If you are considering giving blood, you should ask your Rav. There are those who hold you shouldn’t give blood in a general blood drive, but only give when it is designated for someone specific. The feeling is that if you give blood, and then a few weeks later someone in the community needs blood, you won’t be able to donate.
That being said, I have given blood in the past and I wasn’t scared or nervous. I don’t remember why I decided to give. And I’ve done it more than once. I don’t think I’m able to give currently because of medication I’m on. But if someone needed blood, I would certainly find out.
I know I can’t give platelets. That require better veins than I have. I tried once and they couldn’t get them from me.
I have no idea how I’d feel about giving a kidney.