CORE TOPIC:

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oxygenation of the blood

excretion of carbon dioxide

It is also involved in voice production and the senses of smell and taste.

The system includes (1) the lungs where gaseous exchange occurs and a system of elastic airways which links the lungs to the external environment. This is a transverse section of the thorax at the level of the sternal angle which shows how the lungs fill the space within the chest. At this level, the central part, between the lungs, contains (2) the trachea which is dividing here into right and left bronchi, very large blood vessels (e.g. (3) ascending aorta) and (4) the oesophagus. The heart lies at a lower level than this section and therefore is not visible in this picture.

Respiratory movements

Increase in the volume of the thoracic cavity, caused mainly by contraction of the diaphragm and supported by intercostal muscle action, lowers intrapleural pressure. This increases the transmural presure of the lungs, so that air moves down the airway and expands the lungs (inspiration). When the muscles relax, the elastic recoil of the lungs is enough to reverse this flow (expiration). In quiet breathing, therefore, inspiration is active process, and expiration is passive. When we need to exhale forcefully (e.g. coughing) we use the muscles of the abdominal wall.

There is abundant elastic tissue in the walls of the respiratory tract, which provides flexibilty and ensures the ability to spring back after distension. The process of inspiration stretches the elastic tissue of the airways and when the muscles relax during expiration, the elastic tissue recoils and the the lungs regain their original volume.

Functionally the respiratory system can be divided into two parts which merge into one another.

The conducting portion consists of a series of airways which carry air into the respiratory portion where gaseous exchange occurs.