Apparently his view was that there was a sort of ebb and flow in both systems--and yet, he uses language just such as we would, speaking of the venous system as ".

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a conduit full of blood with a multitude of canals large and small running out from it and distributing blood to all parts of the body." He compares the mode of nutrition to irrigating canals and gardens, with a wonderful dispensation by nature that they should "neither lack a sufficient quantity of blood for absorption nor be overloaded at any time with excessive supply." The function of respiration was the introduction of the pneuma, the spirits which passed from the lungs to the heart through the pulmonary vessels.

Galen went a good deal beyond the idea of Aristotle, reaching our modern conception that the function is to maintain the animal heat, and that the smoky matters derived from combustion of the blood are discharged by expiration.
I have dwelt on these points in Galen's physiology, as they are fundamental in the history of the circulation; and they are sufficient to illustrate his position.