The tiniest of human blood vessels - capillaries - are far more important in tackling heart disease than previously thought, say scientists.

While the transport of blood around the body is principally the job of arteries and veins, to supply individual cells, smaller branches are needed.

There are as many as 40 billion capillaries in the human body - and they are so narrow, that red blood cells have to pass through single file.

We believe that it opens up exciting possibilities for improving the blood supply to the heart muscle Dr Barbara Ward, William Harvey Research Institute

It was previously thought that these tiny tubes did not have the ability to constrict and widen as arteries do to regulate the flow of blood.

However, scientists at Queen Mary's School of Medicine, Bart's and the London Hospitals, and City University, all in London, have shown that even these tiny blood vessels can do this.

New role

This means that they could potentially play a much bigger role in the part of a heart operation where the organ is taken off bypass and normal blood flow has to be reestablished.

This has often proved problematic because "reperfusion" occasionally shuts down capillaries - preventing proper flow of blood to the delicate tissues.

This situation is called "no reflow" and can cause heart cells to die, hampering long-term recovery.

The finding that they have the ability to open up and close again suggests that a drug might be found to force them open when doctors need to reperfuse a heart after surgery.

Dr Barbara Ward, from the William Harvey Research Institute at Barts and the London, said: "This research is at an early stage, but we believe that it opens up exciting possibilities for improving the blood supply to the heart muscle in coronary heart disease."

The research was published in the journal Cardiovascular Research.