The gills on a fish enable the animal to survive under water, without them, like humans they would not be able to live in water as they would not be able to breath. If it wasn't for the gills on a fish or if a fish's gills got severely damaged, the likelihood is they would drown.
HOW DO THEY WORK?
Gills have small rows and columns of specialised cells grouped together called the epithelium, this is one of the four basic types of animal tissue, the epithelium is thin tissue that forms the outer layer of a body's surface, however it also lines the alimentary canal (the passage food passes through from the mouth to the anus during digestion) and other hollow structures on the body.
Deoxygenated blood in the fish is supplied directly from the heart to the epithelium via arteries, and even smaller arterioles. As seawater is forced across the epithelium membranes, dissolved oxygen in the seawater is taken up by tiny blood vessels and veins, while the carbon dioxide is exchanged. The process begins when the fish gulps water through its mouth, the water enters the mouth and then passes through the filaments in the fish's gills. These filaments absorb the oxygen from the water and move it into the bloodstream. The fish's heart then pumps the oxygenated blood around the body, distributing oxygen all over as it goes round. At the same time carbon-dioxide in the blood passes through the gills and back out into the water.
Gills have a car radiator-like appearance. Most fish have 4 gills on each side, consisting of a main bar-like structure that has numerous branches as that of a tree, and those branches consisting of even smaller branch-like structures. This arrangement of cells allows for a very large surface area when the gills are immersed in water.
https://www.google.co.uk/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=what%20is%20a%20epithelium
http://www.todayifoundout.com/index.php/2011/09/how-fish-gills-work/
https://www.google.co.uk/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=what%20is%20a%20alimentary%20canal
https://www.google.co.uk/search?q=what+is+a+epithelium&espv=2&biw=1280&bih=728&source=lnms&tbm=isch&sa=X&ved=0ahUKEwiPuvaNvrbJAhWBPxoKHTwTDMMQ_AUIBigB#tbm=isch&q=what+is+a+epithelium+in+fish&imgrc=bUv152fOZiqYSM%3A
http://www.dkfindout.com/uk/animals-and-nature/fish/how-fish-breathe/
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