Gills
The underside of a Walleye’s gills (photo credit: USFWS).
Someone may tell you that you look “green around the gills” when you look ill but the expression doesn’t translate well to fish gills…
No matter if a fish is sick or not, fish gills are often red – blood red, to be exact. That is because gills have blood vessels very close to their external surface. As the primary mode of gas exchange for most fish (but not all!), gills absorb oxygen from water that passes over them and release carbon dioxide usually facilitated through a counter-current exchange of blood and water flowing in opposite directions. Gill lamellae provide increased surface area for gas exchange, which is particularly important because there is less dissolved oxygen in water than there is in air. For fish that breathe air, lungs and skin provide alternative methods for oxygen absorption.