Page 16 - iScience K to 12 Curriculum Series

U getting into your lungs. Air from the larynx This process of breathing is inhalation.
N enters the trachea through an opening known
I as the glottis. It is supported by C-shaped During inhalation, the diaphragm
T cartilage rings, which ensure it is always contracts and moves down, and the rib cage
kept open. The lower end of the trachea moves up. When the chest expands, the air
divides into two tubes, the bronchi (singular: pressure in the chest cavity drops causing air
1 bronchus), one for each lung. The bronchus to rush into the lungs.
divides into smaller tubes called bronchioles.
2 The bronchioles end in clusters of tiny air sacs
or alveoli inside the lungs. The alveolar walls
are thin, moist, and well-supplied with blood
3 capillaries, all perfect for gaseous exchange.

The lungs lie in the upper chest cavity. The
4 lungs are divided into sections called lobes.
Each lung lies in the pleural cavity; it’s almost
filled when both lungs expand. The pleural
cavity is lined with a transparent elastic
membrane called pleura that encloses each
lung. The airways in the lungs end into millions
of tiny air sacs called alveoli. A network of
capillaries covers each alveolus. The oxygen
diffuses from the air in the alveoli into the
blood. Capillaries in the body tissue carry out
oxygen to all parts of the body cell. This body
cell produces carbon dioxide and is mixed
with the blood. The ribs support the chest wall
to protect the lungs from mechanical injury.
A dome-shaped sheet of muscle and elastic
tissue called the diaphragm lies on the floor
or at the lower portion of the chest cavity.
When the diaphragm muscle contracts, the Inhalation
diaphragm flattens downward, and when the
muscle relaxes, the diaphragm arches upward In exhalation, muscles return to their
again. relaxed position. When the air pressure inside
the lungs equals that outside the lungs, the
Inhalation and Exhalation muscles relax and return to their original
position. This movement reduces the size of
Inhalation involves muscle contraction the chest cavity. The size of the chest cavity
in the chest. Breathing begins when the decreases, and the air pressure inside the
diaphragm, the dome-shaped muscle below chest cavity gradually becomes greater than
the chest cavity, contracts and moves the air pressure outside the body. Air leaves
downward. The muscles between the ribs the lungs, again equalizing the pressure. This
also contract, causing the rib cage to move up part of breathing is called exhalation.
and out. Together, these muscle contractions
cause the chest cavity to enlarge. When the When the diaphragm relaxes and moves
chest expands, the air pressure in the chest up, exhalation happens. In this case, the size
cavity drops. The air pressure outside the of the chest cavity decreases, and the air
body is greater than in the chest, causing air pressure increases, thus forcing the air out of
to rush into the lungs to equalize the pressure. the lungs.

4 iScience 9: K to 12 Curriculum Series iScience 9: K to 12 Curriculum Series 5