The Real Reason Blood Makes Some People Faint
If the mere sight of blood makes you dizzy and light-headed to the point of passing out, you're not alone. Fainting at the sight of the blood is a fairly common reaction known as vasovagal syncope (via Health). Triggered by an overreaction to certain stimuli, vasovagal syncope overstimulates the vagus nerve, which causes your heart rate and blood pressure to drop suddenly. This reflexive response lowers the amount of blood flow to your brain, causing you to faint.
Common symptoms of vasovagal syncope prior to fainting include nausea, tunnel vision, clammy or sweaty skin, and feeling light-headed. They're also most likely to occur when you're on your feet. "When you're standing up, you're most vulnerable to that drop in blood pressure, which would lower [blood flow] to the brain and cause you to pass out," Larry A. Chinitz, M.D., a cardiologist and the director of the NYU Langone Heart Rhythm Center, told Health.
While people experiencing vasovagal syncope usually regain consciousness in a matter of seconds, it may take a few hours for any lingering symptoms of dizziness or fatigue to completely disappear (via Verywell Health). That's why it's important to get plenty of rest after an episode. Until all your symptoms go away, you may be at risk of fainting again.
How to stop yourself from fainting
Although vasovagal syncope is not a harmful condition, fainting can potentially result in injury, depending on where you are and how you fall. Luckily, there are a few things you can do to try to prevent yourself from passing out (via Healthline). While the easiest way to stop yourself from fainting is to avoid any triggers, this is not always possible. You'll most likely have to donate blood or get vaccinated at some point in your life.
That's why you should make sure you drink plenty of fluids throughout the day, but especially before you do something like give blood. This will help you avoid getting dehydrated, which can trigger a fainting episode. It also might be a good idea to look away when you're getting a shot or having blood drawn. If you feel like you're going to faint, however, you should lie down and elevate your legs. If you're unable to lie down, you can sit with your head between your legs. While you're doing this, take slow, deep breaths and clench your fists. This will help calm you down and raise your blood pressure back to its normal level.