just follow me ^_^

1. Alveoli

2.  Alveoli exchange oxygen with red blood cells
when the heart's Sinoatrial and
Atrioventricular nodes send electric signals to
the hearts ventricles. The right side of the
heart then contracts twice,[source?] sending
blood shooting into the pulmonary circulatory
system, where capillary beds are located in
the lungs. The Alveoli then exchange oxygen
and discard carbon dioxide with the red blood
cells, which then return to the heart's left
atrium.

3.  The singular of alveoli is one alveolus. Alveoli are tiny
grape-like sacs where gas exchange takes place in the
lungs (oxygen and carbon dioxide). When you breathe out,
the body delivers carbon dioxide to the alveoli, and you
release it in your exhalation. When you breathe in, oxygen
fills the alveoli and then enters the blood, so it can be
delivered to the rest of the body. In asthma there is no
damage to the alveoli, which is different from another
common lung disease which is called: chronic obstructive
pulmonary disease, in which alveoli are damaged. A
network of blood capillaries surround walls of each
alveolus.The walls are extremely thin(one cell thick)and
moist,thus allowing gaseous diffussion through them.
There are over 300 million alveoli in each lung. If you were
to spread one person's alveoli across a tennis court, they
would cover over half the court!