Medically speaking, why should you try not to speak after reciving a potentially!

Medically speaking, why should you try not to speak after reciving a potentially fatal wound?

I know in movies, whenever somebody is doomed to die, the Hero/Protaganist/Whoever always says something along the lines of "Try not to talk." when the said victim is trying to reveal something to them. Obviously, it makes you die faster, but why?

I know that the red blood cells carry oxygen, but I'm not sure if that has any relevance.

You're on the right track. The simple act of talking requires the use of many muscles in relation to the lungs, larynx and jaw. All these movements subsequently require more blood to be pumped harder which is usually the last thing that needs to be done when blood or oxygen is oozing out where it isn't supposed to be oozing.