I noticed this in a movie today and it made me wonder, what is it with blood appearing in people’s mouths when they’ve been shot or badly hurt in certain movies? Is this real and if so, why does it happen?
Happens a lot in wuxia films, when a character has suffered so much internal trauma that his organs rupture and blood is expelled from the first available orifice. As the mouth is the closest to the chest cavity where death blows usually land, the blood would logically be expelled from there.
I thought it was mostly expository. Hey look, this guy is dying. See the blood?
I imagine a punctured lung could lead to coughing up blood.
A corollary to this question would be, “How come dying people in movies never shit their pants?”
They do on South park 
How do you know they don’t? They rarely show the cops taking off the person’s pants.
As for the smell, the cops are just too polite to mention it.
Venture Bros. did it (first)!
Having blood dribble out of a mouth is more appealing than vomit and defecation.
A friend of mine who was a combat medic in Viet Nam in the early 70s told me that the blood from the corner of the mouth is a hollywood invention. A punctured lung from a gunshot produces blood from the mouth, but it’s voluminous, frothy and pinkish.
That’s what Bob told me anyway.