ickle.org » capillaries

Capillaries are the tiniest mechanisms for blood delivery in the body. Far far smaller than their larger cousins - the veins and arteries - they're just 5-10 microns in diameter

and thus only allow red blood cells though one at a time. They're also responsible for that beetrooty look post-exercising, as they release excess heat delivered by the warmed blood into the surrounding tissue. So now you know why Santa's face is always ruddy ... it's not the gin, honest.

DP