How do gills work?
Gills work in a similar way to lungs, they allow a fluid which contains oxygen to pass close to the blood stream which in turn allows the oxygen to move into the blood stream through the thin layer of cells which separates the blood from the outside fluid (air for lungs and water for gills). This moving of a soluble substance through the cells is called diffusion.
For gills specifically, the fish opens its mouth and fills it with water. Then it shuts the front of its mouth and blows the water out through tubes at the back of its mouth over the gills and out through the gill flaps (the visible, external parts of the gills) and back into the ocean.
Just beneath the gill flaps is where the magic happens, so to speak. The fish has structures there which have a very large surface area, with blood flowing very close to the surface which allows for oxygen to diffuse efficiently through and into the fish's blood.
A side note for sharks!
A lot of people believe that if sharks stop swimming they will drown. As with bony fish, the process of respiration occurs as oxygen moves over the gill openings and is exchanged for carbon dioxide (a waste product, just like humans) in a process called the countercurrent principle. The gills need a constant supply of water and so muscles work to draw water into the mouth before squeezing it out through the gills when the mouth is closed. Water is prevented from entering the throat via valves and gills via gill flaps. Swimming forward aids this process by the natural movement of the water.
The common tale of sharks drowning if they stay still comes from the large ocean faring sharks that really rely on the movement of the water induced by swimming and are barely able to use the pumping mechanism. If they are then trapped in a net, for example, they get into severe difficulties and eventually drown. The majority of sharks fall into this category.
However there are other sharks that rely more on the muscles to do the work and there are even a number of species that spend the majority of their time sitting on the sea floor, not moving at all. These species rely solely on being able to pump oxygen using their musculature.
A couple of species have some further wacky adaptations but that is a general summary!
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