Breathing In (Inhalation)
When you breathe in, or inhale, your diaphragm contracts (tightens) and
moves downward. This increases the space in your chest cavity, into
which your lungs expand. The intercostal muscles between your ribs also
help enlarge the chest cavity. They contract to pull your rib cage both
upward and outward when you inhale.
As your lungs expand, air
is sucked in through your nose or mouth. The air travels down your
windpipe and into your lungs. After passing through your bronchial
tubes, the air finally reaches and enters the alveoli (air sacs).
Through the very thin walls of the alveoli, oxygen from the air passes
to the surrounding capillaries (blood vessels). A red blood cell protein
called hemoglobin (HEE-muh-glow-bin) helps move oxygen from the air
sacs to the blood.
At the same time, carbon dioxide moves from
the capillaries into the air sacs. The gas has traveled in the
bloodstream from the right side of the heart through the pulmonary
Oxygen-rich blood from the lungs is carried through a
network of capillaries to the pulmonary vein. This vein delivers the
oxygen-rich blood to the left side of the heart. The left side of the
heart pumps the blood to the rest of the body. There, the oxygen in the
blood moves from blood vessels into surrounding tissues.
(For more information on blood flow, go to the Health Topics How the Heart Works article.)
Breathing Out (Exhalation)
When you breathe out, or exhale, your diaphragm relaxes and moves
upward into the chest cavity. The intercostal muscles between the ribs
also relax to reduce the space in the chest cavity.
space in the chest cavity gets smaller, air rich in carbon dioxide is
forced out of your lungs and windpipe, and then out of your nose or
Breathing out requires no effort from your body unless
you have a lung disease or are doing physical activity. When you're
physically active, your abdominal muscles contract and push your
diaphragm against your lungs even more than usual. This rapidly pushes
air out of your lungs.