Let the blood run free

By Michael Day GIVING grafted blood vessels corkscrew-like turns might add years to the lives of heart bypass patients, concludes a study presented at last week’s heart disease meeting at the Royal Academy of Engineering in London. Vessels with twists are better because they encourage swirling blood flow which evenly scours the insides of arteries, reducing the build-up of plaque that can clog them, says Colin Caro, a biophysicist at Imperial College, London. Caro got his inspiration from studying the natural shapes of blood vessels in humans and animals. Scientists used to think that vessels were essentially the biological equivalent of a cylindrical water pipe, with blood sweeping straight through like river water. But Caro found that the vessels, rather than being perfect cylinders, were actually slightly helical. To understand the ramifications of the helical shape, his team developed a new imaging system combining computers, magnetic resonance imagers and software developed to study airflow around aircraft. They used this system to study blood flow in three dimensions, through both twisting and straight vessels. They found that gentle corkscrewing encourages blood to move at an even velocity throughout a vessel. In contrast, straight vessels develop areas with fast and sluggish flow, says Caro’s collaborator, Spencer Sherwin, a fluid dynamics engineer at Imperial College. The differences can be dramatic. For example, the researchers found that when vessels meet at a T-junction, the shearing force of the blood—which increases with the unevenness of flow—can be three and half times higher if the vessel before the junction is completely linear, rather than helical. “We’re hopeful this could turn out to be very important in bypass surgery,” Caro says. Vessels inserted to bypass a diseased artery often thicken and become clogged, requiring the risky surgery to be repeated within ten years. Other research has shown that a smooth blood flow also encourages the production of protective substances” (New Scientist, Science, 5 October 1996,
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