When you breathe in, 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 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. (Oxygen is especially drawn to hemoglobin.)
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 artery.
Oxygen-rich blood from the lungs is carried through
a network of capillaries, which become 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, see the
Diseases and Conditions Index
"How
the Heart Works" article.)
Breathing Out (Exhalation)
When you breathe out, your diaphragm relaxes and
moves upward into the chest cavity. The intercostal muscles between the ribs
also relax to make the chest cavity size smaller.
As 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
mouth.
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 even
more so against your lungs. This pushes the air in your lungs out rapidly.
The animation below shows how the lungs work. Click
the "start" button to play the animation. Written and spoken explanations are
provided with each frame. Use the buttons in the lower right corner to pause,
restart, or replay the animation, or use the scroll bar below the buttons to
move through the frames.
The animation shows how the lungs
inhale oxygen and transfer it to the blood. It also shows how carbon dioxide (a
waste product) is removed from the blood and exhaled.