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Good on you for donating blood, though! I used to be the chairperson of the blood drives at my old workplace and it's great when folks donate. You are saving lives!
For future reference, I've had good luck working out a few hours before donating blood and was also able to work out the next day.
After a period of not donating due to some health issues, I returned to being a regular blood donor last year (age 66) as well as being a regular (almost every day exerciser). I also got a heart rate monitor (best thing I did). When donating blood, I usually took one day off and did not notice any problems. Yesterday, I did not exercise and donated "double red" (the first time I did that). They take double the red cells of a pint of blood and return the plasma and platelets to you. It only takes 15-20 minutes more and you need to wait 16 weeks (double the normal 8 weeks) to donate again.
I did weight and aerobic exercise this morning (rowing machine and versaclimber) and did notice a significantly higher heart rate, so I eased back just a bit on intensity, but did the normal times. I guess that makes sense, since the "double red" donation would significantly reduce the ability of the blood to carry oxygen, and therefore need higher heart rates to carry blood to the body.
No workout after giving blood. Too risky. You could very easily pass out and get injured.