The heart is located in the mediastinum behind the sternum and lies in the "pericardium", which is formed by the two layers of the pericardium. Most of the cardiac mass is formed by the heart muscle (myocardium), which is lined with endocardium toward the lumen, covered externally by the epicardium.The supply to the heartis carried out by arterial blood via two large coronary arteries, the artery coronary artery and the arteria coronaria sinistra. With each other they form numerous anastomoses. However, these anastomoses are not enough to form a complete collateral circulation. We therefore speak of functional end arteries. In case of closure of a branch there is a coverage area of ischemia followed by necrosis of myocardial tissue.The transport of venous blood takes place via the coronary veins and the coronary sinus.