c. Lungs. A lung is an individual organ composed of tubular structures and
alveoli, bound together by fibrous connective tissue. In the human, there are two lungs-
-right and left. Each lung is divided into lobes. A pulmonary lobe is a major subdivision
of a lung marked by fissures (deep folds). Each lobe is further partitioned into
(1) The right lung is larger in volume than the left lung since the left lung
must leave room for the heart. The right lung is divided into three pulmonary lobes
(upper, middle, and lower) and ten bronchopulmonary segments (2+3+5). The left lung
is divided into two pulmonary lobes (upper and lower) and eight bronchopulmonary
(2) Each lung is supplied by a primary or mainstream bronchus leading off
the trachea. Each lobe is supplied by a secondary or lobar bronchus branching off of
the primary bronchus. Each segment is supplied by a tertiary or segmented bronchus,
a branch of the lobar bronchus.
d. Pleural Cavities. Each lung is encased in a serous cavity called the pleural
cavity. Each serous cavity has inner and outer membranes. The serous membranes
secrete fluid that act as a lubricant between the membranes, allowing freer motion for
the organs. The pleural cavities allow the lungs to move freely with a minimum of
friction during the expansion and contraction phases of breathing. Located in the
middle of the thorax, between the two pleural cavities, is the mediastinum ("I stand
between"). The mediastinum is a tissue and organ-filled space. Within it is the heart,
which is located at the same level as the lungs.
BREATHING AND BREATHING MECHANISMS
a. Boyle's law (named after Robert Boyle, British physicist, 1627-1691) states
that as the volume of a gas-filled container increases, the pressure inside decreases.
Conversely, as the volume of a closed container decreases, the pressure inside
increases. When two connected spaces of air have different pressures, the air moves
from the space with greater pressure to the one with lesser pressure. In regard to
breathing, we can consider the air pressure around the human body to be constant.
The pressure inside the lungs may be greater or less than the pressure outside the
body. Thus, a greater internal pressure causes air to flow out; a greater external
pressure causes air to flow in.
b. The human trunk can be compared to a hollow cylinder. This cylinder is
divided into upper and lower cavities by the diaphragm. The upper is the thoracic cavity
and is essentially gas-filled. The lower is the abdominopelvic cavity and is essentially
water-filled. By changing the size (volume) of the thoracic cavity, air can be forced into
and out of the cavity. The size of the thoracic cavity is changed by movement of the rib
cage (coastal or thoracic breathing) and by movement of the diaphragm (diaphragmatic
or abdominal breathing).