The lungs are separated into lobes by invaginations of the pleura, which are often incomplete. The right lung has three lobes, whereas the left lung has two. The upper lobe lies mainly in front of the lower lobe and therefore physical signs on the right side in the front of the chest are due to lesions of the upper lobe or the middle lobe. Because of the contrast in density between healthy and diseased lung, plain radiography enables accurate localization of disease processes, especially if postero-anterior (PA) and lateral views are taken.
Each lobe is further subdivided into bronchopulmonary segments by fibrous septa that extend inwards from the pleural surface. Each segment receives its own segmental bronchus.
The bronchopulmonary segment is further divided into individual lobules approximately 1 cm in diameter and generally pyramidal in shape, the apex lying towards the bronchioles supplying them. Within each lobule a terminal bronchus supplies an acinus, and within this structure further divisions of the bronchioles eventually give rise to the alveoli.
--------------------------------------------------------