Book 4 - Self Instructional Manual for Cancer Registrars - SEER ...

Heartand PericardiumThe 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 (see Figure 14B).• Right and left atriaThe atria are small receiving chambers much like big veins. The right atrium, which receivesblood from the systemic circulation, is larger than the left atrium, which receives blood fromthe lungs only. Both atria have thinner walls than do the ventricles since they have lesspressure exerted on them than do the ventricles, just as veins have thinner walls than doarteries.• Right and left ventriclesThe ventricles are large, contracting chambers with thick walls. The walls of the left ventricleare 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 bycuspid valves:• The tricuspid valve controls the opening between the right atrium and the right ventricle.• The mitral (bicuspid) valve controls the opening between the left atrium and the left ventricle.Semilunar valves control the opening from the right ventricle into the pulmonary artery and theopening from the left ventricle into the great aorta.The heart muscle (myocardium) receives its blood supply from the branches of the right and leftcoronary arteries (Figure 14B). These vessels come off the ascending aorta just as this structure exitsfrom the left ventricle.A small mass of modified cardiac muscle called the sinoatrial node and an additional mass nowtermed the atrioventricular junction, along with an abundant nerve supply, contribute to thestimulation and regulation of heart action.103