system
Given the following post, respond as if you're commenting on Reddit:
human
Would any doctor really accept a blood donation from somebody who had just run 26.2 + 2 miles?

I heard this talked about on the news. It seems to me that while heroic, it also sounds like a downright TERRIBLE decision to go give up a liter of your blood after running in a marathon.  

The blood is important for circulating oxygen, and I have to imagine that a runner's body must be very oxygen deprived after such extended physical exertion. If they have less blood, won't they be even more susceptible to fainting than marathon runners already are after the race? 

Furthermore, wouldn't a hospital have enough blood on hand to need donations there at the time?  Donated blood must be tested for diseases and whatnot before it is transfused, so even if the hospital did not have enough blood on reserve, it would not be able to use the blood from the runners but instead blood probably brought in a helicopter from another hospital in Boston.
gpt
I was wondering the same thing.  Probably not a good idea, but the reasons are varied.

You would have to consider first the risk to the marathoner (donor), and then the person receiving their blood.  

The risk to the marathoner would be great.  Throughout the course of the race, their bodies have accrued an oxygen debt that needs to be "repaid" as they recover.  While their muscles have tried to continue aerobic respiration to the greatest extent possible, there is likely to be a large amount of circulating lactic acid from anaerobic respiration. That lactic acid will be combined with oxygen in the liver to reform glucose.

While there is a very effective buffer system in place to maintain blood pH within a range, decreased pH (increased acidity, increased conc. of protons) from the lactic acid, as well as increased body temperature and partial pressure of CO2, will slightly lower hemoglobin's affinity for oxygen, making it a little harder to repay the oxygen debt.  The runners are also likely dehydrated as well as deficient in plasma electrolytes (hypokalemic, etc.).  Essentially, their bodies are working hard to re-equilibrate their chemical balances, and losing blood volume would make that more difficult by decreasing their capacity to exchange gases as well as redistribute various compounds to the tissues where they are needed.

That being said, on any old NON-race day, a marathoners would be able to handle the loss of a pint of blood much better than less athletic people, as they likely have a higher hematocrit and cardiovascular systems that are more efficient. 

As for the safety of the recipient, they would probably be unaffected because usually donated blood is fractionated into erythrocytes, platelets, and plasma.  It is probable that lactic acid and other wastes would be removed and the pH adjusted. Also, during a MCE (mass casualty event), hospitals gather together their supplies of O- blood (universal donor) that can be pretty safely given to anyone.  All the marathoner would be doing is replenishing the supplies they used that they already had on hand.  It's unlikely that WHOLE blood donated right then and there would be used for an emergent case.  If that somehow were the case, post-marathoner blood would be a BAD idea because acidosis is part of the "trauma triad of death" that leads to death in trauma patients.  The lactic acid in the donated whole blood would exacerbate the acidosis and probably decrease their chance of survival.