system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why can seeing a drop of blood or a needle make me pass out, but seeing something that could actually kill me like a lion or a knife can’t?
gpt
The short answer is that we don't actually know for sure. What we do know is that some triggers such as seeing blood or a needle can cause your blood pressure to drop. This is called vasovagal syncope and it's why you pass out.

But, seeing something that truly scares you such as a lion or a knife will cause adrenaline to be dumped into your bloodstream which causes your blood pressure to spike, not drop.

But, there's not a clear explanation for why some things trigger a vasovagal response and why some trigger an adrenaline response.