Has it happened to you yet? You get started a brutal workout with light weights and easy movements - so easy, it doesn't make sense to stop and rest because your muscles don't quite fatigue. If you're mentally strong enough to pick the bar up and keep going, the only other limiting factor are the lungs you've neglected to train for the past entire lifetime.
So you keep pushing. Your lungs are killing you. You want to stop and breathe. But, your mental game is strong today. Something is motivating you to get back on the bar. Is your bro there? Is he back on the bar again? Or are you crushing him and want to create a lead so large that he will never be able to see what once resembled gains under the shadow you're casting over him like the Mountain. So you rush back to the bar to increase your lead. Or catch up.
Personally I can get defeated pretty easily if I fall back. It's gaining a lead that makes me more motivated to keep going. Ashamed to say this, but it's not very Rocky Balboaish. He was a man that took beating and asks for more. He took a beating from a roided up Russian then spat back, "ain't so bad" as he takes haymakers to the chin and keeps moving forward. I strive to be more like Rocky. Ain't so bad.
And you pick the bar up. Lungs burning, everyone around you can vividly hear you gasping for oxygen. But it ain't to bad. Just finish this set, no matter what, then you can collapse on the floor, assume the fetal position, kick off your shoes because somehow that will improve our oxygen uptake and make the intense pain relinquish its control over our central nervous system.
We did well. Beat out PR. Kept up with that person. Had the best time. Pushed ourselves to the limit. Found out what we were really capable of... Whatever you wanted to do, you did. And now you're on your feet, urging others to finish their workout. You know what they are going through and you do your best to help them get through it. Then you cough.
"Almost there!" you cheer. Coughing again you notice a taste of blood. Holy shit did i just rip my lungs open? bad ass, you think to yourself. In that moment you may feel how I feel, accomplished, proud & slightly nervous that something serious might be going on, but screw it - no pain no gain. So you go on to let everyone know how badass you are. Like a gladiator in the arena that just won your life yet you're still a slave so you don't brag too much. It's more like a humble brag. "Is it normal to taste blood after dominating this workout?" you say trying not to sound like a tool. You don't really care what the answer is, you just want them to know you taste blood - aka - victory.
What is happening though? Why are you tasting blood? Some people may also get a metallic taste - it's the same thing. Blood contains iron. Iron is in weights. You eat weights for breakfast. And you just had breakfast. Except our weights are rubber so this analogy doesn't really work, does it?
TLDR; you're tasting blood because fluids containing red blood cells leak into lung air sacs.
For the bros: be warned, it gets sciency for a bit.
This is referred to as Exercise Induced Pulmonary Edema (EIPE). It's relatively common in box gyms (Group Fitness Training Programs), marathon runners, triathletes, cyclists, endurance athletes and fighters.
There is a bit of permeability in the "blood - gas barrier" through which "gasses and blood are exchanged through the alveolar air & the blood in the pulmonary capillaries... High levels of stress compromise this barrier which allow fluids containing red blood cells to leak into lung air sacs". You can read a bit more about it on Stack.
If you really want to hear yourself sound smart, just memorize this quote and blurt this out to anyone that ever tried the humble brag on you.
"Elevated pulmonary artery and left atrial pressures, coupled with a decreased intrathoracic pressure during inspiration, translate to increased capillary transmural pressures and the exudation of fluid from the capillary to the airspace," wrote the authors of a 2011 study entitled "The Curious Question of Exercise-Induced Pulmonary Edema."
They will not only taste blood but see it as well because you just blew their freaking mind.
It's not a trophy to taste blood and it might mean you pushed yourself to the limit. This shouldn't happen often and is probably more common for those people that can't leave their egos at the door. This won't kill you or leave any negative long term effect (that i know about) but this shouldn't be your goal. When you're healthy, in shape and consistently been to the gym, you shouldn't get this experience. But if you take 2 weeks off, eat garbage and try to jump back in right where you left off - you might be tasting blood your next workout.