The lungs are composed of thin-walled alveoli whose sacs are covered by a meshwork of capillaries. This is where oxygen and carbon dioxide are exchanged.
The trachea carries air from the nose and mouth to the bronchi, which branch to each lung. These divide several times to become very small bronchioles, which directly supply the alveoli.
The airways are lined with a ciliated mucosa which carries debris upward to the mouth on a layer of mucous, where it is swallowed. These mucosal membranes can swell in reaction to allergens, bacteria and viruses, leading to narrow airways and respiratory symptoms.