system
Given the following post, respond as if you're commenting on Reddit:
human
Why do severe injuries often not hurt the instant they happen?

When I touch something unbearably hot, I feel the pain almost instantly. I've heard many people who have been shot often don't realize it. I understand that adrenaline masks pain, but how quickly does it take to kick in? 

Another example, I smashed my face into the ground (unintentionally) and broke my nose some months ago. The initial impact was painless; I obviously could tell I hit a hard surface, but the only sensation I experienced was the pressure of the ground. Was adrenaline released as I was panicking and falling? Did my body not have enough time to process the pain? I've heard it takes a couple of minutes for adrenaline to be in effect, but of course I'm not an expert. 

I'm not sure of this is caused by something else, but I'd love to understand it better.
gpt
Self preservation, an evolutionary survival response. 

A minor injury hurts to act as a warning to avoid further injury. 

A catastrophic injury (say from a predator) and the body shuts off the pain response to give the animal a chance to escape first, worry about the injury later. 

Both have the same goal, self preservation.