We have receptors, called the carotid bulbs, in the carotid arteries in our neck. One on each side. Their job is to maintain good blood flow, that is, blood pressure, to the brain. If the pressure drops, their job is to fix the flow. By fixing the pressure. It goes black in front of your eyes first and then you faint. Presumably you will fall down, which will help with that pressure.
Except when you don't fall down. The first person I saw faint was my cousin, on a train. We were 17. She was sitting across from me between two people. I looked at her and her eyes were rolled back in her head. I elbowed my other cousin, her older sister. She looked up, said "Fainted." and reached across and shoved her sister's head down to her knees. My cousin (her sister) promptly woke up.
In residency a friend was visiting. We went to "Inner City Hot Tubs" in Portland. We hot tubbed for a good while. He was an athlete but had nearly no body fat. He and my then husband went in the sauna for more time. I don't love saunas. Too hot. They came out and we were showering. The friend said, "I feel strange...." I whirled and shoved my hands under his arms from behind as he went down. Got him. Which is good because he was headed for a head bang against tile corners. I laid him down and put his legs up. All of the other people sitting around in towels gave me loads of advice. He woke up and got water and salt.
In clinic I am stitching an older teen's forehead. Scalps bleed, messy. He is holding on to his mother with his left hand. I am on his right, gloved, hopefully following sterile technique as I stitch. My nurse steps out. I ask his mother something. She doesn't answer. I look up. She has fainted dead away, but her son has a death grip on her arm. "Um. Don't let go of your mother, ok?" My nurse comes in and lies mom down and puts her legs up on a chair. She revives.
In clinic I am describing a procedure that we are about to do to a young woman. Who faints. We wake her up. Her blood pressure is fine. So we make her talk during the procedure, asking questions about her beau, and she stays awake.
In clinic I am on the phone, trying to set up a colonoscopy. My patient is a bit fidgity, standing up. He makes a sound and then goes down right in front of me. "Uh." I say, "My patient fainted. I'll call you back." He stays down for a while. He has lost a lot of weight, over one hundred pounds. I had cut his blood pressure medicine in half, but that wasn't enough. We keep cutting it in half and now he's off. For every kilogram lost, the blood pressure drops about a point. Ok, if you lose over a hundred pounds, you don't drop 100 points, but you WILL need your blood pressure medicine cut back. That was his second visit with me. Some other clinic was not paying attention.
I have nearly fainted once. Medical school, pregnant, emergency room rotation, Friday night drunk tank with the car wreck drunks lined up on stretchers. I was with the orthopedic resident who was checking that all the tendons were working in an arm. "Wiggle this finger." All of the drunks were dripping blood and none of them knew they'd been in a wreck and all were talking. I felt all my blood drain south. "I need to use the ladies room." I walked out, to the bathroom, put my head way down. I did not faint.
In pregnancy the blood volume is increased by about 1/5. So if the woman doesn't drink enough fluid, she will complain of dizziness standing. Or she will faint.
Heart arrhythmias are not that common as a cause of fainting, but can be a sign of a heart attack, or a prelude to a stroke, or a medication complication. The history and exam and labs (and a urine tox screen and pregnancy test) all help, but history is most important.
We can faint from dehydration, from loss of blood, from a vasovagal response: that's the faint at the sight of blood or too hot or stress....