system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do fish gills actually work?

I know they filter out oxygen and all that, but I'm really interested in knowing how exactly they do it.
gpt
Pretty simple. Dissolved gasses diffuse through membranes across conccentration gradients. That's how you're able to breathe with your lungs. A gill is basically a really high-surface-area membrane that allows lots of water with dissolved O2 in it to diffuse into capillaries in the gill. This diffuses O2 in and CO2 out across their gradients. In order to keep this going at a good level in fish, the water must be more or less continuously flowing over the gills. There are two main strategies for this. One involves gulping water into the mouth and pumping it out through where the gills are and out the gill flaps. Another is to keep swimming and never stop. Sharks in particular employ this method, and will suffocate if kept from moving for a sufficient amount of time.

Gills aren't that dissimilar to lungs in principle, but one is designed to work with water and another designed to work with air.