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The structure of the healthy heart

The adult heart is about the size of a clenched fist and weighs between 250 and 390 grams in men, and 200 to 275 grams in women. It is located in the centre of the chest cavity and is tipped diagonally, with about two-thirds of its bulk left of the body’s midline and the apex pointing left. The heart is suspended in the chest cavity in a multilayered membrane called the pericardium which helps keep it in place. Viewed as if through the opened chest, it is possible to distinguish the large muscular masses of the left and right ventricles (the main pumping chambers) and above them the smaller and thinner walled left and right atria. On the surface of these chambers are grooves along which several of the major arteries run, including the coronary arteries. Approaching the heart are many large blood vessels, some returning blood from the body to the heart, and others carrying blood away to the lungs and other parts of the body. If we look inside the heart, it is immediately clear that it is a hollow structure with a vertical wall of muscle (the septum) dividing it into its right and left sides. Blood within these two halves does not mix. Rather, they operate as a double pump, that on the right handling deoxygenated blood and that on the left the blood which has been oxygenated in the lungs.

The deoxygenated blood flowing back towards the heart through the superior and inferior vena cava enters the right atrium and then passes through the tricuspid valve into the right ventricle. Similarly, oxygenated blood from the lungs enters the left atrium and passes to the left ventricle through the mitral or bicuspid valve. A second pair of valves, the semilunar valves, guard the exits of the ventricles to the aorta and the pulmonary arteries. The tricuspid and mitral valves close when the heart contracts to prevent the back-flow of blood while the semilunar valves open, and vice versa. The valves themselves are not free flaps, but are tethered to the ventricular or arterial walls by fibrous strands.

Blood is supplied to the heart itself (coronary circulation) along the left and right coronary arteries which are the first branches off the aorta. Most blood goes to the left side, where the greatest amount of work is done. It should be noted that there is some collateral circulation – some blood passing along the left coronary artery goes to the right heart and vice versa. A blockage does not necessarily mean a total loss of blood supply – that will depend on the exact location of the blockage. The heart beat is regulated throughout life, whether you are awake or asleep, and in response to physical exercise or emotional experiences, by an internal system of electrical conducting fibres. These are modified heart muscle cells that function like the nerves in the rest of the body and are quite separate from the brain and central nervous system.