Why do our body cells require oxygen?
Our body cells require oxygen to release energy. The oxygen that we take in during respiration is used to break down the food we eat to release energy from it. When we breathe in oxygen, it diffuses into blood from the lung alveoli. The lung alveoli have very thin walls in order to make the diffusion of oxygen into blood possible. The oxygenated blood is then taken to the cells where the cells use it for the breakdown of energy. After the function is complete, the carbon dioxide generated during the release of energy diffuses into blood and is again taken to the lung alveoli. And then, the carbon dioxide is thrown out of the body when we breathe out. So, this is why cells require oxygen and this is how the complete process of respiration occurs.
PS: Diffusion is the movement of a substance from a region of higher concentration to a region of lower concentration.