The people who donate blood donate a much smaller amount than the people who lose it and need a transfusion. Also, the people who donate blood need to be in perfect health and they can't take meds. The blood needs to be 'pure'. You get asked a lot of questions before you donate blood and if you're eligible, then they give you a physical and test your haemoglobin. That's basically the iron. If it's too low, or if your blood pressure is too low, they won't let you donate blood because you can pass out and it's not healthy. Also, after some surgeries and accidents resulting in bloodloss, you don't need a transfusion. It really depends on the amount.