system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do injured people cough up blood?

It's often seen in film/books/etc. that when someone receives a chest injury, is smashed against a wall or similar, he coughs up blood or has blood coming out from his mouth. Does this actually happen? Why?

If the lungs get damaged it makes sense that blood gets coughed up, but what about injuries that affect other organs?
gpt
They don't unless there's some lung damage. 

But if you show how real injuries look, viewers whose minds haven't been warped by the far edges of the internet get grossed out.  

Even then, making practical, believable injuries for the camera is expensive and takes a ton of time to set and reset. Plus, if you don't get it all done in one day, then the makeup folks have to make sure tomorrow's injury looks JUST like yesterday's.  

Coughing blood just requires the actor to crunch a blood capsule in their mouth, and cleans up with paper towels. Plus, movie culture has already accepted that as a shorthand for "Internal/Critical Injuries".