Figure 7-4. Infralaryngeal structures ("respiratory tree").
b. Alveoli. The alveoli (alveolus, singular) are tiny spherical (balloon-like) sacs
which are connected to the larger tubes of the lungs by tiny tubes known as alveolar
ducts and bronchioles. The alveoli are so small that there are billions in the adult lungs.
This very small size produces a maximum surface area through which external
respiration takes place. External respiration is the actual exchange of gases between
the air in the alveolar spaces and the adjacent blood capillaries through their walls.
c. Lungs. A lung is an individual organ composed of tubular structures and
alveoli bound together by fibrous connective tissue (FCT). In the human, there are two
lungs--right and left. Each lung is supplied by a primary or mainstem bronchus leading
off of the trachea. The right lung is larger in volume 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 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