A college I worked at had a blood drive every year. It was interesting to see the big boys go down from fainting every year. Had several Dr's and nurses tell me there is no phisilogical reason to pass out.
One time at the end of the day I just gave my unit and we were sitting at the table eating the last cookies and drinking juice and a worker had just finished giving and was walking over to the table. The puncture in his arm gave way and blood started squirting like crazy. It looked like some Halloween trick, a real fountain. Needless to say, that caused several people sitting around to get light headed and get sick.
I felt like a blood giving machine. Vains stuck right out, nurses loved them. A few taps and they were ready to go. One time I thought I could pump out a unit or blood in short time by squeezing the ball harder. Big mistake. Nurse looked at my arm and said "what did you do?" It swelled up big time, black and blue and took days to go down.
Another time a buddy wanted to give blood for the first time. I was experienced by this time. He was scared spitless that he would pass out. I was teasing him about it and all of a sudden I got very light headed and the next thing I know we are both laying down with wet cloths on our heads.
Of course students would often call in sick the next dayday af giving blood for the first time. Ah, the good ole days.