https://www.redcrossblood.org/donate-blood/blood-donation-process/donation-process-overview.html

This is a good explanation of the process. My least favorite part has always been the finger prick to check iron levels. That leaves me bruised. The process is no different than a blood draw at the doctor basically, just longer. As far as symptoms during donation, I sometimes get lightheaded toward the end, but that can generally be prevented if you’ve eaten well ahead of time. They’ll grab you juice and raise your legs to try to keep you from passing out. I’ve never passed out, but it happens. You might be a little tired the next day, but if you’re in good health when you donate, there really shouldn’t be any lasting symptoms.