I used to pick up extra money selling/donating my plasma a couple times a week. The only anxiety I felt was because I have small veins, so I would request a few of the drawers (phlebotomists?) that were really good.
Then the economy turned to crap, and the wait time from in the door to out the door was stupidly long and I quit.
But giving blood and selling plasma are very different and perhaps there is some primal anxiety to "losing" whole blood.