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.
Wallflower, wallflower
Won't you dance with me?
I'm sad and lonely too.
Wallflower, wallflower
Won't you dance with me?
I'm fallin' in love with you.
Just like you I'm wondrin' what I'm doin' here.
Just like you I'm wondrin' what's goin' on.
Wallflower, wallflower
Won't you dance with me?
The night will soon be gone.
I have seen you standing in the smoky haze
And I know that you're gonna be mine one of these days,
Mine alone.
Wallflower, wallflower
Take a chance on me.