by Jon Clark

Created on: September 12, 2009   Last Updated: September 17, 2009

The human heart is a miracle of design and without doubt the strongest and most durable pump in existence. A human life span of 80 years at an average beat rate of 70 beats per minute would have a total of about 2 billion heart beats.

It would be difficult to come up with an organ better suited to its function than the human heart. As mammals we have double circulation - that is, one circulation to the lungs and another to the body. So there are two sides to the heart, one which carries deoxygenated blood and pumps it to the lungs to pick up oxygen - the right side. The left side carries oxygenated blood and pumps it to the body. The two sides of the heart are separated by the septum; if the oxygenated and deoxygenated blood were to mix, then not enough oxygenated blood would circulate to the body.

This double circulation is a great improvement on the single circulation found in fish, for example. The problem with a circulatory system is that at some point, it has to go over a large respiratory surface to pick up oxygen and, in doing so, loses a lot of pressure. So, in a fish, after the blood has gone over the gills (the respiratory surface) and lost pressure, it still has to make the journey around the fish's body. Due to the single circulation, fish hearts do not have separate sides. The blood is only pumped out of the heart once; it just moves straight through. This single circulation results in somewhat inefficient circulation of blood.

Mammals' double circulation means that the blood is pumped twice, once to the lungs and once again to the body, so any pressure that is lost at the lungs is made up for by the second pump from the left side. In fact, the left side is made of much thicker muscle than the right side and can develop the high pressure necessary to pump blood around the body.

That is just the basics though. It is a much more elegantly designed and constructed organ than I have so far illustrated.

In order to pump blood around the body, it is obvious that the heart must first fill with blood, then pump it. Therefore, the heart has two chambers on each side, the atria at the top, that fill with blood and the ventricles at the bottom that receive blood when the atria contract and then pump the blood out through the arteries at high pressure.

Even more clever, there are valves that lie between the atria and ventricles that prevent the blood from flowing backward when the ventricles contract. There are also valves in the arteries that stop the blood from being sucked back into the heart when the ventricles relax. It is the closing of these valves that results in the "lub-dup" sound of the heartbeat.

Taken together with its own intricate blood supply - the coronary vessels (hence the term coronary heart attack - when they get blocked) and its astonishing electical wiring - the heart is a wonder of biological engineering and design. But who or what did the engineering and design? That's a topic for the debaters to get their teeth into!

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