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Respiration describes breathing and the utilisation of oxygen within the cells of the body. Traditionally the term external respiration has been used to describe the processes of ventilation and the transfer of oxygen from the lungs into the blood. Ventilation is the process of physically moving air in and out of the lungs. Internal respiration describes how oxygen is used within the cells of the body to oxidize food in order to generate energy.
Nose and mouth
Air passes in and out of the nose via two openings called the external nares or nostrils. As air passes through the nose it is warmed when it comes into contact with the lining of the internal nasal passages as they are well perfused with warm blood. The vascular nature of the nasal cavities explains why a lot of blood can be lost through a nose bleed. Warming the air prevents the lower airways being chilled. As the lining of the nasal passages are moist, air passing through the nose is also humidified. This protects the lower airways from possible drying effects of air. The mucous lining, in combination with nasal hair, has the effect of filtering air passing through the nose, removing foreign bodies such as small insects.
Strangely the sense of smell provides much of the sensation of taste; this is why food seems to lose much of its taste when we have a cold. Smell also alerts us to the dangers of poisonous gases and bad food. Olfaction describes the sense of smell and there are olfactory sensory receptors found in the lining of the nasal cavity. These receptors are continuous with sensory nerve fibres which pass into the temporal lobes of the brain where smells are experienced.
A septum describes a structure which divides an area into two, so the nasal septum divides the nasal cavity into left and right sides. The back part of the septum is made of bony tissue and the front part of more flexible cartilage. Posteriorly the nasal cavity communicates with the nasopharynx through two openings called the internal nares. The mouth can increase the size of the airway opening to increase the volumes of air which can be inhaled. However, young babies breathe exclusively through the nose which means if their noses are blocked for any reason they will asphyxiate.

Sinuses
These are small air filled cavities in some of the frontal bones of the skull. They are lined with mucus which drains into the nasal cavity via small passages. The function of the sinuses seems to be to lighten the weight of the skull and to give resonance to the voice. Infection may spread from the nose into the sinuses giving rise to the common painful condition of sinusitis.
Pharynx
This is the passage which connects the back of the nose and mouth with the trachea and oesophagus. It is composed of three sections; the nasopharynx, oropharynx and laryngopharynx. The nasopharynx is behind the nasal cavity, below this is the oropharynx behind the mouth. The palatine tonsils are located in the oropharynx and can be seen through the open mouth. The lower section of the pharynx is called the laryngopharynx which leads down to the oesophagus and trachea.
Larynx
The larynx extends from the laryngopharynx to the start of the trachea. Because the structure is mostly composed of cartilage it can easily be felt in the middle of the neck, where it is commonly referred to as the ‘Adam’s Apple’.

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