Feeling Faint - Information for donors
It is important to be aware that some donors feel faint after giving blood. During blood donation 8-10% of the blood volume is removed. Most people adapt quickly to this loss and their system is not upset by it, but a few individuals adapt less well and consequently feel faint. We hope this will help explain why some donors feel faint after giving blood.
The effects of blood donation are aggravated by various factors.
People who weigh less than 7st 121bs (50kg) are more likely to faint if they give blood because they lose relatively more of their blood volume than larger people. Therefore we advise them not to donate, in the interests of their own health and well-being.
- People who smoke soon after giving blood are more likely to feel faint because of the effects of nicotine.
- People who take alcohol within a few hours of giving blood are more likely to feel faint because alcohol dilates the blood vessels. This causes less blood to be available to circulate to the brain leading to dizziness and fainting.
- Being in a hot room also causes the blood vessels to dilate and thus has a similar effect to alcohol.
- Rushing about, or vigorous exercise can also produce similar effects.
- Missing meals and not replacing fluids means a delay in the recovery from blood donation.
- Standing still for long periods of time can lead to pooling of blood in the legs, a situation similar to soldiers on parade. This reduces the amount of blood available to the brain.
If you rush about, miss a meal, have a 'liquid lunch', a cigarette, or get overheated you may feel faint even if it is several hours since you gave blood.
If you do feel faint it is important to:
- tell someone, either a friend or a member of our staff if you are at a session.
- lie down, or bend forward with your head between your knees until the feeling passes. Make sure that when you get up you do so slowly. If you still feel faint, lie down again.
Feeling faint can happen to anyone. However, fainting can often be avoided by following our advice.
The main considerations are:
Follow the advice about resting after your donation
- Take plenty of fluid
- Do not smoke or drink alcohol within 2 hours of giving blood
- Avoid rushing around, prolonged standing, hot environments, or vigorous exercise
- With these general measures donors can minimise the chances of fainting.
If any problems persist you can obtain advice by ringing our donor helpline on 0300 123 23 23.
If you need any more information or advice after reading this, ring our donor helpline on 0300 123 23 23. Our donor helpline is open for general enquiries 24 hours a day, every day of the year. If you have a non-urgent medical enquiry, please try to contact us between 8.00am and 5.00pm Monday to Friday.
Feeling faint Welsh (PDF, 520K)
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