Respiratory system
Humans posses a pair of lungs located in the
chest, or thoracic, cavity. The two lungs hang free within the cavity,
being connected to the rest of the body only at the one position where
the lung's blood vessels and air tube enter. This air tube is called
a bronchus. It connects each lung to a long tube called the trachea,
which passes upward in the body, past the voice box, or larynx, and
opens into the rear of the mouth. Air normally enters throught the nostrils,
where it is warmed. In addition, the nostrils are lined with hair that
filter out duct and other particles. As the air passes throught the
nasal cavity, an extensive array of cilia on its epithelial lining further
filter the air and moistens it. The air then passes through the back
of the mouth, crossing the path of food as it enters first the larynx
and then the trachea. From there it passes down through the bronchus
to the lungs. The human respiratory apparatus is simple in structure,
functioning as a one cycle pump. The thoracic cavity is bounded on its
sides by ribs, which are capable of flexing, and on the bottom by a
thick layer of muscle, the diaphragm which separates the thoracic cavity
from the abdominal cavity. Each lung is covered by a very thin, smooth
membrane called the pleural membrane. A second pleural membrane marks
the interior boundary of the thoracic cavity, into which the lungs hang.
Within the cavity the weight or the lungs is supported by water, the
intrapleural fluid. The intrapleural fluid not only supports the lungs,
but also plays another important role by permitting an even application
of pressure to all parts of the lung. The inner balloon opens out to
the atmosphere, so that two forces act on this inner balloon: air pressure
from the atmosphere pushes it outward, and water pressure from the intrapleural
fluid pushes it inward. During inhalation, the walls of the chest cavity
expand. The rib cage moves outward and upward, and the diaphragm moves
downward by stretching taut. In effect, we have enlarged the outer balloon
by pulling it in all directions. This expansion of the air pressure
within the inner balloon. As a result, the wall of the inner balloon
is pushed out. As the inner balloon expands, its internal air pressure
on the fluid. This pressure is transmitted uniformly by the fluid over
the entire surface of the lung, forcing air from the cavity back out
to the atmosphere.
Breathing, the active cycle of inhalation and
exhalation, thus depends on the regular contraction of the muscles surrounding
the thoracic cavity. The rate of breathing is regulated by a respiratory
center in the brainstem. Chemo receptors in the artrial walls detect
changes in carbon dioxide levels and transmit nerve impulses to the
respiratory center, which sends appropriate signals to the muscles of
the diaphragm and rib cage.
