system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What exactly causes pupil dilation/constriction after ingesting drugs?

I understand that our pupils dilate when we’re in the dark, and they become constricted when we look directly at the sun for example. But why do certain drugs alter our pupils, and for what reason?
gpt
Certain drugs can change pupil size by activating or blocking the sympathetic/parasympathetic systems. If a drug causes sympathetic activity then it will directly cause the pupil to dilate. If it activates parasympathetic activity then the pupil constricts. Here's another interesting part: The drug can also block one system and allow the other to take over unopposed. Meaning you'd get the effects, but would not actually be activating that system. 



The reason for the systems is due to our ability to collect light and focus. Sympathetic activity requires us to pull in more light and have a better immediate surrounding visual field versus a better focused field further away.