An alveolus (plural: alveoli, from Latin alveolus, "little cavity") is an anatomical structure that has the form of a hollow cavity. Found in the lung parenchyma, the pulmonary alveoli are the terminal ends of the respiratory tree, which outcrop from either alveolar sacs or alveolar ducts, which are both sites of gas exchange with the blood as well. Alveoli are particular to mammalian lungs. Different structures are involved in gas exchange in other vertebrates. The alveolar membrane is the gas-exchange surface. Carbon dioxide rich blood is pumped from the rest of the body into the alveolar blood vessels where, through diffusion, it releases its carbon dioxide and absorbs oxygen.
The alveoli are located in the respiratory zone of the lungs, at the distal termination of the alveolar ducts and atria. These air sacs are the forming and termination point of the respiratory tract. They provide total surface area of about 75 m2.
A typical pair of human lungs contain about 700 million alveoli, producing 70m2 of surface area. Each alveolus is wrapped in a fine mesh of capillaries covering about 70% of its area. An adult alveolus has an average diameter of 200 micrometres, with an increase in diameter during inhalation.
i think of all the things we did
back when we were alive
i watched around the corner
as you rode up on your bike
i sat with you and held your hand
when movies made you cry
i think of all the things we did
back when we were alive
so many things to believe in
so much room to spread our wings
the joy we found in every little thing
i never will forget the day
you heard your brother died
i'm sorry now i couldnt help more
while you sat and cried
the letters that you sent me
that summer you were gone
i couldnt wait for your return
i couldnt wait that long
swimming holes to sing in
all the joy we found
open fields we'd go to dream
in every little thing
every little thing
i think of all the things we did
back when we were alive
and kissed you when you cried
when we made love i held you close
im sorry that it had to end
im sorry youre not mine
ill be with you until the end
in dreams of you tonight
ill be with you until the end
in dreams of you tonight
i think of all the things we did