Alveoli are tiny capillary-rich sacs located in the lungs. The thin epithelial walls and capillaries of each alveolus turn this balloon-like sac into a gas exchange chamber, where oxygen contained in the air we breathe is exchanged for the carbon dioxide contained in the venous blood circulating in the venous capillaries (blue) of each alveolus; these capillaries are tiny branches of the pulmonary artery; carbon dioxide-rich blood is pumped into the lungs by the right ventricle through the pulmonary artery. In the alveoli, oxygen is picked up by iron-rich hemoglobin in the red blood cells of arterial blood circulating in the arterial capillaries (red) of each alveolus; this oxygen-rich blood is then drawn into the heart left atrium during diastole, to be pumped to the rest of the body by the left ventricle during sistole. There are appoximately 300 million alveoli in an adult's lungs.
Alveoli are found in grape-like clusters, which are connected to the bronchi through tiny branches called bronchioles. Typically, the air in two adjacent alveoli is separated by a single alveolar wall. Most of the airfacing surface(s) of the wall are lined by a continuous layer, one cell thick, of flat epithelial cells termed type I alveolar cells. Interspersed between these cells are thicker, specialized cells termed type II alveolar cells that produce a detergent-like substance, surfactant. The alveolar walls contain capillaries and a very small interstitial space, which consists of interstitial fluid and a loose meshwork of connective tissue. In many places, the interstitial space is absent altogether, and the basement membranes of the alveolar-surface epithelium and the capillary-wall endothelium fuse. Thus the blood within an alveolar-wall capillary is separated from the air within the alveolus by an extremely thin barrier. The total surface area of alveoli in contact with capillaries is roughly the size of a tennis court. This extensive area and the thinness of the barrier permit the rapid exchange of large quantities of oxygen and carbon dioxide by diffusion.