I posted last year about the effect of giving blood on your fitness. Basically, they take about 13% of your blood volume, which has a corresponding effect on your aerobic capacity, as you lose red blood cells. Plasma, white cells and platelets replenish themselves in a day or two, but red cells take weeks.
After I gave blood last year, I had a letter from the blood people asking if I would consider switching to donating platelets instead of whole blood (my blood group is AB-), so the next time I donated (in November) they took a platelet sample too, to check that I had sufficient platelets to make donation viable.
It was, so yesterday I went to a transfusion centre and made my first donation. They hook you up to a machine that extracts the platelets from the donor's blood (it draws out a quantity of blood, centrifuges it and collects the platelets, then returns the blood and goes through further cycles). The whole process takes about 80-90 minutes. There's very little blood lost through the process, and no loss of the other blood components, so no effect on fitness! You can donate up to 24 times a year, so I'm going to do it again in two weeks.
It's a very valuable form of donation - mainly helping cancer patients and unborn babies. I feel quite privileged to be one of the just 12-14,000 donors in the country.