BLOOD vessels are just tubes, right? And blood just washes through them like a river, right? Even though that's how it looked in the film Fantastic Voyage (a fanciful tale of miniaturised medics doing brain surgery from close up), the answer is - wrong, and wrong. Actually, the arteries have a helical twist, like the rifling on a gun barrel, and the blood pulsing through them swirls as it flows, according to new research by a team of medical scientists working with aeronautical engineers at Imperial College, London.

Why would nature do that? Because it might help stop heart attacks, caused by blockage of the blood supply to regions of the heart.

We already know that blood vessels can become blocked, or furred up, with fatty deposits on their walls. Traditionally, medics thought that blood flows like a river, with stagnant regions where "silting" could lead to blood clots and blocked arteries.

However, nature is rather cleverer than that. Professor Colin Caro, describing the research last week, said: "A helical pipe is a much better model for blood in arteries than a two-dimensional structure. Like so many natural systems, blood flow turns out to be non-linear, and is very sensitive to arterial geometry."

The Imperial College scientists used body scanner equipment, and computational fluid dynamics techniques originally developed for aircraft design, to construct a new three-dimensional model of blood flow. They realised that swirling blood flow may help to prevent arteries furring up - since the blood velocity along the walls is greater for a given overall blood flow than a "river" form.

Knowing this could help heart patients in the future. At the moment, when heart surgeons graft a new vessel to replace a blocked one, they don't take any note of planar or non-planar geometry - they just stitch it in, performing up to 400,000 bypass grafts each year in the UK. But a recurring problem is one called "intimal hyperplasia", where the vessel's inner surface thickens and becomes blocked again - which happens particularly in regions where the blood flow is stagnant.

"Surgeons have not systematically made non-planar grafts previously," said Professor Caro. "We are trying to see whether using a non-planar geometry might improve the long-term results," says Professor Caro. "Nature seems to go to so much trouble to make our arteries non-planar and avoid stagnant areas that there must be a good reason for it."