system
Given the following post, respond as if you're commenting on Reddit:
human
Why can't salt water fish breathe in fresh water and vice-versa? It's all water right?

I feel like I should have learned this at some point but I haven't.
gpt
saltwater fish are less salty than their surroundings to stay physiologically stable so they constantly drink saltwater to keep hydrated while using chloride cells in the gills to excrete excess ions. continuing this process as needed keeps their body osmolarity stable. on the other hand, freshwater fish are hyperosmotic to their surroundings and will have water diffuse across skin and gills directly to their system. to keep from swelling up and dying they have advanced kidneys that allow them to quickly get rid of this excess water. salt and freshwater fish cannot live in each others environment without having the others adaptations. This however has nothing to do with breathing as they will both receive the same amount of oxygen across the gills from either water supply. fish who are brackish have features of both and can go into either system.