Science Questions

Why do babies have a quick heartbeat and elephants a slow one?

Sun, 27th Jan 2008

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Question

George Campbell asked:

My wife and I are expecting our first child and we’ve been using a Doppler machine to listen to the baby’s heartbeat. It’s very fast, at least 150bpm whereas ours is about 70. Why is this? Does an elephant have a very slow heartbeat and does it take longer for the blood to circulate in bigger animals, for example?

Answer

You’re on the right lines there, George. The reason is that if you think of a heart as almost a sphere and the volume of a sphere is given by the mathematical formula 4/3πr3. That’s the volume of blood that a heart could hold. You could work out how much output’s coming out of your heart by multiplying the number of times per minute the heart beats (the heart rate) by what’s called the stroke volume (how much blood the heart squeezes out with every beat). Given that a heart that’s small, instead of it just decreasing its volume by a small amount when you shrink the heart a bit it will actually decrease the volume of blood proportionately by the radius cubed. For every shrinkage of the heart, if the heart gets smaller and smaller – in fact there’s a very big decrease in the amount of volume it can pump. You can compensate for the reduced volume by increasing the rate at which you pump. If you have a smaller heart you have to make it go faster. A mouse’s heart, for instance, can be pumping at two-four hundred times a minute whereas a big whale could have a heartbeat of say 20 or 10 times a minute but it has got a heart which is the same size as a Volkswagen beetle. That’s why.

On a related note, if you’ve got a very big heart it’s going to be almost impossible to empty that very, very quickly. It’ll be impossible to empty a whale’s heart which is several feet across 400 times per second. You just couldn’t shift the blood quick enough.  It's likely that the dynamics of muscle contraction wouldn’t be able to occur fast enough to make the muscle get shorter quickly enough en masse to move that amount of blood at that speed. It is a phenomenal thing to have achieved.

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