Raynaud phenomenon

Raynaud phenomenon is a reaction of the blood vessels to cold or emotional stress: for a while, they narrow. This happens mostly in the digital arteries, which carry blood to the fingers, and in the cutaneous arterioles, the tiniest branches that bring blood to the skin. That narrowing is called vasoconstriction. Sometimes Raynaud's shows up on its own, and it's called primary. Other times it's the result of another disease, and it's called secondary.

Here's what happens inside. Artery walls contain smooth muscle, a type of muscle that works without you having to think about it. When it's cold, the sympathetic nervous system, the part of your nerves that runs on autopilot, releases norepinephrine, a chemical messenger that tells that muscle to tighten. Picture a fist squeezing a garden hose: the hose is still there, but the opening gets narrow. In Raynaud phenomenon, the vessels in the hands and feet clamp down like that very quickly and take a long time to let go.

That's why an attack hits all at once. The fingers get cold and turn white or blue. Then, as the blood comes back, they turn red, like when the fist finally lets go of the hose and the water flows again.