ok ive actually been wanting to ask this for a while; can getting stabbed in the stomach/abdomen really cause blood to come out of the mouth or is that just a dumb trope for The Drama?
Yes, but also, yes.
There are a lot of injuries that can result in blood in your respiratory system. The most obvious would be puncturing a lung, or bruising a lung (called a pulmonary contusion.) Also, bleeding from the sinuses that gets into the throat can result in coughing up blood (though this one is pretty benign.) As a general rule, you do not want fluid in your lungs at all, and fluid that is prone to coagulating, blocking your lungs' ability to absorb oxygen is a very bad thing.
And it gets worse, for your body to effectively expel a large glob of blood, the way you usually see in films, you would need a lot of blood in your lungs. On their own, even relatively minor pulmonary contusions have a pretty significant mortality rate. What this means is, if you're ever coughing up blood, there's about a 30% chance you're going to die.
Also as a quick aside, particularly violent coughing can result in some minor spotting from broken blood vessels. It's not inherently life threatening, but it is something that should be checked out quickly, because it can be a symptom of something worse. (Such as lung cancer.)
The way these injuries tend to get inflicted is a mixed bag. When someone's been getting beaten (particularly in the upper torso), they've been in an explosion, or a car accident, then it's not that unreasonable for them to be coughing up blood. A pulmonary contusion is a pretty plausible injury in those circumstances. And again, that pulmonary contusion could easily end up being the fatal injury.
When a character has been shot or stabbed in the stomach, then it's less plausible. There's a bit of a distinction here, because if you have serious hemorrhaging from internal injuries and that gets into your stomach, it's quite possible you'd vomit blood. You don't see this very often in film, coughing is far more popular, and in a lot of cases, you'll see characters coughing up blood when they should be vomiting. As with pulmonary contusions, this is not a symptom to screw around with, if someone's vomiting blood, they're at very serious risk of dying, and need immediate medical care.
So, is it just used for drama? Yeah. It is. Stage coughing is pretty easy for an actor, and a little bit of slight of hand with a blood pack can let them get a nice death scene in. It's not about the realism of the injury, and in most cases, aside from coughing up blood, they're not exhibiting any of the other associated symptoms (including the ones that are more common.) When someone's coughing up blood like this, they're also suffocating, because their lung can't absorb oxygen out of the air. As a result, they'll frequently start hyperventilating. Which, you'll never see in one of these scenes. The actor will lay there, deliver their lines, feign pain, and then spit up a red dye pack and relax. Which, that's not how this injury works at all.
So, yes, it is a real injury, but also, yes, this is something that is used for dramatic effect and not out of any interest in realism.
-Starke
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