system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 Vasovagal response and what prompts it

My son (16) fainted the other day when he had a blood draw. He has never had a similar response to the sight of blood or anything.
gpt
The autonomic nervous system is responsible for maintaining a lot of the normal day to day functions of your body, including blood pressure. When someone vasovagals, the autonomic nervous system misfires and is temporarily knocked out of balance which causes not enough blood to get to the brain thus leading to fainting. 


There are many things that can cause someone to vasovagal. The blood pressure sensors in the arteries can be too sensitive in some people to the point where wearing a shirt with a tight collar or tie could cause someone to vasovagal. Exercise could cause someone to vasovagal. Situational vasovagal syncope is when a specific situation causes someone to faint. Some common examples are during urination, after coughing, or fear of a painful stimulus (like blood draws). It doesn’t necessarily happen every time and there are some factors that could contribute to it being more likely to happen, like being dehydrated and having a slightly lower blood pressure.