Why Do People Pass Out While Giving Blood?

Written by april kohl
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Why Do People Pass Out While Giving Blood?
Giving blood lowers blood pressure and can lead to fainting. (blood sample image by Glenn Jenkinson from Fotolia.com)

When giving blood, up to 10 per cent of your body's blood supply is removed. This affects not only your ability to absorb oxygen but also the pressure in your blood vessels. As a consequence, many people will feel the effects of blood loss until the body stabilises, and some people pass out while giving blood if they do not follow the right safety precautions.


Because a considerable part of the body's blood supply has been removed, the pressure in the blood vessels is reduced significantly. This means the heart has to do more work to pump blood to the head and other parts of the nervous system when a person is standing up or attempting anything energetic. The body compensates for the reduced pressure by increasing the fluid content of your blood, but this takes a little time.

Time Frame

The effects of blood loss can take hours to fully overcome. It takes even longer if you follow a vegetarian diet because of the lower iron content of many vegetarian diets. According to the National Blood Service, it is possible for people to still feel faint even after several hours, especially if they don't eat right, drink alcohol or smoke soon after giving blood.


Symptoms of low blood pressure due to giving blood are dizziness and associated nausea, feeling lightheaded, and in some cases, fainting. These symptoms are due to low blood pressure preventing sufficient oxygen from reaching the brain. The symptoms will often come on suddenly and as a result of either standing up quickly, or being too hot. Overheating causes expansion of the blood vessels and therefore further reduces blood pressure.


Prepare for giving blood to avoid passing out. Because the body uses water to temporarily replace the lost blood and needs nutrients from food to create new blood, eat a healthy breakfast and drink plenty of fresh water (not caffeinated drinks) the morning of your donation. After donating, drink juice and have a light, sugary snack to replace your lost blood sugar.


Even if you prepare fully before giving blood, it is still possible for you to pass out. Avoid too much exercise, drinking alcohol, smoking or staying too long in a hot room for at least two hours after giving blood to allow your body time to recover. If you follow a vegetarian diet, remember that it may take you a lot longer to replace the iron content in your blood.

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