When you hit your head why do you get a bump?

Answer 1

AnswerWhen you bump your head you cause damage to various cells that make up your tissues. Consequently, the cells release prostoglandins, and other molecules including vasodilators and constrictors, that activate an immune response. Specifically they modulate a non-specific inflammatory response, which in a nutshell causes increased blood flow to the damaged area (simplified version). The whole purpose of the increased blood flow is to allow the transport of critical immune molecules (neutrophils, monocytes, lymphocytes) to the site of damage so that they may begin the repair process.