People shot in the back or chest (in the movies)...

…inevitably have blood dripping from a corner of their mouth, before they fully realise they’ve been shot.

Is this realistic / where does the blood come from?

Well, I guess theoretically if you got shot in the lungs you could cough up a bit of blood, or if your esophagus were punctured that could be another route for the blood to get there. It’s never seemed very realistic to me though. It’s one of those movie/TV drama things. There’s probably a TV tropes page for it somewhere.

Never mind. Answered the wrong aspect of the question.

I always assumed the torso was 90% saliva glands.

I just figured it meant a bullet penetrated a lung or two.

You can be shot and not realize how significant the event is for a few moments…or longer.

With chest cavity penetration, intrapulmonary bleeding might show up as bloody sputum, although probably not before someone even realizes they have been shot.

A projectile which penetrates the esophagus is much more likely to be a serious trauma because of the other adjacent structures. This would not present as blood dripping from the corner of one’s mouth.

Perhaps biting your tongue or lip from anxiety or surprise might cause a little dribbling of blood from the corner of the mouth in such a situation.

From the prop department.

It’s not just a made-up thing for TV.

Below is a link to extremely graphic footage of the death of Neda Agha-Soltan who was shot in the chest in 2009 during the protests in Iran. The wound to her chest caused copious bleeding from the mouth and nose a few seconds after she hit the ground.

Seriously. You really don’t want to watch this.WARNING – Very Disturbing Video of A Real Person Dying – WARNING

I just happened to see a show last night about the attempted assassination of Ronald Reagan. As you probably recall, he had a nearly fatal shot to the chest, and did not know it. His immediate handlers and the first few doctors who saw him didn’t realize it either. It turned out to be a bullet in the lung. He did not bleed from the mouth.

Lung injury can cause a variable amount of blood to come out the airway, usually as ‘coughing up blood’. Injury to the mediastinum (the bit in the middle where the lungs aren’t but the heart is) can hit the heart, the aorta, the pulmonary arteries, but these aren’t going to communicate with the mouth unless there is also an injury and entry point to the mouth, esophagus, stomach or bowel, or, more horribly yet, a communication with the airway. Obviously bullets or blades can do any of these things.

In general, it’s just a way to show someone is gravely injured without having them look ooky or the area strewn with unsavory internal organs. It’s so common in kung-fu movies I once started to write a paper on it, calculating the predictive value for fatal injury.