Aerobic Respiration Cells utilise $O_{2}$ from atmospheric air or from water to oxidise the nutrients. It involves (i) External Respiration Gaseous exchange of $O_{2}$ and $CO_{2}$ between the blood and air (or water) (ii) Transport of gases to tissues (iii) Internal Respiration Gaseous exchange between the blood and tissues (iv) Cellular Respiration Oxidation of nutrients in the cells and liberation of energy