The Respiratory System

The respiratory system extends from the nose and mouth to the alveoli within the lungs.

Upper respiratory tract

upper respiratoryThe upper respiratory tract includes the nose, nasal cavity and pharynx. It is responsible for collecting and delivering air to the lungs, and it is also responsible for warming, filtering and humidifying the air.

Lower respiratory tract

The lower respiratory tract includes the larynx, trachea, bronchi, bronchioles and alveoli.

The trachea begins immediately below the larynx (voice-box), runs down the centre of the front part of the neck and ends behind the upper part of the sternum (where it divides to form two branches which enter the lung cavities). It is flexible (similar to a vacuum tube), which allows a person’s head and neck to twist and bend without closing it off. It consists of about 20 “C” shaped cartilaginous rings which prevent it collapsing or overexpanding as pressures within the respiratory system change.

lower respiratorySmooth muscle in the wall of bronchioles constricts and dilates to alter the resistance to airflow through the lungs. Alveoli are small sac like structures where the exchange of gases between the lungs and the circulation occurs. Gas exchange occurs at the level of the alveolus. Oxygen and carbon dioxide cross the alveolar membrane down their concentration gradients. Oxygen crosses into the circulation while carbon dioxide is removed.

A human lung contains about 480 million alveoli, with a total surface area of about 70-90 square metres.

Mechanics of respiration

Inspiration is an active process. The diaphragm contracts down, the ribs rotate up and out and the sternum moves forward. This increases the intrathoracic space and decreases the intrathoracic pressure, so air moves in.

Expiration is a passive process. The respiratory muscles relax and the elastic tissue in the lung recoils. This creates a smaller intrathoracic area and therefore increases the pressure. As a result, air moves out.

Respiration is controlled by the central nervous system. The respiratory rate is altered to maintain blood oxygen and carbon dioxide levels. When the carbon dioxide level in the blood rises, the respiratory rate also increases.


For more information on the respiratory system click here.