Functional anatomy

The primary function of the respiratory system is the exchange of oxygen and carbon dioxide between the body and the environment. In addition, the lungs also have a metabolic role, act as a filter for small emboli in the circulation, play a part in acid–base balance and contribute to the immune defences of the body. These functions are all reflected in the anatomy of the components of the respiratory system.

Upper airway and larynx

In respiration, the function of the nose, mouth and pharynx is to conduct fresh gas to the larynx, which marks the entrance to the conducting airways. These structures also warm, humidify and filter the gases.

During quiet nasal breathing this section of the airway can provide two-thirds of the total resistance to airflow of the respiratory system. Since the pharynx is a muscular tube without rigid structures to maintain its patency, it can increase the flow resistance considerably, even to the point of total obstruction, depending on the tone of its muscular wall, the associated muscles and the transmural pressure.

The larynx has three main functions:

• Regulation of expiratory airflow (expiratory braking). This is important for vocalisation, coughing and control of end-expiratory lung volume.

• Protection of the lower airway. Vocal cord closure prevents aspiration of foreign material or objects and expiratory braking enables the cough reflex to expel foreign material and secretions.

• Vocalisation.

Conducting airways

The respiratory system is traditionally divided into gas-conducting and gas-exchanging components.