Heart: The heart is a hollow muscular organ about the size of a human fist lying within the mediastinum between the lungs, with its apex on the diaphragm. It has four chambers. Right and left aria: The atria are small receiving chambers much like big veins. The right atrium, which receives blood from the systemic circulation, is larger than the left atrium, which receives blood from the lungs only. Both atria have thinner walls than do the ventricles since they have less pressure exerted on them than do the ventricles, just as veins have thinner walls than do arteries. Right and left ventricles: The ventricles are large, contracting chambers with thick walls. The walls of the left ventricle are thicker because the left ventricle must pump blood to the most distant parts of the body; the right ventricle pumps blood only to the lungs. The openings between the atria and ventricles, the atrioventricular orifices, are controlled by cuspid values. The tricuspid valve controls the opening between the right atrium and the right ventricle. The mitral (biscuspid) valve controls the opening between the left atrium and the left ventricle.