The heart, actually, is positioned in the guts of the chest, behind the sternum (or the breastbone). <br />However, part of it is slightly offset left, as you can find out from the picture over. For understanding this, I believe prerequisite knowledge of the heart's basic anatomy is required. <br />Okay, for starters, I would need to refute Andrew's tips vis-a-vis the evolutionary motive and the sternum offering resistance. <br />There is no reason why the center should be a much better place for the heart to be compared to the right or the left sides of the upper body. When our body was evolving, characteristics had to fit complex organs in the most efficient of ways in the tiny space of a body. Thus, our bodies are full of small asymmetries. Some simple things needed to be devote the centre, some plain things in the left and some things in the proper. The heart is merely at the heart because there happen to be lungs on the either area and therefore, there was no space kept on the sides in the thoracic cavity. <br />Regarding the real point you raised about the sternum, Andrew, the heart is certainly enclosed in a double walled sac called pericardium. The pericardium's primary features are to protect the heart and anchor it to the structures around it, to lessen friction although it beats and to secure it from overdilating (or overexpanding). So there is absolutely no relevant concern of the heart pushing against the sternum anytime. The discomfort which would build if your bare heart pushed against the sternum would be unbearable. <br />The pericardium limitations the movement of the heart. The sternum doesn't. <br />Now, to learn why the heart is offset to the left slightly, one must know about the basic functioning of the heart and how circulation can be caused in our body. <br />The human heart has 4 chambers, namely the Left Atrium (LA), the Left Ventricle (LV), the proper Atrium (RA) and the Right ventricle (RV). The atria are the obtaining chambers and the ventricles are the discharging chambers. <br />In mammals, the function of the right side of the heart is to accumulate de-oxygenated blood, in the proper atrium, from the body which, via the right ventricle, is pumped to the lungs for oxygenation then. The left side collects oxygenated bloodstream from the lungs in to the left atrium then. From the still left atrium the blood movements left ventricle, through the which pumps it out to the physical physique. <br />On both sides, the low ventricles are thicker and better than the upper atria. Nevertheless, the muscle wall structure surrounding the remaining ventricle is a lot thicker than the wall surrounding the right ventricle due to the higher force needed to pump the blood through the whole of the body (systemic circulation). However, the right ventricle has to pump blood only to the lungs, which are close enough. <br />Consequently, the increased muscle mass of the LV triggers it to offset a little bit to the left and since it may be the largest the main heart, it gives a considerable impression of the center being more to the left. This is as well the reason that Where is the heart located are feeling the heart beat on the left side of the chest since the LV generates the highest pressure.