a. Thoracic Cavity. The thoracic cavity is the space of the trunk above the
diaphragm. It is open to the outside by way of the neck and head. Since the wall of the
thorax is reinforced by special muscles, bones, and cartilages, we can consider the
thorax to be a "solid-walled container" filled with gas.
b. Abdominopelvic Cavity. The abdominopelvic cavity is the rest of the trunk
cavity below the diaphragm. The abdominopelvic cavity is a closed system. Its walls
are "elastic" since they are made up of musculature. The abdominopelvic cavity is filled
with a fluid continuum. This fluid continuum consists primarily of water contained in the
soft tissues of the abdomen and the pelvis.
Section II. INTRODUCTION TO HUMAN BREATHING
Breathing is basically the process of moving air into and out of the lungs.
7-6. USE OF PRESSURE GRADIENTS
Breathing is accomplished by manipulating the pressure gradient between the
surrounding atmosphere and the thoracic cavity. For all practical purposes, the
pressure of the surrounding atmosphere can be considered a constant. Thus, the
desired pressure gradients are achieved by changing the pressure within the thoracic
cavity. The pressure in the thoracic cavity alternates so that it is less and then greater
than the pressure of the surrounding atmosphere.
7-7. TYPES OF HUMAN BREATHING
The two types of human breathing are costal and diaphragmatic. They may be
used individually and independently, or they may be used in combination.
7-8. LUNG CAPACITIES
a. Total Lung Capacity. From the instant of the "first breath," the lungs have a
certain total volume called the total lung capacity. This is the entire volume of air in the
lungs after one inhales as much as one can. Total lung capacity equals the sum of the
residual volume and the vital capacity.
b. Residual Volume. After the "first breath," the lungs are never completely
emptied. Thus, there is a certain portion of air that is always present in the lungs. After
one exhales as much air as possible, the portion remaining in the lungs is called the
residual volume. In actuality, this is not "dead air," because air circulation continually
refreshes the air of the residual portion.