Seen from the side, the heart is located between the 2nd and 6th rib. The broad base of the heart with the atria is situated dorsally, roughly at the level of the shoulder joint. The apex of the heart, which is formed by the left ventricle, points ventrally towards the breastbone. The heart is enclosed in a tough sac known as the pericardium. Between the pericardium and the surface of the heart there is a narrow space filled with clear pericardial fluid; this helps to prevent friction between the heart and the surrounding structures as the heart beats.