Components - Hypothesis

Serious injuries and deaths from accidents in cars have more than halved over the last 20 years. It's not because we've got any better at driving, it's because we've got better at safety engineering. Things like seat belts and air bags.

Whilst we've got really quite good at protecting heads in accidents, we're getting to a stage where protection for other parts of the body are starting to lag behind. In fact, the second most likely cause of sudden death in a car accident is something called an aortic rupture.

The aorta is the largest blood vessel in your body. It comes straight out of your heart and carries oxygenated blood up to your brain and around to the rest of your body. It sits inside the chest cavity, in the middle there, with the aorta arching out of the heart and back and down, right in front of the spinal column.

If you're driving along a road like this, at 60 miles an hour like I am, and the car crashes, it comes to a very sudden stop. And everything in the car has to come to very sudden stop. So if you're wearing a seat belt, that has to put a very large force on your chest to stop your chest moving.

Now, the internal organs, your heart, your lungs, are still moving forward. And they've got to be stopped. My heart sitting in my chest, weighing about 300 grams. When you have a crash, the accelerations are enormous, probably of the order of about 70G. What that means is the forces your heart experiences are 70 times normal. So something more like the weight of 21 kilograms.

That can do some damage.

But there's something really odd about aortic ruptures that might give us a clue as to why they happen in the first place.

Whether you're in a car accident, on a motorcycle, or even falling out of a high building, the aorta ruptures in the same place. If you do have an aortic rupture, with the heart beating and pumping blood out at that sort of rate, it's enough to empty your entire blood volume out in about a minute. And that's the reason why most aortic ruptures are fatal.

The place it ruptures is just a bit where the arch turns the corner to a place called the isthmus, there. And this is really a very interesting place for that to happen. It suggests a hypothesis that the reason it bursts there is because there's a weak point there.

That corresponds to a feature of the circulatory system in foetuses, where there are a couple of short circuits between the circulation to the lungs and the circulation to the body. Foetuses don't need to use their lungs, because they get the oxygen from the placenta. So these short circuits divert the blood back to the body.

One of them is a short tube that joins the aorta on to the artery that comes out of the heart to the lungs, closes up at the moment of birth, but leaves a small scar, just at the point where we find aortic ruptures occur. That's why we have the hypothesis that the weak point in the aorta is located there.

Because the people who die from aortic ruptures tend to be young and healthy, we usually use young and healthy aortas from the butcher. But that's a bit messy for today. We'll just use a balloon.

To test whether the aorta ruptures at a weak point, we took samples of aorta from that place, and from other places, to see if it really was weaker. Now to simulate accident conditions, we have to do that at high strain rates. And that's where the experiment starts to get fun.

This is how we test a section of aorta that's clamped across a hole there. We can pop it like a bubble with compressed air that we let in with this valve. And we can measure the pressure and watch how it deforms, so we can quantify its mechanical properties.

So that's all that's left of my bit of balloon. When we did the experiment with the aortas, we found that the rupture pressure at the point where we thought the weak point was was just the same as the rupture pressure higher or lower down the aorta.

So we can't confirm that hypothesis. Back to the drawing board.

Best you can do at the moment is just to wear a seat belt.