Cardiovascular training is exercise for your heart and lungs. During this type of training you are increasing and decreasing you heart rate and increasing its endurance as well as expanding lung capacity.
Your lungs bring oxygen in and your blood cells swing by and pick up this oxygen delivering it throughout your body. If you happen to be riding a bike or running, the muscles in your legs are working hard and burning up the oxygen quickly in their cells. Your muscles have to have oxygen to keep moving. The more stress on the muscles the more oxygen needed.
By bouncing your heart rate up and down and moving through your target zones, you are training your system to respond accordingly. Pick up the pace the heart pumps faster to bring in the oxygen rich blood to the muscles working. The faster the circulation, the less oxygen the blood cells are able pick up but what it does pick up is delivered quickly. Slow your pace and you slow your heart rate and circulation allowing the blood cells to be engorged with oxygen within the lungs.
In short, when I need you I need you now and when you get the chance to refuel take it and be ready to explode again