The trachea, bronchi and bronchioles enable the flow of air into and out of the lungs. Trachea and bronchi are similar in structure, with the exception of size – bronchi are narrower. They are composed of several layers which together make up a thick wall. The wall is mostly composed of cartilage, in the form of incomplete C rings. Inside surface of the cartilage is a layer of glandular and connective tissue, elastic fibres, smooth muscle and blood vessels. This is referred to as the ‘loose tissue’. The inner lining is an epithelial layer composed of ciliated epithelium and goblet cells.