So what can happen is the blood inside the right atrium. If the heart is actually undergoing some type of rigorous activity, so the person, let's say, running, and the heart has to pump more, then what can happen is the pressure inside the right atrium can be slightly greater than inside the left atrium. And so some of that blood in the right atrium can move into the left atrium and then directly into the left ventricle and into the systemic circulatory system. And this bypasses the lungs. Now, why is this a problem? Well, as it turns out, the lungs not only oxygenate the blood, but they're also pretty good at breaking down blood clots.