William Harvey is the 17th century physician who hypothesized that blood moved in a circle. This was quite different from the accepted view of the time. Galen had postulated 1700 years previously that there were two kinds of blood, arterial and veinous, each with their own pathway to the tissues where it was consumed.
Harvey wrote: Since all things, both argument and ocular demonstration, show that the blood passes through the lungs and heart by the force of the ventricles, and is sent for distribution to all parts of the body, where it makes its way into the veins and porosites of the flesh, and then flows by the veins from the circumference on every side to the centre, from the lesser to the greater veins, and is by them finally discharged into the vena cava and right auricle of the heart, and this in such a quantity or in such a flux and reflux thither by the arteries, hither by the veins, as cannot possibly be supplied by the ingesta, and is much greater than can be required for mere purposes of nutrition; it is absolutely necessary to conclude that the blood in the animal body is impelled in a circle, and is in a state of ceaseless motion.
That is one hell of a sentence!
Harvey knew blood moved in a circle but he was unable to determine exactly how. He did not have the technology to observe the small capillaries that connected the arterial and veinous vessels.
Harvey was one of the first to use the scientific method in his work. He said:
I profess to learn and teach anatomy not from books but from dissections,not from the tenets for Philosophers but from the fabric of Nature.
You can learn more about William Harvey here.