About 3 % of the people will faint after donating blood for analysis or eye inspection. In case you suffer from this, you could try to do something to prevent it, but it is all in vain. You could try going to toilet in order to avoid being seen by anyone, but you could faint on the way and get hurt.
The cause of fainting is the so-called vasovagal reaction, involving the vague nerve (vagus in Latin means wandering). It is believed to be an impairment related to the regulation of the blood flow, as when you change position from being seated to standing up (the postural fainting).
Vasovagal fainting can be triggered by scary, embarrassing or uneasy situations or during blood drawing, coughing or urinating. The fainting can also be caused by heart related conditions (cardiac fainting) and neurological conditions (neurological fainting). Fainting can also be caused by low blood glucose levels and lung conditions like emphysema and a pulmonary embolus.
In some situations, when you see blood or a medical tool is put closer to your eyes, your vegetative nervous system behaves like you were laid in bed, while you were seated or standing. At the beginning, the heart beats fast because of the anxiety, then the heart rhythm drops suddenly and the blood vessels from the feet dilate. That's why the blood quantity increases in the feet, but lowers at the head level. The brain does not get enough oxygen and you faint. How can this be avoided?
When blood is taken from you, you could look somewhere else or lay on a bed. The beginning of a vasovagal reaction is preceded by some warning signals, when you can act so that you won't faint. Many physicians suggest laying on the bed and leaning your feet on a chair or a wall. This way you prevent the blood from reaching your feet and the reaction could be stopped. In a few minutes you will probably feel better.