The heart is a completely muscular organ that plays an important role in the circulatory system by transporting materials throughout the body. It is located in the thoracic cavity behind the sternum and between the lungs, tilted slightly to the left. The adult human heart is approximately 12cm long, 9cm broad and weighs around 300g. It has a rough conical shape that is broader at the upper end and covered by a double membrane pericardium filled with pericardial fluid to protect and allow smooth functioning of the heart.