Breathing nd exchange of gases

The mechanism by which an organism obtains oxygen from the air and releases carbon dioxide is called breathing. 

The respiration takes place inside the cells of the body so it is known as cellular respiration. The energy released during cellular respiration is immediately used to synthesize a molecule called ATP which is used to fuel all other activities in the cell. In these processes, ATP is broken down giving rise to a fixed amount of energy which can drive the endothermic reactions taking place in the cell.

Diffusion is insufficient to meet the oxygen requirements of large multicellular organisms like humans because the volume of the human body is so big that the oxygen cannot diffuse into all the cells of the human body quickly and oxygen will have to travel large distances to reach each and every cell of the body.

Large organisms contain a respiratory pigment called haemoglobin which carries the oxygen from the lungs to all the body cells very efficiently.
A terrestrial animal has an advantage over an aquatic animal in regard to obtaining oxygen for respiration that it is surrounded by an oxygen rich atmosphere from where it can take any amount of oxygen.