When you donate blood, the amount is usually just less than one pint of blood. That’s not quite a tenth of your blood volume. The removed blood contains both blood cells and plasma (fluid). You can quickly replace the total volume within 24 hours if you are able to drink some extra fluids (such as electrolyte drinks). It will take closer to a week for you to replace the cells that have been removed.

Most people do not feel symptoms after donating blood. But it is possible for you to have lightheadedness or mild fatigue immediately after. You will be less likely to have lightheadedness if you eat well and drink enough fluids before donating, and replenish your fluids quickly after.

Some people can become so lightheaded they faint. It’s very common. It could be triggered by the sight of blood or their blood being removed or both. Doctors call it a vasovagal reaction.

Typically you get lightheaded, dizzy and sweaty when you first try to sit up after donating. And it may cause a momentary “blackout.” Vasovagal reactions do not require treatment. Just lie flat for a bit. And they’ll get better within several minutes.