Medical people - Why do you have to be over 50kg to give blood?

I've just checked the blood donor site, went through the entire questionnaire which said I am fine to give blood, but then said you need to weigh over 50kg.





What MEDICAL reason would that be?

Medical people - Why do you have to be over 50kg to give blood?
The reason for this rule is that no donor should lose more than 13% of their blood volume when donating. This in order to avoid side effects such as fainting, or becoming anaemic. 470ml of blood is taken at one session and, as a very rough guide, approximately 8% of body weight is blood, ie., 50kg weight will have approximately 4000 mls of blood, so 470ml would equal 11.75% of volume. This can only be an estimate, so they have to err on the side of caution and allow for a margin of error.
Reply:Those under 110 pounds may not tolerate a donation as well as those over that weight. The limit is for the donor's safety. Donors should not drink alcohol for a couple days because less blood increases the concentration and effect of alcohol (and there may be other similar effects if blood is too thin). Likely carefully supervised self-donations would be permitted before surgery regardless of weight (because they could take less blood).
Reply:i also went to give blood once, i was told then that your body will take about a week to replace what is taken out, i didnt think it was going to take that long my self, but i suppose if you are light it could take longer, or you might not have enough in you to fill the donnor bag and if you did then not enough left for you, im guessing but it would be something like that.
Reply:The more petite you are, the longer it takes to regenerate blood. And if you are small or underweight you are possibly more vulnerable to lowered immunity yourself.
Reply:There is a certain amount of blood they take from everyone which is just over a pint, therefore if you are smaller (and have less blodd effectively) then they are taking a higher percentage of your bodys blood which is more risky.
Reply:It's a bit arbitrary, granted, but the standard amount of blood withdrawn is fixed, and has to be for practical reasons. Those under 50 kg are trying to donate too large a percentage of their total blood volume.
Reply:also, the less you weigh, the less blood you have inside you, but when you give blood they need you to give certain minimum amount, otherwise it wouldn't be worth it. But it's for your own protection so you don't faint or feel too weak afterwards.