b. Muscles. The larynx serves two functions and there are two sets of muscles-
-one for each function.
(1) One set controls the size of the glottis. Thus, it regulates the volume of
air passing through the trachea.
(2) The other set controls the tension of the vocal cords. Thus, it produces
vibrations of selected frequencies (variations in pitch) of the moving air to be used in the
process of speaking.
1-5. INFRALARYNGEAL STRUCTURES
a. Trachea and Bronchi. The respiratory tree (Figure 1-4) is the set of tubular
structures that carry the air from the larynx to the alveoli of the lungs. Looking at a
person UPSIDE DOWN, the trachea is the trunk of the tree and the bronchi are the
branches. These tubular parts are held open (made patent) by rings of cartilage. Their
lining is ciliated to remove mucus and other materials that get into the passageway.
b. Alveoli. The alveoli (alveolus, singular) are tiny spherical (balloon-like) sacs
that 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 millions 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 from 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 3 pulmonary lobes (upper,
middle and lower) and 10 bronchopulmonary segments (2 + 3 + 5). The left lung is
divided into 2 pulmonary lobes upper and lower) and 8 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. A tertiary or segmental bronchus, a branch
of the lobar bronchus supplies each segment.
d. Pleural Cavities. Each serous cavity has inner and outer membranes. In the
case of the lungs, the inner membrane, is known as the visceral pleura which very
closely covers the surface of the lungs. The outer membrane is known as the parietal
pleura, forming the outer wall of the space. The pleural spaces are the potential spaces
between the inner and outer membranes. The opening between the pleural layers
contains a slick fluid called pleural fluid. The pleural fluid serves as a lubricant and
allows the lungs to move freely with a minimum of friction.