Lungs and circulation

During normal breathing, air enters and leaves the body through the nasal passages or the mouth.

During inspiration (breathing in) air is warmed in the nasal passages and it also takes up moisture (humidification).

The air then passes through a serious of branching (dividing) airways. These decrease in size and are termed:

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• Trachea (the windpipe)
• Bronchi
• Bronchioles
• Terminal bronchioles
• Respiratory bronchioles
• Alveolar ducts
• Alveolar sacs

There are 23 divisions of airways and 300 million alveoli in an average adult. The total surface area of an average adult’s lungs is about 70 m2.