MECHANICS OF BREATHING
Breathing is accomplished by changes in the size of
the chest cavity.
Surrounding-and guarding-the lungs have twelve pairs
of ribs. They are joined to the spine at the back and curve around the chest to
form a cage. In front, the top seven pairs are connected
to the breastbone. The next three pairs are connected to the rib above.
The
last two pairs, unconnected in front, are called floating ribs. The entire cage
is flexible and can be expanded readily by special muscles. In addition to the
rib cage, which forms the wall of the chest, there is the dome-shaped
diaphragm, which forms the floor of the chest cavity. The diaphragm is attached
to the breastbone in front, the spinal column in back, and the lower ribs on
the sides. The lungs do not suck in air; all the work is done for them by the
diaphragm and muscles of the rib cage. When you inhale, muscle fibers of the
diaphragm contract and the sheet of tissue is drawn downward; at the same time,
rib muscles pull the ribs upward and outward.
This expands the chest cavity and lowers the pressure
within it to below that of the atmosphere. This causes the lungs to expand,
too; and the tiny air sacs in the lungs also become a little bigger. Air then
rushes into the lungs to fill the extra space. When you exhale, just the
opposite series of events occurs. Muscles of the diaphragm relax; so do those
of the rib cage. The chest becomes smaller; the elastic tissue of the lungs
returns to its original shape, making the air sacs smaller; and the air is
driven out.
The rate of breathing varies with age. In a baby the rate is about
45 times a minute; by age 6, it is down to about 25; between ages 15 and 25, it
drops to about 18. There is some tendency for it to increase again with
advanced age.
The rate of breathing is also influenced by the
carbon dioxide content of the blood. If you hold your breath, carbon dioxide
accumulates in the blood until, finally, it so strongly stimulates the
respiratory control center of the brain that you are forced to breathe again.
The length of time the breath can be held varies from 25 to 75 seconds. Mothers
sometimes become alarmed when children hold their breath during crying spells
or temper tantrums. There is no real danger of suffocation; when the child
really needs air, he will be forced to inhale. In any condition leading to
elevation of body temperature, the respiration rate is always increased.
