors; phlegm, black bile, yellow bile, and blood. It was as an anatomist that Galen's true originality lay. He stressed the importance of dissecting personally and frequently, and combined his anatomical studies with a number of neat and conclusive experiments. Thus he clearly demonstrated the falsity of Erasistratus's view that the arteries carry air not blood by placing ligatures both above and below the point of incision and noting the absence of any air escaping before the discharge of blood. Also, by similar experiments he was able to show that urine passes from the kidneys to the bladder irreversibly down the ureters.

Galen however was less successful in describing the complete system and operation of the body. He thought that blood was made in the liver from the food brought by the portal vein from the stomach. This was then transported by the venous system to nourish all parts of the body. Some of the blood however passed along the vena cava to the right side of the heart where it passed to the left side by some supposed perforations in the dividing wall or septum. Belief in such perforations persisted well into the 16th century. In the left ventricle the blood was mixed with air brought from the lungs and distributed round the body via the arterial system carrying the vital spirit or innate heat. Some of this blood was carried by the arteries to the head where in the rete mirabile (a vascular network not actually found in man) it was mixed with animal spirit and distributed to the senses and muscles by the supposedly hollow nerves. It was this spirit or pneuma that produced consciousness.

Thus instead of a single basic circulation Galen has a tripartite system in which the liver, heart, and brain each inject into the body three different spirits – natural, vital, and animal, which travel through the body via the venous, arterial, and nervous channels respectively. Although such a scheme is now recognized as totally misguided it possessed sufficient plausibility and experimental support to persist into the 17th century.