The Structure of the Respiratory System.

The respiratory tract begins at the nose. During inspiration air is drawn into the nasal cavity where it is warmed and humidified. The nasal cavity contains microscopic hairs known as cilia, there are also cells that produce mucus. Together these serve to trap dust, bacteria and other foreign particles that are in the air we breath and prevent them from reaching the lungs. The nasal cavity also contains a large number of capillaries in order to warm the air before it passes to the pharynx http://www.cdli.ca/~dpower/resp/struct~1.htm

From the nasal cavity air moves into the pharynx (throat) and then into the larynx. The larynx contains the vocal chords, and it is the act of air flow (expiration) across these that cause them to vibrate, enabling humans to make sounds for speech. The entrance to the larynx is blocked by the epiglottis, this prevents food and liquid from entering and diverts it down the oesophagus http://www.cdli.ca/~dpower/resp/struct~1.htm

The nasal cavity, pharynx and larynx are known as the upper respiratory tract. Their main function is to warm and humidify the air, as well as removing dust and bacteria from the air before it moves into the lower respiratory tract.

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The larynx leads directly into the trachea. This is a tube approximately 12cm in length and 2.5cm wide. The trachea is kept open with 16 rings of cartilage that form a 'c' shape. Like the nasal cavity the trachea is lined with cilia which move mucus back to the pharynx http://www.bbc.co.uk/dna/h2g2/A27019505.

At the centre of the chest the trachea splits into the left and right primary bronchi. The right bronchus is wider than the left and is often where inhaled foreign objects end up. These two primary bronchi divide into secondary bronchi which divide further into tertiary bronchi, with eight on the left and ten on the right http://www.bbc.co.uk/dna/h2g2/A27019505.

These tertiary bronchi then split into a network of bronchioles each ending in an alveoli. This network is what makes up the lungs. Each lung is split into lobes with two on the left and three on the right. The reason the left only has two is because the heart sits on that side of the body.

Alveoli are tiny air sacs at the end of bronchioles, it is here that gaseous exchange takes place. Alveoli are only one cell thick, this is so that air can diffuse quickly into the blood stream. There are thought to be around 600 million alveoli in an adult human giving a surface area of 100 square meters. This provides a hugh area for gaseous exchange.http://www.bbc.co.uk/dna/h2g2/A27019505