The main purpose of respiration is to supply O₂ to cells for their metabolic needs and to remove CO₂ from cells.

The survival of an organism depends on:

• The integrity of the intrinsic physiological systems. The numerous physiological systems present in each individual must not only function individually but are interdependent where each system depends on the other systems for optimal function.
• A supply of nutrients, water, and oxygen. The essentials of life are food, water and air. Energy for everyday work is derived from the oxidation of food, and this can be either used immediately or it can be stored in high energy chemical bonds for later use. Water is the internal solvent which is essential for all the internal chemical reactions to take place. Whilst the body has an ability to store nutrients and water (humans can survive for weeks without food and days without water) the ability to store oxygen is very limited (humans can survive for only a few minutes without air or oxygen. Thus a constant supply and transport of oxygen is required for the survival of the individual.