Air first enters the body through
the mouth or nose, quickly moves to the pharynx
(throat), passes through the larynx (voice box),
enters the trachea, which branches into a left
and right bronchus within the lungs and further
divides into smaller and smaller branches called
bronchioles. The smallest bronchioles end in tiny air sacs, called alveoli,
which inflate during inhalation, and deflate during exhalation.
Gas exchange is the delivery of oxygen from
the lungs to the bloodstream, and the elimination
of carbon dioxide from the bloodstream to the
lungs. It occurs in the lungs between the alveoli
and a network of tiny blood vessels called capillaries,
which are located in the walls of the alveoli.
The walls of the alveoli actually share a membrane
with the capillaries in which oxygen and carbon
dioxide to move freely between the respiratory
system and the bloodstream. Oxygen molecules
attach to red blood cells, which travel back
to the heart. At the same time, the carbon
dioxide molecules in the alveoli are blown out
of the body with the next exhalation.
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