All living things require energy. Most cells obtain energy by the process of cellular respiration which involves the reaction of oxygen with glucose sugar to release energy, as well as carbon dioxide as a waste product. (see Plant systems B: Photosynthesis, nutrition and respiration).

The function of the respiratory system is to facilitate the intake and absorption of oxygen, so that it is available to the cells for cellular respiration in which energy will be released.

Worms absorb oxygen through their moist skin. Fish use their blood-rich gill plates to absorb oxygen from the surrounding water.

Large endothermic (warm-blooded) animals such as land-dwelling humans breathe air (which is approximately 20% oxygen) through internal specialised organs called lungs.

The lungs of humans are situated in the pleural cavity (chest area), surrounded on the sides by the ribs and below by the diaphragm.

Airways
Air entering the nose is warmed and foreign particles filtered out of it by fine hairs called cilia. The air then passes through the throat (pharynx). Food is prevented from entering the lungs by the epiglottis, a flap of tissue that covers the windpipe during swallowing. The trachea (windpipe) branches into two bronchi, one to each lung. In the lung, each bronchus divides into increasingly smaller bronchioles ending in lobed, thin-walled sacks called alveoli.

Each alveolus has a capillary network covering it which provides an enormous surface area for oxygen to enter the blood from the lungs and carbon dioxide to leave. There are approximately 300 million alveoli in each lung, with a combined surface area for exchange of about the size of a tennis court.

The oxygen combines with the red pigment haemoglobin carried by the red blood cells, while the carbon dioxide is carried in solution.

Breathing
When the muscles between the ribs (intercostals) contract along with the diaphragm, the chest cavity enlarges in volume. This reduces the air pressure in the lungs below that of the external atmosphere so air is forced in from outside bringing in fresh oxygen (inspiration).

When the intercostals and the diaphragm relax, the chest volume decreases and air is forced out removing waste carbon dioxide (expiration).