It sometimes depends first on what your usual blood pressure has been through your life. My blood pressure is low and has gone down to 90/50. I donu2019t pass out but if my BP was normally 140/80 and suddenly dropped to 90/50 I might.If a person is dehydrated, has not eaten anything for several hours, is out in the hot sun without a hat, has heart disease history or any combination of a couple of these factors the risk for fainting is higher.When a person actually passes out, their blood pressure has become extremely lowu2026.they arenu2019t getting enough oxygen to their brain because of low or no BP and they go down. Once they are flat, the blood goes to their head and brain and they usually wake right up. Given water and some food, or a piece of hard candy or orange juice their blood pressure returns, low at first then back to normal.There are many other reasons for blood pressure to drop drastically but passing out is caused by a low enough blood pressure that causes poor venous return and therefore not enough oxygenated blood for your heart to circulate to your brain.