Breathing and the Respiratory System

lungs4  air-sac

 Diagram showing the structure of the lung
The lungs consist of a network of thin tubes that each end up in a tiny air sac that is surrounded by blood capillaries

The capillaries and air sac both have very thin walls only one cell thick so gases can easily pass backwards and forwards

When you breath in:

Table showing a comparison between the composition of inhaled with exhaled air.

Gas Air entering lungs – Inhaled air Air leaving lungs – Exhaled air
Oxygen 21% 16%
carbon dioxide 0.03% 3%
Nitrogen 78% 78%
water vapour small amount significantly moist

Exhaled air has less oxygen
Exhaled air has more carbon dioxide
Nitrogen is not needed by our body so stays the same
Exhaled air has more water vapour