Your heart moves your cardiovascular system, right?
Yes it does, but did you know your muscles play an important role in driving your cardiovascular system? Muscles contract and relax and as they do, they drive blood into the smallest and remotest of your capillaries. The more muscles used to pull blood throughout your body, the less work your heart has to do to pump your blood.
Instead of thinking you need to do a ton of cardiovascular exercise for your heart you can think of your stretching program as lengthening (and growing) more muscle fibers. Two big muscle groups that are short on almost everyone (because of how much we sit) are the hamstrings and calf muscles.
So, as you do your calf and hamstring stretches, imagine your muscles growing in length and the new capillaries this creates. Now imagine these capillaries drawing blood into them much the same as a wick draws up oil in a lamp. And, you are doing a cardiovascular workout.