Your heart is about the size of your fist. It is shaped like an upside-down pear. The heart is a muscle; it acts like a small pump. This pump is divided into four cavities, two of them are called atria and the other two are called ventricles. The left side of the heart houses one atrium and one ventricle. The right is the same. The smooth inside lining of the heart is the endocardium. The middle of the heart is called myocardium. And the fluid filled sac that surrounds the myocardium is the pericardium.