1.31. Respiration http://www.ck12.org
FIGURE 1.55
Being able to control breathing is impor-
tant for many activities, such as swim-
ming. The woman in the photograph is
exhaling as she exits the water.bottom of the rib cage. The diaphragm and rib muscles contract and relax to move air into and out of the lungs.
During inhalation, the diaphragm contracts and moves downward. The rib muscles contract 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 lower 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 similar to letting the air out
of a balloon.
How does the inhaled oxygen get into the bloodstream? The exchange of gasses between the lungs and the blood
happens in tiny sacs calledalveoli. The walls of the alveoli are very thin and allow gases to pass though them. The
alveoli are lined with capillaries (Figure1.56). Oxygen moves from the alveoli to the blood in the capillaries that
surround the alveoli. At the same time, carbon dioxide moves in the opposite direction, from capillary blood to the
alveoli.
FIGURE 1.56
The bronchi and alveoli. During respi-
ration, oxygen gets pulled into the lungs
and enters the blood by passing across
the thin alveoli membranes and into the
capillaries.