Cupid and his bow, Valentine and his cards, not to mention New York City and its T-shirt logo, have all conspired to misrepresent the appearance of the heart. It looks more like a boot than the traditional symmetrical appearance, in fact the hypertensive heart is called boot shaped.
Generally supposed by the public to be in the left side of the chest, the heart in fact sits behind the sternum (breast bone) in the midline of the thorax, and its several valves are all located behind the sternum. It does, however, extend more to the left side of the midline than the right. Many persons these days bear a central scar on their chests, the mark of heart surgery, When the surgeon splits the sternum down its center to enter the thorax, he first finds the pericardium, a cone-shaped and firm fibrous sac, attached to the diaphragm below and enclosing the great vessels above. This tough sac which encloses the heart, is separated from its muscular substance by a thin film of fluid, and the transparent serous pericardium that secretes this fluid.
The heart developed from a tube, by undergoing a complicated series of convolutions the tube became a four chambered muscular structure, but it should be borne in mind that the heart is muscle, specialized muscle histologically different from other muscle fibers, but bearing out the axiom in treating heart attacks, time is muscle. There are two sides to the heart, they work in co-ordination but have different functions. The right side receives the venous, deoxygenated blood via the superior and inferior vena cava, and passes it on to the lungs. The left side receives the oxygenated blood from the lungs and passes it on to the rest of the body.
Muscle fibers need an attachment, elsewhere in the body they are attached to bones by ligaments, in the heart a similar but less visible structure serves this function. The heart has in effect a skeleton, composed of a pair of tough fibrous rings, joined like the figure 8, only it lies on its edge between the right and left portions of the heart on the fibrous inter-ventricular septum. To these rings are attached the whorls of muscle fibers that compose the bulk of the heart, each side of the heart has its own distinct fibers which do not cross over. The valves that lie between the chambers of the heart, that is between atrium and ventricle of each side, also find stability by attachment to this fibrous skeleton.
The right side of the heart is flat. The great veins, superior and inferior venae cavae (Latin singular ending adds an “e” in the plural) empty their contents into it, and from there the blood passes almost horizontally into the right ventricle. There is a clinically important side chamber, left over from the development of the heart, the auricular (shaped like a dog’s ear) appendage, which can be a back water in which clots may form and subsequently dislodge as emboli that get passed on to the lungs. Most of the smooth walled right atrium is in fact the vein incorporated into the heart, a small ridged area is the original portion of the developing heart, as is the auricle.
The right ventricle is horizontally to the left of the atrium, their cavities are separated by the three leafed tricuspid valve; the leaves are attached at their fixed base to the fibrous ring described, and on their free ends are attached by chordae tendinae to the walls of the ventricle. The interior upper portion of the ventricle becomes cone-shaped as it becomes the pulmonary artery, their interiors are separated by the three leafed pulmonary valve. It should be noted that the vessel is called an artery, designated by the fact it leads from the heart, not by the degree of oxygenation of blood. In the same sense, the vessel passing from the lungs to the heart is designated as a vein.
The left atrium lies at the back of the heart. Like the right atrium, it is really a coalescence of the four veins it receives from the lungs, and there is a vestigial auricle left over from the original formation of the heart. Blood flows from the left atrium to the left ventricle through the opening guarded by the two-leafed mitral valve (anatomy students learn, “the bishop is never right”).
The left ventricle does the hardest work of the heart, pumping the blood around the body. The wall, which is muscle, is three times as thick as the right ventricle, and when it becomes harder to pump blood around the body because of thickened vessels, the ventricular muscle becomes thicker and the ventricle elongates to the left making the “boot-shaped” heart as seen on the chest X-ray. Normally the heart shadow on the X-ray is less than half as wide as the chest, when it widens beyond that, excessive load on the heart and potential failure is considered. Blood is pumped from the left ventricle into the aorta, through the three-leafed aortic valve.
Two coronary arteries leave the aorta immediately above this valve, the right coronary artery descends on the anterior surface of the heart in the groove between the right atrium and ventricle; the left descends down the back of the heart between the ventricles. The coronary arteries are considered “end-arteries” in that the territory supplied by one is not also supplied by the other, there is very little cross-flow possible, hence when an obstruction occurs in a coronary artery the muscle it supplies is deprived of oxygen – the typical heart attack. Unlike the usual arrangement in the body where veins run with arteries, in the heart the veins, or sinuses, are distinctly separate; the lymphatic drainage parallels the arterial supply.
The nerve supply of the heart is by the autonomic nervous system, put very crudely, the parasympathetic system (vagus nerve) slows it down, the sympathetic system speeds it up, so in the fight or flight principle of the sympathetic system the heart beats faster. The heart does not have visible nerves of the type seen going to other muscles, but has a specialized zone of muscle fibers, the pacemaker in the right atrium, and then the wave of electrical excitation is picked up by, effectively a nerve, the Bundle of His which lies in the inter-ventricular septum and sends branches to the cardiac muscle.