When we breathe we take in oxygen (O2), which goes to our lungs and increases oxygenation in our blood. Blood flows in one direction, just like a mountain spring, through valves that open one way only. From the lungs and through the pulmonary veins, oxygenated blood goes into the left side of the heart, filling up the left atrium, which contracts and pulses our blood through the mitral valve to the left ventricle.