 As already explained in a previous video on the seven life processes, respiration is carried out by all living things. Animals need oxygen to carry out aerobic respiration and they need to get rid of the other waste product called carbon dioxide. This is called gas exchange, swapping one gas for another. Animals have evolved different mechanisms for carrying out gaseous exchange. Look at this polar bear. It is a mammal and all mammals have lungs in order to exchange gases between the blood and the atmosphere. This shark is a fish. Fish have internal gills to get oxygen out of the water as they swim along. Some animals, such as the Mexican salamander, have their gills on the outside. What about this locust? It is an insect and a lot of insects use a system of tubes called trachea for exchanging gases. Amphibians, such as this poison dart frog, also have lungs like mammals but can get some oxygen through their moist permeable skin as well. These are all special in their own way but they also have a few things in common which make them good at exchanging gases. The most important thing is to create a large surface area. The bigger the surface area, the faster the diffusion of the gases can occur. The total surface area for a human set of lungs is about 75 metres squared which is as big as a badminton court and it's all packed into your chest. Secondly, they all have a good blood supply. You want to get oxygen into the blood and carbon dioxide out as quickly as possible so the blood travels very close to the exchange surface to once again maximise diffusion. You can see that they have a good blood supply as the gills and the fish are bright red. The blood needs to be taken away from the exchange area quickly so that you maintain the concentration gradient between the two areas. Diffusion happens quickly when there is lots of gas in one place and not very much in the other. This is called the concentration gradient. So as soon as oxygen enters the blood it needs to be transported away so that more can enter the blood next time you breathe in. Being moist also helps dissolve the gases and speed up exchange. So there you have it. Animals have developed different systems to achieve the same thing. Take oxygen out of the air and put carbon dioxide in it.