  The heart was once considered the centre of human emotion, and the location of your soul. We now know that the heart is simply an organ that pumps blood throughout the body, and can be replaced if necessary by donor hearts, and occasionally animal or synthetic hearts. But the heart is still an incredible mechanism. Made from muscle tissue and located between your two lungs, the average heart beats 72 times per minute, over 100,000 times per day, almost 38 million times per year, and by the time you die, on average, it will have beaten 2.5 billion times. An average heart pumps about 70 ml of blood per heartbeat. It will pump 7,200 litres of blood per day, about 2.6 million litres per year, and a total of 184 million litres by the time you die. Considering that this little pump weighs just 300 grams, that's an impressive performance!
Here are the important parts that make up the heart.The muscles surrounding the heart squeeze each inner cavity in turn, as valves open and close, to pump blood around the chambers of the heart, on its way in and out from various places in the body. The left ventricle squeezes blood that is full of oxygen out an artery called the aorta, where it is distributed through the artery system to all the cells of the body. After distributing its load of oxygen, the used blood returns to the right atrium of the heart via the superior and inferior vena cava veins. The right ventricle then contracts and pushes it out the pulmonary arteries, to the lungs. After receiving a new load of oxygen at the lungs, the blood returns via the pulmonary veins to the left atrium of the heart, and into the left ventricle, from where it is pumped back out into the body. All of these contractions and expansions of the cavities within the heart happen within about a second, or one heart beat.
During a heart beat, the heart muscle contracts; it does this in two steps. In the first step, the two atria contract at the same time, pumping blood to the right and left ventricles. Then the ventricles contract together, to push blood out of the heart. Finally, the heart muscle relaxes before the next heartbeat. This allows more blood to enter the heart.
 What causes blood clots, or a coronary thrombosis? The problem starts with a build-up of fatty material on the inside of the arteries, called plaque. When the buildup of plaque becomes quite thick in the coronary arteries, ('hardening of the arteries'), the flow of blood is reduced and the heart doesn't get enough oxygen. The resulting pain is called angina, and is similar to (but more severe than) to the 'pins and needles' pain you feel in your arm after sleeping on it all night, depriving the tissues of oxygen.As the plaque continues to thicken, and the heart begins to falter due to lack of oxygen, doctors can clean out the arteries by running a wire and small balloon through them (called coronary angioplasty) cleaning the plaque off the artery walls. If the arteries are too weak, they can also implant new ones around the blockage (a bypass) with arteries from somewhere else in your body.  Thick plaque on the walls of your arteries, which is composed of cholesterol, calcium, clotting proteins, and other substances, also causes another problem ... bits of plaque can flake off the walls, and be carried with the blood to the tiny coronary arteries, where they will get stuck, and cause blood cells to pile up and clot. Or a region of the coronary artery itself can begin flaking, and because it is no longer smooth, will also cause blood cells to stick and clot. Either blockage may be severe enough to stop the flow of blood altogether, in which case you will suffer a heart attack.
Thickening of the coronary arteries due to plaque is called atherosclerosis, and it usually begins when a person is about 20, and gets worse with increasing age. Reducing the cholesterol in your diet, getting sufficient exercise, treating hypertension (high blood pressure), and quitting smoking can all lessen the likelihood of this disease. Heart disease is the leading cause of death in North America; it accounts for as much as one third of all deaths each year. |
Here is the process shown as an animation. If you've listened to a heartbeat with a stethoscope, or seen it displayed on a heart monitor, you know that a single heart beat is really four sounds - 'bump-bump bump-bump'. The sounds are caused as the blood speeds up and then slows down, first through one set of valves, and then through another set. (Valves are muscular 'doorways' that allow blood to flow one way, but prevent it from going backwards). When each chamber contracts, the exit valve opens so the blood can leave. When it's finished contracting, the valve closes so that blood can't flow backwards.
