explain the importance of the following.
a The trachea wall contains C-shaped rings of cartilage.
b The distance between the air in an alveolus and the blood in an alveolar
capillary is less than 1/1000th of a millimetre.

c The lining of the trachea contains mucus-secreting cells and cells with cilia.
d Smokers have a lower concentration of oxygen in their blood than
non-smokers.
e Nicotine patches and nicotine chewing gum can help someone give up
smoking.
f The lungs have a surface area of about 60 m² and a good blood supply.