Heart location, membranes and chambers

Where the heart sits

The heart lies in the mediastinum, the central compartment of the thoracic cavity between the two lungs. It sits behind the sternum and in front of the vertebral column, above the diaphragm, and roughly two-thirds of it lies to the left of the midline. Its broad upper part, the base, faces upward and to the right and is where the great vessels enter and leave; its pointed lower end, the apex, points down, forward and to the left. This orientation explains several clinical facts: the heartbeat is felt best at the apex, between the fifth and sixth ribs, and the heart sits close to the front of the chest, which is why it can be examined with a stethoscope, ultrasound and X-ray. Because it lies between the lungs, its position changes with breathing and posture.

For example: Placing a stethoscope at the apex, on the left side below the nipple line, gives the clearest sound of the mitral valve closing.

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Pericardium and the heart wall

The heart is enclosed by the pericardium, a sac with two parts. The outer fibrous pericardium is tough connective tissue that anchors the heart to surrounding structures and prevents it from overfilling. Inside it, the serous pericardium forms two layers: the parietal layer lines the fibrous sac, and the visceral layer, also called the epicardium, is attached directly to the heart muscle. Between the two layers is the pericardial cavity, a thin space containing a small amount of serous fluid that lets the heart move without friction. The heart wall itself has three layers. The epicardium is the outer covering; the myocardium is the thick middle layer of cardiac muscle that does the pumping; and the endocardium is the smooth inner lining that contacts the blood and continues into the vessels. The myocardium is thickest in the left ventricle because that chamber pumps against the highest pressure.

For example: In pericarditis, the inflamed layers rub together and cause sharp chest pain, because the fluid that normally lets them glide is insufficient.

The four chambers

The heart has four chambers arranged in two pairs. The right atrium receives deoxygenated blood from the body through the superior vena cava, the inferior vena cava and the coronary sinus, which drains the heart’s own muscle. The right ventricle receives blood from the right atrium and pumps it into the pulmonary trunk towards the lungs. The left atrium receives oxygenated blood from the lungs through the pulmonary veins. The left ventricle receives blood from the left atrium and pumps it into the aorta towards the body. A septum separates the two sides: the interatrial septum between the atria and the interventricular septum between the ventricles. Small ear-like extensions of the atria, called auricles, increase their volume slightly. Because the left ventricle must generate systemic pressure, its muscular wall is much thicker than that of the right ventricle.

For example: A blood sample taken from the right atrium is deoxygenated, while one taken from the left atrium is oxygenated — the same heart, two different circuits.

Common misconception: The atria are the receiving chambers and the ventricles are the pumping chambers. Blood does not flow directly from the body into a ventricle.

The great vessels

Each chamber connects to specific great vessels, and knowing them makes blood flow easy to follow. Into the right atrium: the superior vena cava from the head, neck and arms; the inferior vena cava from the trunk and legs; and the coronary sinus from the heart muscle itself. Out of the right ventricle: the pulmonary trunk, which divides into the right and left pulmonary arteries. Into the left atrium: four pulmonary veins, two from each lung. Out of the left ventricle: the aorta, the largest artery, which arches and descends to supply the whole body. The pattern is simple once the circuits are understood: veins bring blood to the atria, and arteries carry it away from the ventricles. The only apparent exception, the pulmonary trunk carrying deoxygenated blood, follows the direction rule explained in section 5.2.

For example: The superior vena cava enters the right atrium from above and the inferior vena cava from below, which is why both are named for their position relative to the heart.

Chambers and the circuits they serve

The four chambers are best remembered by the circuit each serves. The right atrium and right ventricle form the right pump, which sends blood to the lungs; the left atrium and left ventricle form the left pump, which sends blood to the body. Because the two circuits are in series, the two pumps must move the same volume over time, which is why the right and left ventricles have matched outputs. Their walls differ because their workloads differ: the right ventricle pumps into the low-pressure pulmonary circuit, while the left pumps into the high-pressure systemic circuit. This is why the left ventricular wall is roughly three times thicker and why diseases that raise pressure in one circuit change the shape and workload of the corresponding ventricle.

For example: In pulmonary hypertension, the right ventricle must work against higher resistance and its wall thickens over time, just as the left ventricle does in systemic hypertension.