Everything from the 2 lessons of this topic on a few pages: the key idea of each lesson, the diagram to remember, the points to learn and the traps examiners set. Read it, cover it, test yourself.
Air passes through the trachea, which divides into two bronchi, one to each lung; these branch into many bronchioles, which end in clusters of alveoli surrounded by a capillary network. Cartilage is found as C-shaped rings in the trachea and irregular plates in the bronchi, but not in bronchioles or alveoli. Ciliated epithelium with goblet cells lines the trachea, bronchi and larger bronchioles. Smooth muscle and elastic fibres are in the walls of all the airways, with relatively most smooth muscle in the bronchioles. Squamous epithelium forms the alveolar walls, which are wrapped in capillaries. Goblet cells and mucous glands secrete mucus that traps dust, pollen and bacteria, and the cilia beat to sweep the mucus up to the throat, where it is swallowed.
Bronchioles have no cartilage; that is the quickest way to tell a bronchiole from a bronchus on a slide. Do not say the trachea has complete rings: the rings are C-shaped, and the gap at the back, next to the oesophagus, is bridged by smooth muscle.
Cilia do not trap particles and they do not make mucus. Mucus (from goblet cells and mucous glands) traps; cilia move the mucus. Keep the two jobs separate in your answer.
In the alveoli, oxygen diffuses from the air (high partial pressure, about 13 kPa) into the blood in the capillaries (about 5.3 kPa), and carbon dioxide diffuses from the blood into the alveolar air, each down its own concentration gradient. Diffusion is fast because the surface area of the millions of alveoli is huge, the diffusion distance is less than 1 μm (the squamous epithelium of the alveolus plus the capillary endothelium), and the gradients are kept steep by ventilation, which replaces the alveolar air, and blood flow, which carries oxygenated blood away. Cartilage holds the trachea and bronchi open; smooth muscle contracts or relaxes to change the diameter of the airways, especially the bronchioles; elastic fibres stretch as the alveoli fill and recoil to help push air out.
Gases cross by diffusion, not active transport, and each gas moves down its own gradient. Say down a concentration (partial pressure) gradient, and name both layers of cells: the alveolar epithelium and the capillary endothelium.
Bronchioles have no cartilage, so it is the smooth muscle and elastic fibres in their walls that control their width. Cartilage does not contract; muscle does. Elastic fibres do not contract either: they recoil passively after being stretched.