Cells utilise oxygen for metabolism and produce energy along with substances like carbon dioxide which is harmful. Animals have evolved different mechanisms for the transport of oxygen to the cells and for the removal of carbon dioxide from there. We have a well developed respiratory system comprising two lungs and associated air passages to perform this function.
The first step in respiration is breathing by which atmospheric air is taken in (inspiration) and the alveolar air is released out (expiration). Exchange of O2 and CO2 between deoxygenated blood and alveoli, transport of these gases throughout the body by blood, exchange of O2 and CO2 between the oxygenated blood and tissues and utilisation of O2 by the cells (cellular respiration) are the other steps involved.