Most of the energy required by the cells of the body is derived from chemical reactions that can only take place in the presence of oxygen (O2). The main waste product of these reactions is carbon dioxide (CO2).

The respiratory system provides the route by which the oxygen present in the atmosphere is taken into the body and through which carbon dioxide is expelled.

Blood acts as the transport system for these gases between the lungs and the cells of the body. The exchange of gases between the lungs and the blood is called ‘external respiration’ and between the blood and the cells it is called ‘internal respiration’.

Pulmonary Respiration or External Respiration

Oxygen is taken in through the nose and the mouth. It then flows down to the lungs along the trachea and the bronchial tubes. In the alveoli the life-giving exchange between oxygen and carbon dioxide occurs: where fresh oxygen enters the circulatory system to be carried throughout the body by haemoglobin in the blood and where the blood for elimination through exhalations returns gaseous waste products such as carbon dioxide.

Tissue Respiration or Internal Respiration

It is in the cells and more particularly in the mitochondria, where oxygen is used to release the energy stored in food. This chemical transformation occurs when oxygen reacts with carbon (from food). The energy released within all the cells of the body by the reaction between oxygen and carbon is used to drive other parts of the metabolism, allowing the digestion of food, blood circulation and movement. Waste products, such as carbon dioxide are returned to the venous blood and ultimately to the lungs and back into the atmosphere.