Circulation and gas exchange
Arteries carry blood away from the heart, veins return it, and all exchange happens across thin capillary and alveolar walls.
A worked example
Cardiac output is stroke volume times heart rate, so 80 mL per beat at 60 beats per minute gives 4 800 mL/min, or 4.8 L/min.
The mistake people actually make
Arteries and veins are named by direction of flow, not by oxygen content, which is why the pulmonary artery carries oxygen-poor blood. Breathing rate is driven by rising carbon dioxide, not by falling oxygen.