Syncope [pronounced sin-co-pee]: a sudden brief loss of consciousness In the case of fainting at the sight of blood, syncope results from non-cardiovascular causes, particularly due to problems with the autonomic nervous system, which is responsible for many of our bodily functions, including regulating blood pressure, heart rate, and our responses to anxiety, fear and emotional stress. Vasovagal syncope, typically precipitated by unpleasant physical or emotional stimuli (eg, pain, fright, sight of blood), usually occurs when a person is standing upright. The feeling of fear or anxiety at the sight of blood is the cause of the onset of fainting: * As the heart beats faster and stronger [a response of the autonomic nervous system to person seeing blood], it sends a wrong message that tells receptors in the heart's chambers that the ventricle is filled with blood. * The receptors send a message telling the nervous system that blood pressure is too high. In fact, the ventricle is not full and the blood pressure is too low. * When the brain receives the faulty message, it slows the heart rate and further dilates the blood vessels. * The blood pressure drops even lower, less blood is pumped to the brain and the risk of fainting increases. (Retrieved from http://hrspatients.org/patients/signs_symptoms/fainting/non-cardiovascular_syncope.asp and http://www.merck.com/mrkshared/mmanual/section16/chapter200/200b.jsp)

Thanks i have another question how can we avoid faiting when we see blood?

I have vasovagal and I am currently studying to become a doctor, astonishing, I know, someone who used to faint from the mere sight of blood now surrounded by it daily. In order to overcome my fainting, with constantly affected my life (at blood tests, when exposed to accidents, etc) I followed a rigid plan of desensitizing myself to blood. In order to do this I pushed myself to lay down and and watch hours of surgeries and look at graphic images etc. I started small googling "stitches," moving on to cataract surgery, plastic surgery, and eventually the most graphic show "ER true trauma." From there I worked to expose myself to other bloody situations, when my little sister had a nose bleed I was sure to go over and look, at blood tests I would watch, after my mom had surgery I looked at her open wounds. The most important part of this process was remembering deep calm breathing. By slowing down my heart beat and eventually becoming so used to seeing blood I overcame vasovagal. This made a lovely essay for college admissions, and I realized that this goal was attainable after fainting at age 13 and speaking the the doctor at the ER, who had overcame vasovagal as well.