The idea that venous return is equal to arterial outflow resulted from the work of William Harvey, published in 1628. He argued the case persuasively, even though he did not have a microscope and could not see the capillary links between the arteries and the veins.
Ordinarily, the veins hold 60 - 70 % of the blood volume. As such they play a very important role in supplying preload to the heart. Some investigators, including Arthur Guyton, would have us believe that the veins alone are important in controlling stroke volume. In their view the heart is a purely passive pump, simply pumping out that blood which it receives.
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