c. Lungs. A lung is an individual organ composed of tubular structures and
alveoli bound together by porous, spongy, elastic type, fibrous connective tissue. In
humans, there are two lungs: the right lung and the left lung. Each lung is supplied by a
primary or mainstem bronchus leading off the trachea.
(1) Structure. The right lung is larger in volume and shorter than the left
lung. The left lung must leave room for the heart. The right lung is divided into three
pulmonary lobes (upper, middle and lower) and 10 bronchopulmonary segments
(2 + 3 + 5). The left lung is smaller and narrower than the right lung and is divided into
two pulmonary lobes (upper and lower) and eight bronchopulmonary segments (4 + 4).
A pulmonary lobe is a major subdivision of a lung marked by fissures (deep folds).
Each lobe is further partitioned into bronchopulmonary segments. Each lobe is supplied
by a secondary or lobar bronchus. Each segment is supplied by a tertiary or segmental
bronchus, a branch of the lobar bronchus.
(2) Lung lobules. Each bronchopulmonary segment of the lung is divided
into many small compartments called lobules. See figure 1-8. Elastic connective tissue
is wrapped around each lobule. Each lobule has a lymphatic vessel, an arteriole, a
venule, and a branch from a terminal bronchioles. Terminal bronchioles further
subdivide into tiny branches called respiratory bronchioles. Respiratory bronchioles
subdivide into several (2 to 11) alveolar ducts. Many alveoli and alveolar sacs surround
the alveolar duct.
Figure 1-8. Diagram of a lung lobule.