Although they look very different, they both do the same thing – provide a large surface area for gas exchange. This means they take up oxygen that we breathe in, and get rid of carbon dioxide which is a waste product.
So why do humans have lungs but fish have gills? It all comes down to our way of life and the kind of oxygen we breathe.
Like all animals on land we breathe air. Lungs are adapted to receive air through our nose and mouth. Oxygen is then taken up and carbon dioxide diffused out. Our lungs also have lots of other functions to do with our life on land – they help us produce sound and get rid of dust and other particles from the air.
Fish on the other hand have gills. This is because fish breathe oxygen that is dissolved in water so they need a different structure that is specialised to extract this dissolved oxygen. Gills are like a very fine comb- water passes through the mouth and over the surface of the gills where oxygen is extracted. Water then leaves through the gill slits. Gills also help for an aquatic way of life. Fish can get rid of waste products through the gills such as nitrogen.