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Your heart will pump around about 200,000 tonnes of blood around your body in your lifetime. (Source: Getty Images)

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How does the heart work?

Tuesday 29th March 2016 12:07 pm

Your life depends on the regular beat of your heart. Dr Karl explains how this mighty four-stage pump works.

The heart is the organ of love — but it’s also a mighty pump. In an average 75-year lifespan, it will pump about 200,000 tonnes of blood. That’s the weight of one of those super-giant luxury liners.

So how about this for a bit of a shocker? It might be possible to make your heart stronger by zapping it with carefully timed jolts of electricity.

In the short term, you would mess with the internal rhythm of the heart and make it less efficient — but in the long term, you might be able to make it stronger.

Now we humans go through life with only one heart (and it’s absolutely essential), so it’s not the kind of messing around that you would do lightly.

So let’s examine the heart.

Your heart has four separate chambers — the right atrium and right ventricle, and the left atrium and left ventricle.

There is a detour after the second chamber (the right ventricle). Instead of going directly into the third chamber (the left atrium), the deoxygenated blood is pumped to the lungs where it picks up oxygen — and only then, flows into the third chamber (and then the fourth, the left ventricle). (If you want to get all mechanical, just think of the heart as a four-stage pump.)

These chambers are very carefully timed in how they perform. Each of the four chambers accepts blood coming into it, squeezes in a beautifully synchronised manner, and pushes that blood, at a higher pressure, onwards. (All this, without a second thought from you.)

The four-stage pump

The process starts with deoxygenated blood from the veins flowing, at the low pressure of about 5 mmHg, into the first heart chamber (the right atrium). The right atrium pumps it up to about 10 mmHg, and sends it (with exactly the right timing) to the second chamber, the right ventricle.

The right ventricle pumps the blood up to about 25 mmHg, and sends it off to the lungs. Inside the lungs, there is a massive process of gases being swapped around.

Oxygen leaves the air in the lungs and crosses the very thin alveolar membrane to enter the blood — and that’s how the blood gets oxygenated. At the same time, and at the same place, the carbon dioxide in the blood crosses the alveolar membrane (in the opposite direction) and enters the lungs.

So you breathe in air that contains about 20 per cent oxygen, and you breathe out air that now contains about 16 per cent oxygen and 4 per cent carbon dioxide.

This gas exchange happens with each of those 15 breaths you have every minute. So the lungs are literally awesome. They have about 2,400 kilometres of airways for the air to travel in and out of with each breath, about 400 million alveoli with very thin walls to allow the oxygen and the carbon dioxide to go into and out of the blood, and a total surface area of about 60 square metres to make it easier for the gases to cross over. Sixty square metres is about half a tennis court.

As you might imagine, there is a bit of resistance to the blood flow inside the lungs. So while it enters the blood vessels of the lungs with a pressure of 25 mmHg, it leaves at a much lower pressure — about 10 mmHg. But it is now well and truly oxygenated.

This oxygenated blood now returns to the heart, and flows into the third chamber of the heart — the left atrium. It leaves the left atrium, and is forced into the left ventricle — the fourth and last chamber of the heart. The left ventricle is probably the hardest working chamber of the heart.

About four seconds after the low pressure deoxygenated blood entered the heart at the right atrium, the left ventricle pumps this now-oxygenated blood at the high pressure of about 120 mmHg to the aorta — and then, to your general circulation.

For your heart to run at peak efficiency, the timing of the pumping of the blood at all stages between the first and fourth chambers has to be very precise.

Problems of the heart

There’s a whole range of different problems you can have with the heart. One of them is called heart failure. In heart failure, for a variety of reasons, one or more of the chambers of the heart no longer squeezes as effectively as before. About five million Americans suffer from heart failure.

Now about one quarter of these five million Americans have their heart failure caused by a timing problem. More specifically, the various chambers have lost their exact synchrony with each other. So the blood doesn’t get moved as efficiently.

It was the results of a new treatment for this variety of heart failure that led to an unexpected discovery – and a possible new way to make your heart stronger, by temporarily weakening it. And I’ll talk about that, next time …

 

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This blog first appeared on Dr Karl's Great Moments in Science

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