and cause the ribs to move outward. This causes the chest volume to increase. Because
the chest volume is larger, the air pressure inside the lungs is less than the air pressure
outside. This difference in air pressures causes air to be sucked into the lungs. When the
diaphragm and rib muscles relax, air is pushed out of the lungs. Exhalation is normally a
passive process, similar to letting the air out of a balloon.
The walls of the alveoli are very thin and are permeable to gases. The alveoli are lined with
capillaries, the walls of which are also thin enough to allow gas exchange. These capillaries
are shown inFigure19.4. Oxygen diffuses from the alveoli to the blood in the capillaries
thatsurroundthealveoli. Atthesametime, carbondioxidediffusesintheoppositedirection,
from capillary blood to the alveoli. At this point, the pulmonary blood is oxygen-rich, and
the lungs are holding carbon dioxide. Exhalation follows, thereby ridding the body of the
carbon dioxide and completing the cycle of respiration.
Figure 19.4: The bronchi and alveoli; during respiration, oxygen gets pulled into the lungs
and enters the blood by passing across the thin alveoli membranes and into the capillaries.
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Breathing and Respiration
When you breath in, oxygen is drawn in through the mouth and down into the lungs. The
oxygen then passes across the thin lining of the capillaries and into the blood. The oxygen
molecules are carried to the body cells by the blood. Carbon dioxide from the body cells is
carried by the blood to the lungs where it is released into the air. The process of getting
oxygen into the body and releasing carbon dioxide is calledrespiration.