What is volume expansion after blood donation?

After you donate blood, the amount of blood taken isn't in your bloodstream anymore. Now you have to replace the volumn of the loss by hydration. This will ensure that you are not walking around with 500cc or so of blood loss. Most of the blood in your bloodstream is water anyways, so most of the hydration will replace it pretty quick. If you are a small frame person, small amounts of blood loss could cause you to be "anemic" temporarily, so you may pass out or feel dizzy until your body replaces the volumn thus maintaining the normal blood pressure your body is used to.