In literature and poetry the heart is regarded as the seat of vitality and the centre of emotions such as love, happiness, guilt and sadness. In real life the heart pumps. It is under the control of the brain and the in the stream. The heart is vital to life. It never stops pumping at an average rate of just more than one beat per second for 70 years or more, perhaps totalling 3,000 million beats in a lifetime.
The average human heart weighs around 300 grams, a little more in men than women. It is a conical organ situated in the chest, slightly to the left of centre. You can feel the point or apex just below the left nipple. In the unusual condition known as dextrocardia, the heart lies to the right of centre. The whole heart and its connecting veins and arteries are reversed as if they are a mirror image of the usual position. People with dextrocardia generate an ‘upside-down’ electrocardio- gram, but usually the condition does not affectand is only discovered by accident.
Two pumps in one
Anatomically the heart consists of two sides, right and left, although these sides do not lie neatly on the right and left of the organ as it sits in the chest. The two sides are in fact twisted around each other. Each side is divided into two chambers: an atrium on top, a ventricle below. Each atrium plus ventricle acts as a separate pumping system so the heart is really two pumps, side by side.
The heart’s right side (on the left at the diagram below) receives blood from the large collecting veins, the venae cavae). The superior vena cava collects blood from the head and the arms while the inferior vena cava drains blood from the lower body. Both veins enter the right atrium. From here blood passes into the right ventricle, which squeezes with each heartbeat to pump it out through an artery, the pulmonary artery (pale blue) to the lungs, where it is oxygenated. Blood returning from the lungs travels through the pulmonary veins to the left side of the heart. These veins enter the left atrium and from there blood passes into the left ventricle. It is then pumped out into the body again via a large artery, the aorta (red), which divides to form the network of arteries carrying blood to all parts of the body except the lungs.
The atria are thin-walled chambers separated from each other by a dividing wall, the atrial septum. The ventricles have much thicker walls which are almost solid muscle, because they have the job of pumping blood around most of the body. They too share a common central wall – the interventricular septum. The left ventricle is thicker walled, being up to 1.5 cm thick compared to 0.5 cm in the right ventricle. Each ventricle contains about 140 ml of blood when its muscle is fully relaxed.
The heart) is affected by a rheumatic condition. In mitral stenosis, the valve’s two flaps tend to stick together and restrict the flow of blood through the valve. In the rarer condition known as mitral incompetence, the valve does not close properly. As a result, when the heart contracts some blood passes back into the atrium instead of being pumped into the aorta. Mitral disease may be treated with drugs or by surgery to repair the valve flaps. Fortunately doctors now have the expertise to replace poorly-functioning valves with artificial ones made of plastic, called prosthetic valves, should the need arise.
The walls of the heart are made up of very powerful and highly specialized muscle known as cardiac muscle or myocardium. This muscle works continuously without us having to think about it – contracting regularly, faster or slower, stronger or weaker, depending on the demands of the body. Unlike most other muscles in the body, heart muscle never tires and never rests.
Separating each atrium from its corresponding ventricle is a valve. The valves are made of soft but tough cup-shaped membranes called cusps, with the open ends facing downwards. They are made chiefly from the endocardium, the smooth and glistening innermost layer of the heart muscle. When the ventricle contracts, the blood inside is under great pressure and it pushes up under the cusps, ballooning them out so that they meet in the centre and prevent blood from flowing the wrong way. The valve on the right side has three such cusps and is known as the tricuspid valve; the left valve has only two cusps and is known as the mitral valve. Similar but smaller valves exist at the exits to the large arteries, doing the same job. When the ventricles relax after they have pumped the blood in the arteries, the valves prevent blood re-entering the ventricles again. The pulmonary valve is at the entrance to the pulmonary artery, and the aortic valve is at the entrance to the aorta. Each valve is attached firmly to its chamber or vessel by a valve ring around its rim. The larger mitral and tricuspid valves also have fibrous strands holding down their inner, free edges, like the strings of a parachute. These are called the chordae tendinae and are attached to muscular projections, the papillary muscles, sticking up from the inner walls of each of the ventricles.
If a valve becomes loose and leaky, or narrowed by disease, it disturbs the normal flow of blood and this can often be heard with the stethoscope as a heart ‘murmur’. In some cases the murmur is insignificant and harmless, but in others it indicates possible trouble to come. The commonest condition is mitral disease, in which the mitral valve (on the left side of
The entire heart is surrounded by a double layer of fine membrane called the pericardium, forming the pericardial sac that fits snugly between the left and right lungs. The inner layer of the pericardium sticks closely to the surface of the heart, the outer layer to the lungs, and between the two is a thin layer of fluid that lubricates the movements of the beating heart within its cavity.
The pericardium may become inflamed, a condition called pericarditis. It usually follows a serious infection, such as pneumonia, pyaemia or Bright’s disease (nephritis, or inflammation of the kidney).