Blood picks up oxygen from the lungs and flows directly to the heart. This insures that the heart always has plenty of oxygen, so it does not get tired. The one exception is if the blood vessels that lead to your heart get blocked. Then the heart muscles run low on oxygen and get tired. The pain that you feel is what tells you that you are having a heart attack.
Athletes exercise regularly to increase the blood flowing to their muscles.
If the muscles get more blood, they get more oxygen. Then they can work harder and longer before they get tired. Right now, I could use some extra oxygen, so I could unload the rest of our stuff. Have a good week.