He checked that by tying off and blocking different veins in animals he experimented on the veins always bulged on the side of the block away from the heart. As though the blood as trying to flow toward the heart and to accumulate just below the block because it simply couldn’t flow away from the heart. This was true of all veins

In the arteries, the blood bulged on the heart side of any block he put in, as though it were trying to flow away from the heart and couldn’t move in the other direction

Harvey now saw what was happening. The heart pushed blood into the arteries, and the blood returned by way of the veins. It did this for both ventricles. The blood had a double circulation. If one started from the right ventricle, it left by way of the arteries to the lungs, and returned by way of the veins to the left atrium is known as pulmonary circulation and from there into the left Ventricle.From the left ventricle, it left by way of the arteries to the rest of the body and returned (in a “greater circulation”) by way of the veins to the right atrium and from there into the right ventricle. It is known as systemic circulation. As the blood reaches the heart two times, once through pulmonary cycle and again through systemic cycle, it is called double circulation

Harvey also showed that it was impossible, suppose that the blood was used up in the body and that new blood was formed. He measured how much blood the heart pumped in one contraction and also counted the number of contractions

He found that in one hour, the heart pumped out a quantity of blood that was three times the weight of a man. The body couldn’t use up blood and form new blood at such a rate. The same blood had to circulate and be used over and over again

Harvey still had some problem. The smallest arteries and veins that could be seen had to be connected by vessels too small to see. Were they really there?

In the 1650s, scientists had learned to put lenses together in such a way that objects too small to see with the naked eye could be magnified and made visible. Marcello Malpighi (1628-1694), with the microscope, he could see tiny blood vessels that were invisible with naked eye.

In 1661, four years after Harvey’s death, Malpighi studied the wings of bats. He could see blood vessels in their thin membranes and, under the microscope; he could see that the smallest arteries and veins were connected by very fine blood vessels.

He called these blood vessels “capillaries” from the Latin word for “hair”, because they were as thin as the finest of hairs.

With the discovery of capillaries, the idea of the circulation of the blood was complete, and it has been accepted ever since.