Reynaud's phenomenon is a rare condition that affects the circulatory system. A lot of our color comes from the blood flowing under the skin. Messing around with that can make dramatic changes - like half a hand turning one color and the other half turning another.
You know that you're going to get something dramatic when a medical issue is called a "phenomenon," but perhaps you don't expect patchwork hands. The mechanics behind the condition are fairly simple. They were understood all the way back in the middle of the 1800s, when Maurice Raynaud wrote about them. Raynaud was, by all accounts, one of those people who was good at everything they do. He was a lecturer, a physician, and a writer, and it didn't tax him too much to realize what was happening when someone came in complaining that every time it got cold, their hands turned colors along a definite line.
In Reynaud's phenomenon, certain circulatory passage constrict at a certain point. usually this is just around the tips of the fingers or toes. At other times, especially if it's not natural but induced by a primary condition, it can cut seemingly random lines across the extremities. The attacks are brought on by cold, exertion, or even extreme emotion. They usually fade, but they can cause tissue damage if they are prolonged. This is not just a cosmetic condition, causing pain either at the site of the accumulated blood or in areas that have been starved of blood and oxygen. For most people, though, it's a minor problem, and means that they have to be careful in very cold weather. Those who live under conditions that cause long spates of the phenomenon, enough to be dangerous, doctors usually prescribe vasodilators, which open up the blood vessels and let the blood through again.
Top Image: Jan Huňát