Why does skin go red when you slap it?
Skin goes red during a vasodilation, when there is increased blood flow to that region. It’s similar to when a person blushes, although the initiating events (a slap versus an emotional trigger) are different, of course.
The mechanism goes something like this: the skin is slapped, and there is an immediate inflammatory response, whereby at least two things happen. First, veins dilate, leading to a recruitment of more arterial blood (this is why the skin goes red). Second, white blood cells within the blood are directed via a process known as chemotaxis to the site of inflammation. Eventually, unless there are additional signals (such as would be the case if the skin was torn and a scar needed to form), regulatory factors kick in and the blood supply returns to baseline.