The Heart: Two Pumps in One Fist

How the heart actually sits in the chest

Almost every drawing of the heart lies about its position. The heart is drawn upright and central; in reality it lies obliquely, tipped forward and to the left, with about two-thirds of its mass to the LEFT of the midline. It occupies the middle mediastinum, wrapped in its own serous sac — the subject of the pericardium — inside the middle compartment of the mediastinum. Orient yourself by two poles. The APEX points down, forward and to the left, and lies in the fifth intercostal space in the mid-clavicular line; it is formed entirely by the LEFT VENTRICLE, and it is what you feel as the apex beat. The BASE faces backwards, upwards and to the right, and is formed mostly by the LEFT ATRIUM (with a small contribution from the right atrium) — which is why the left atrium, the chamber you can least easily picture, is the one lying directly against the oesophagus and the vertebral column.

Between those two poles the heart presents surfaces and borders, and each one is dominated by a different chamber — a fact that turns a plain chest radiograph into a readable map. The anterior (sternocostal) surface is mostly RIGHT VENTRICLE: the chamber closest to your breastbone is not the left ventricle everyone imagines, but the right, which is why a sternal blow or a needle passed under the xiphoid reaches the right ventricle first. The inferior (diaphragmatic) surface, lying on the central tendon of the diaphragm, is formed by both ventricles, mainly the left. The left (pulmonary) surface, filling the cardiac notch of the left lung, is left ventricle. The right border is RIGHT ATRIUM; the left border is LEFT VENTRICLE with the left auricle; the inferior border is chiefly right ventricle; and the superior border is the two atria and their auricles, with the great vessels leaving between them.

The grooves on the outside — and the crux

Look at a heart from outside and you see grooves, filled with fat, that mark exactly where the internal walls lie — and, conveniently, carry the vessels described in the coronary arteries and cardiac veins. The coronary (atrioventricular) sulcus runs around the heart like a slanted belt, separating the atria above from the ventricles below; it lodges the right coronary artery, the circumflex artery and the coronary sinus. The anterior interventricular sulcus runs down the front from that belt to a notch just right of the apex, marking the interventricular septum and carrying the anterior interventricular (LAD) artery. The posterior interventricular sulcus does the same on the diaphragmatic surface. Where the coronary sulcus, the posterior interventricular sulcus and the interatrial groove all converge on the back of the heart is the CRUX — the anatomical crossroads where all four chambers meet, and the point from which the artery to the atrioventricular node usually arises. A blocked vessel at the crux therefore threatens not just muscle but the wiring.

The wall itself is three layers, and each one has a disease named after it. From inside out: the ENDOCARDIUM, a smooth endothelial lining continuous with the endothelium of the great vessels, which also covers the valve cusps — inflame or infect it and you have endocarditis, with vegetations growing on those cusps. The MYOCARDIUM, the thick middle layer of cardiac muscle arranged in spiralling sheets that wring the ventricles rather than merely squeezing them; this is the layer that dies in a myocardial infarction and hypertrophies in hypertension. And the EPICARDIUM, which is not a separate coat at all but the visceral layer of serous pericardium applied directly to the heart, with the coronary vessels and their fat running just beneath it. Understanding that the epicardium IS the visceral pericardium is what makes the pericardial cavity, and the tamponade that can fill it, make sense.

The right atrium: a chamber of two halves

Open the right atrium and you find, unexpectedly, two completely different interiors welded together — a fossil record of how it was built. The posterior part is the SINUS VENARUM: perfectly smooth-walled, developed from the embryonic sinus venosus, and it is the collecting basin. Into it drain the superior vena cava (from above, no valve), the inferior vena cava (from below, guarded by the rudimentary valve of the IVC — the Eustachian valve, which in fetal life directed oxygenated placental blood across the foramen ovale) and the coronary sinus, returning the heart's own venous blood between the IVC orifice and the tricuspid opening. The anterior part, including the ear-shaped right AURICLE, is rough: ridged by parallel bundles of PECTINATE MUSCLES like the teeth of a comb. Between the two runs a vertical muscular ridge, the CRISTA TERMINALIS, matched on the outside by a shallow groove, the sulcus terminalis. That crest is not merely a landmark — at its upper end, near the mouth of the superior vena cava, sits the sinu-atrial node, the pacemaker described in the conducting system of the heart.

On the septal wall of the right atrium is the FOSSA OVALIS, an oval depression with a prominent upper and anterior rim, the limbus. It is the scar of the foramen ovale — the fetal shunt whose closure you watched in the opening scene — and it is the thinnest part of the interatrial septum, which is why cardiologists cross it deliberately to reach the left side of the heart. In roughly a quarter of adults the flap never fully fuses, leaving a probe-patent foramen ovale. Most of the time this means nothing at all. Occasionally it means a great deal: a clot formed in a leg vein, which should end its journey harmlessly in the lung, can be pushed through the flap when right atrial pressure transiently exceeds left — a strain, a cough, a Valsalva — and travel instead to the brain. That is paradoxical embolism, a venous clot causing an arterial stroke, and its entire mechanism is one unclosed door in this wall. Lower on the septal wall, near the coronary sinus orifice, lies the small triangle of Koch, which contains the atrioventricular node.

