The heart

“Have a heart!” “You’re breaking my heart!” “He’s a man after my own heart.” “He showed a lot of heart.” “He wears his heart on his sleeve.” “Hey, wanna play hearts?”

We use the word “heart” in a lot of different ways–so many, in fact, that the Oxford English Dictionary (which admittedly is not known for its brevity) takes five pages to list all the usages and cite references (the Saxon word was “heorte” and was in use at least as early as 1000, if you really want to know).

However, from a scientific point of view, “heart” is quite easy to define: it’s “a hollow muscular organ that by its rhythmic contraction acts as a force pump maintaining the circulation of the blood.”

Most people don’t give much thought to their heart. In a way that’s good, because if we did have to give thought to our hearts, deciding consciously to make it beat, that’s about ALL we’d be able to do. But even though we don’t have to think about our hearts, I hope you will, just for a minute, so you can appreciate what an amazing organ it is.

Above a certain level of complexity, most creatures have some kind of heart, ranging from a simple tube in spiders to the huge pumps of large mammals like elephants. The human heart is about the size of a fist and weighs between 250 and 300 grams. It has two sides divided by a barrier called the septum. Each side of the heart has a receiving chamber for incoming blood, called the atrium, and a pumping chamber called the ventricle, separated by valves to prevent reverse flow. The right valve is called the tricuspid, because it has three “cusps” (flaps), while the left valve is called the mitral valve, because it’s shaped a bit like a bishop’s mitre (hat).

The right side receives blood from the rest of the body and pumps it through the lungs for a dose of oxygen. That blood then flows into the left side of the heart, which pumps it out into the body again. Because it’s harder to pump blood to the whole body than just to the lungs, the left side of the heart is larger and has much thicker walls.

The muscle the heart is made out of is different from the muscles in your arms and legs. Heart muscle doesn’t require a nerve impulse to make it contract; it contracts on its own. However, the various parts of the heart would all contract at different speeds if not regulated. That’s the function of a bit of the heart called the sinu-atrial node, which has the highest inherent rate of contraction, about 100 beats per minute. The sinu-atrial node is affected by physical activity and other factors, allowing the heartbeat in humans to vary from 50 or 60 beats per minute to 180 or 190.

Well, this is all very interesting, I hear you say (at least, I hope I hear you say), but you promised more than interesting, you promised “amazing.”

All right, how’s this? At rest, your heart pumps about five litres of blood per minute–pretty well the whole volume in your body. That can be increased up to five times during heavy exercise–in other words, a trained athlete really pushing hard is pumping all the blood in his body in a complete cycle every 12 seconds or so.

This means that on average your heart pumps between 11,000 and 19,000 litres of blood every day, about 5.3 million litres a year, or close to 400 million litres in a lifetime– more than enough to fill 2,100 Boeing 747 fuel tanks. 

Are you amazed yet?

How about this? The network of blood vessels your heart pushes all that blood through, if strung out in a single line, would be close to 100,000 kilometres long–long enough to stretch almost 2 1/2 times around the Earth.

These vessels dilate in the parts of the body where more blood is needed. For example, a marathon runner would have lots of blood coursing through his legs and very little in his stomach. However, a couch potato resting after a big meal would have lots of blood nourishing his digestive system and far less in his legs and arms.

It’s fortunate the body works that way, because to fill those 100,000 kilometres of blood vessels if all were dilated would require the heart to pump more than 40 litres a minute–and even the world’s greatest athletes can only manage 25. If somehow all those blood vessels dilated at once, your blood pressure would drop to zero and you would literally bleed to death in your own veins.

Fortunately, that doesn’t happen. But there are other things that can go wrong with hearts. A partial list makes for scary reading: arteriosclerotic heart disease, myocardial infarction, angina pectoris, camyocarditis, degenerative cardiomyopathy, endocardial fibroelastosis, corpulmonale, pericarditis, etc., etc.

People suffering from these and other heart diseases HAVE to think about their hearts, probably a lot more than they care to. But it’s a good idea for everyone else to think about these extraordinary biological pumps now, too, while they’re unobtrusively performing their amazing feats, rather than later, when heart disease makes it unavoidable, because a little preventative medicine in the form of proper nutrition and exercise can go a long way to ensure a lot of hearts remain unobtrusive.

So give your heart a little thought–and for a special treat, why not take it for a walk?

It will do you both good.

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