Oxygen is necessary to every living thing. We breathe out oxygen and exhale carbon dioxide. Most organisms (eukaryotic) use oxygen to perform cellular respiration and is important for the organism's metabolism. To create chemical, "oxygen is used as an electron acceptor in the mitochondria."  Diffusion occurs in the lungs after oxygen is inhaled and the blood dissolves the oxygen and the blood with clean oxygen delivers the blood to body tissues that need it. The "dirty" blood from the veins going to the heart release carbon dioxide into the alveoli which are little air sacs in our lungs, carbon dioxide is pumped out of the lungs by the diaphragm(humans). The purpose of aerobic respiration (cellular respiration), is to use glycolysis to create ATP to provide cells with energy. 
Channeled Whelk: How do they breathe?
The Channeled Whelk takes in oxygen that is dissolved in water. They have siphons where they can take water in or out and they obtain oxygen and exhale carbon dioxide. Water currents pass and they use their gills that can be found in between their visceral mask and the mantle cavity and this is where respiration occurs. Ctenidia (gills) is where gas exchange happens, the inhaling of oxygen and diffusing carbon dioxide into the water. Muscle contractions pump the air in and out. Also, they can breathe through their skin since it is permeable. Channeled Whelks are cold blooded and by squeezing their bodies, they regulate body temperature.