The right ventricle: the low-pressure pump

It only has to push blood as far as the lungs — and its whole design says so. In cross-section the right ventricle is a crescent, wrapped around the bulging left ventricle rather than being a chamber in its own right, and its wall is only about 3 to 5 mm thick. That thinness is not weakness; it is economy. Pulmonary vascular resistance is a fraction of systemic resistance, so this pump generates a systolic pressure of roughly 25 mmHg where the left generates 120. Inside, the inflow portion is heavily ridged by coarse TRABECULAE CARNEAE, and three PAPILLARY MUSCLES — anterior (the largest), posterior and a small septal group — rise from the wall. From their tips, fine tendinous cords, the CHORDAE TENDINEAE, fan out to the free edges of the tricuspid cusps. Their job is not to open or close the valve but to tether it: when the ventricle contracts, the papillary muscles contract with it and hold the cusps down so that they cannot prolapse back into the atrium, exactly as detailed in heart valves and heart sounds.

Two structures make the right ventricle instantly recognizable. The first is the MODERATOR BAND, properly the septomarginal trabecula: a stout muscular bridge that leaps across the cavity from the interventricular septum to the base of the anterior papillary muscle. Its old name came from the belief that it moderated over-distension of the chamber, but its real importance is electrical — it carries the right bundle branch across the cavity so that the anterior papillary muscle is excited early and takes up tension before the rest of the ventricle contracts. On an echocardiogram it is the reliable marker that says "this chamber is the right ventricle." The second is the CONUS ARTERIOSUS, or infundibulum: a smooth, funnel-shaped outflow tract in the upper part of the chamber, separated from the rough inflow by the supraventricular crest and leading up to the pulmonary valve. Rough inflow, smooth outflow — a pattern you will meet again on the left.

The left atrium and the left ventricle

The LEFT ATRIUM is the quietest chamber and the hardest to see: it forms most of the base of the heart, lying against the oesophagus, which is exactly why a transoesophageal echo probe gives such superb pictures of it. Its walls are almost entirely smooth, because it develops largely by absorbing the primitive pulmonary vein, and into its posterior wall open the FOUR PULMONARY VEINS — two right, two left — without any valves. Only its small ear-like appendage, the left auricle, retains pectinate muscles, and that little trabeculated pouch matters out of all proportion to its size: in atrial fibrillation, when the atrium quivers instead of contracting, blood stagnates in the appendage, and it becomes the commonest site of thrombus formation in the whole heart. A clot forming there sits in the systemic circuit; when it leaves, it travels to the brain. Anticoagulation in AF, and the rate-versus-rhythm decisions covered in atrial fibrillation and SVT, are ultimately about this one small pouch.

Everything about the left ventricle is an answer to one question: how do you generate pressure? The LEFT VENTRICLE is conical, its cavity circular in cross-section, and its wall is 8 to 12 mm thick — two to three times the right — because it must drive blood through the entire systemic circuit, from the aortic root to the capillaries of your toes and back. Its trabeculae carneae are finer and more numerous than the right ventricle's, and it has only TWO papillary muscles, both large: the anterolateral and the posteromedial. That asymmetry has a price. The anterolateral papillary muscle usually enjoys a dual blood supply (from the LAD and the circumflex), while the posteromedial is typically supplied by a single vessel — so in an inferior myocardial infarction it is the posteromedial muscle that ruptures, tearing the mitral apparatus and producing sudden, catastrophic mitral regurgitation days after the infarct. Above the anterior mitral cusp the wall becomes smooth again: the aortic vestibule, the fibrous outflow tract leading to the aortic valve and the aortic root.

The septa and the fibrous skeleton

Two septa divide the heart. The INTERATRIAL SEPTUM is thin and largely muscular, thinning further at the fossa ovalis. The INTERVENTRICULAR SEPTUM is a different beast: overwhelmingly a thick MUSCULAR part, functionally belonging to the left ventricle and bulging into the right, plus a small MEMBRANOUS part high up and posteriorly, where it meets the fibrous skeleton. That membranous portion is only a sheet of fibrous tissue a few millimetres across, and it does two disproportionately important things. It is the commonest site of a ventricular septal defect — the commonest congenital heart defect of all — because it is the last part of the septum to close in development, formed by the fusion of three separate contributions. And it is the doorway through which the atrioventricular bundle of His passes from atrium to ventricle. A perimembranous VSD, or surgery to repair one, therefore risks heart block, because the hole and the wire share a postcode.

Holding all of this together is a structure students routinely skip and examiners routinely love: the FIBROUS SKELETON of the heart. It is a set of four dense fibrous rings — the annuli fibrosi — encircling the two atrioventricular orifices and the aortic and pulmonary orifices, joined by the right and left fibrous trigones and continuous with the membranous septum. It does three things at once. It gives the valve cusps and the myocardium of both atria and ventricles something to attach to, so the muscle has an anchor rather than pulling on other muscle. It keeps the valve orifices from stretching open as the chambers dilate. And, most elegantly, it is electrically inert — a complete insulating gasket between atrial and ventricular muscle, so that a wave of depolarization sweeping through the atria cannot cross directly into the ventricles. The only permitted route is the atrioventricular bundle, piercing the skeleton at one point. Without that insulation, the atrioventricular delay that lets the atria finish filling the ventricles could not exist, and a fast atrial rhythm would drive the ventricles beat for beat.