This cross section shows the internal structure of a normal heart.

The pericardium is a fluid-filled sac that envelops the heart and acts as a tough, protective covering. It consists of the fibrous pericardium and the serous pericardium. The fibrous pericardium is composed of tough, white, fibrous tissue, which fits loosely around the heart and protects it. The serous pericardium, the thin, smooth, inner portion, has two layers: parietal layer, which lines the inside of the fibrous pericardium visceral layer, which adheres to the surface of the heart. This cross section shows the internal structure of a normal heart. The pericardial space separates the visceral and parietal layers and contains 10 to 30 ml of thin, clear, pericardial fluid, which lubricates the two surfaces and cushions the heart. Excess pericardial fluid, a condition called pericardial effusion, can compromise the heart's ability to pump blood. Heart chambers The heart contains four chambers - two atria and two ventricles. (See Inside a normal heart.) The right atrium lies in front of and to the right of the smaller but thicker-walled left atrium. An interatrial septum separates the two chambers and helps them contract. The right and left atria serve as volume reservoirs for blood being sent into the ventricles. The right atrium receives deoxygenated blood returning from the body through the inferior and superior venae cavae and from the heart through the coronary sinus. The left atrium receives oxygenated blood from the lungs through the four pulmonary veins. Contraction of the atria forces blood into the ventricles. P.5Inside a normal heart This cross section shows the internal structure of a normal heart. This cross section shows the internal structure of a normal heart. The right and left ventricles serve as the pumping chambers of the heart. The right ventricle lies behind the sternum and forms the largest part of the heart's sternocostal surface and inferior border. The right ventricle receives deoxygenated blood from the right atrium and pumps it through the pulmonary arteries to the lungs, where it's reoxygenated. The left ventricle forms the heart's apex, most of its left border, and most of its posterior and diaphragmatic surfaces. The left ventricle receives oxygenated blood from the left atrium and pumps it through the aorta into the systemic circulation. The interventricular septum separates the ventricles and helps them pump. The thickness of a chamber's walls is determined by the amount of pressure needed to eject its blood. Because the atria act as reservoirs for the ventricles and pump the blood a shorter distance, their walls are considerably thinner than the walls of the ventricles. Likewise, the left ventricle has a much thicker