Disclaimer: I am neither a doctor nor a professional theologian, and all information is from Wikipedia. I’m not even Catholic.
Let’s assume you’re a distance runner or some other kind of endurance athlete, and also the Pope. Since low oxygen is the limiting factor on how long you can keep running—even if you’re Pope—increasing the amount of oxygen in your blood when it gets low will increase your endurance.
Step zero: Become Pope.
Several people have accomplished this throughout history. It is therefore doable.
Step one: Water to wine.
As Pope, you get to abuse your power. One of the better-known possible miracles is turning water into wine. Nowhere in Catholic theology does it say that the Pope can do miracles at will, but come on. Just look at that hat.
As it happens, slightly under half your blood usefully carries oxygen. The rest is water. That’s what you’re going to turn into wine.
Warning: This will kill you. You’ll wind up with a BAC of five. Not 0.05. 5. It’ll depend on the wine, but we’re somewhere in the territory of “legally intoxicated, and multiply that by sixty.” Some lucky people have been known to survive a BAC of 1, but you’re way past that. So you’d better do the next step quickly.
Step two: Transubstantiate.
To do this, it is necessary and sufficient to say “this is my body, this is my blood.” The alcohol poisoning would kill you, of course, but it probably won’t kill you instantly. So you can squeak out a couple words and the wine goes away. This is where the good part happens.
The blood of Jesus is often described as “scarlet” or “crimson.” Those are both bright red colors, and bright red means highly oxygenated. Neither of those is applied to the relevant blood by the Bible, but scarlet is the official color according to the Catholic Church. So if it’s the Pope doing it, then by golly the blood is going to be oxygenated.
According to the best source in the world, if you run until you’re exhausted your blood may contain as little as 15% of its maximum oxygen. You just transubstantiated a bunch of fully oxygenated arterial blood already in your body. Adding more arterial blood won’t change the saturation of oxygen in your arteries much (that stays above 95% anyway), but it will in your veins. So, for purposes of numbers, all oxygen percentages from here on out refer to its saturation in venous blood. (To clarify, the percentage is relative to how much oxygen the blood can hold. It’s not saying that scarlet blood is literally made of 99% oxygen.)
The blood in your veins isn’t normally that oxygenated. Having just come from dropping the oxygen off where it needs to be, it’s typically down to 75% at rest. In your case, half your venous blood is even lower than that and the other half, which normally contains none because it’s water, is instead at 98-99% because it’s the scarlet blood of Christ.
Your venous blood is at 55% oxygen saturation, which isn’t great but it’s far better than before you did the miracle. Unfortunately, that’s still dangerously low over the long term. Luckily, there are still some tricks up your sleeve.
What you’re doing is called blood doping and is banned from most sporting events. Of course, usually people do this by giving themselves transfusions of their own blood so they can hold more oxygen on game day. Simply creating the blood has not yet been banned. Technically.
Doing this has some obvious problems. Your blood is half water for a reason, and if you’re turning that half into regular blood, and the quarter that’s still watery into blood, and the remaining eighth and so on, then you will 1) eventually run out, and not have any water left in your bloodstream to turn into oxygen-bearing blood, and 2) die. It’ll get thick and unwatery enough that your heart won’t be able to pump it. The trick we need is a miracle to turn blood back into water.
Surprisingly, this is never explicitly done by any Biblical prophet or, as far as I know, Catholic saint. But we can sort of infer that Moses did it: In Exodus 4, God gives him the at-will abilities (for purposes of proving he’s a prophet) to turn his staff into a snake and back, his hand leprous and back, and water into blood. There’s no “and back” stated, but, you know, parallelism. Therefore you the Pope, as Keeper of God’s Authority on Earth and Wearer of the Really Cool Hat, ought to be able to turn blood into water.
(You may ask why, if Moses could do the transformation both ways, we’re bothering with the wine step at all. The answer is that he had to pour it out to turn it to blood and you’re not in a position to pour out the water you want to transmogrify. Also I didn’t think of him in time. MOVING ON.)
Step three: Blood to water.
Use this on two thirds of the blood in your body. After you’ve done the transubstantiation thing once, your blood is ¾ water where it’s supposed to be ½. This just resets it to normal thickness, but it doesn’t change the fact that you tripled the concentration of useful oxygen in your veins. You have now completed a cycle.
Regular blood: 50% water, 75% of venous oxygen capacity.
After running as long as you can: 50% water, ≥15% oxygen.
Water to wine: 50% wine, 15% oxygen.
Wine to blood (remember, blood is half water and half useful stuff): 25% water, 55% oxygen.
Blood to water: 50% water, 55% oxygen.
The other problem is that this amount of oxygen, while high enough that you’re not about to collapse, is low enough that over the long term you’d need medical intervention.
So you do another cycle: 50% wine, 55% oxygen saturation.
Wine to blood: 25% water, .5*55+.5*98.5=76.75% oxygen.
Two thirds of the blood to water: 50% water, about 75% oxygen.
That means if you run until a marathon runner would drop and then you do two cycles of this, you’ve got as much oxygen going to your muscles and organs and things as you would while sitting in an armchair. And there’s no reason you have to wait that long. You’ll get less extra oxygen per cycle if you do it more often, but you don’t exactly have a limit on how many times you can do it.
Let’s go to the extreme. If you do this continuously, you can replace breathing.
A given blood cell, if it’s going all the way down to your toes, can take 20 seconds to leave the heart and lungs and come back. Let’s give you ten, since most blood doesn’t have that far to go. If you completely replenish the oxygen every ten seconds, it will be every bit as effective as the usual situation with lungs and everything.
The only one of these that takes actual time is the transubstantiation step. You need to say “this is my body” and “this is my blood,” but the Church has placed no limit on how long you need to take to do that. Or in what language: If you’re underwater and don’t want to have to exhale, you could use any of various sign languages.
That would allow you to stay alive and active, without breathing, as long as you want to. Of course, there has to be a catch. The catch is blood type. You’re turning the wine in your bloodstream into blood, but it’s Jesus’ blood. According to the Catholic Church, Jesus’ blood type was AB. (Seriously! They have an opinion!) So if you have a different type, this will kill you.
Of course, you’d need the ability to perform miracles in the first place. And if you’re Pope, you’re probably surrounded by devout and vigilant people who would frown on the idea of using it to give yourself superpowers. You’d have to get away from the spotlight before going out to fight crime as Aquaman, and has anyone seen Benedict?