is the movement of blood from the heart to the lungs for oxygenation, then back to the heart again. Oxygen-depleted blood from the body leaves the systemic circulation when it enters the right atrium through the superior and inferior venae cavae.
Around 2-4 days late is when it can start to be a little worrying....but if u haven't "done it" with in the last few days then you shouldn't have to worry about being pregnant
