• The entry point of the respiratory tract is the nasal cavity. This has an important role in warming and moisturising air as it goes in, and also filtering it

• Below the pharynx sits the larynx – the location of the vocal chords. It has a protective role, continuing to humidify the air inside so it doesn’t cause any damage as it goes further into more delicate respiratory structures

• Next comes the trachea, also known as the windpipe, which is around 10-12cm long and 2.5cm wide. It is rigid courtesy of 16 rings of cartilage and muscle that run along its length

• At the bottom of the trachea, the pipe narrows and splits into two bronchi that run into the lungs. The bronchi continue to divide and taper until they are only 1mm wide, at which point they are called the bronchioles, which consist of smooth muscle that has nerves running through it

• These tubes continue to split and narrow even more until they reach the point at which they have no muscle in them at all. These are known as the terminal bronchioles. At the end of these are alveoli, which have walls only one cell thick and are surrounded by a dense network of tiny blood vessels called capillaries.

Gaseous exchange

Deoxygenated blood comes from the heart via the pulmonary artery into some of the capillaries, and the carbon dioxide in the blood diffuses out into the alveoli where it is replaced by oxygen. This newly oxygenated blood goes into different capillaries that combine many times and eventually form the pulmonary vein, which re-enters the heart so the blood can be pumped around the body. The process that takes place in the alveoli is called gaseous exchange.