Sturcture of Alveoli

In mature adults, the alveoli are found in bunches of anywhere from 2 to 5 alveoli, situated at the terminal ends of the terminal bronchioles in polyhedral sacs called atria (1). Generally each individual alveolus is in the vicinity of 300 µm in diameter (1), and with approximately 300 million alveoli found in the lungs of a 70 kg man, they provide a total surface area of roughly 143 square meters for gas exchange (2). This gas exchange of carbon dioxide and oxygen within the alveoli during respiration, occurs across what is referred to as the air-blood barrier. The pulmonary capillary network is the most extensive network of capillaries within the body, and covers rough 85-95% of the surface of the alveoli (2). In fact, it has a surface area of 126 square meters! Which is 70 times the surface area of skin! (2)

The interalveolar walls of the alveoli are made from an epithelium which is covering a highly vascularized space of connective tissue (1). This connective tissue space includes the following types of structures (1):

• Reticular fibers
• Elastic fibers
• Fibroblasts
• Macrophages
• Mast cells (although not abundant)
• Lymphocytes
• And Eosinophil Leukocytes

Figure 3: Structure of Alveoli (Retrieved from

http://academic.kellogg.cc.mi.us/herbrandsonc/bio201_McKinley/f25-10a_alveoli_and_the_c.jpg)

Also, as a result of their development, alveoli also have pores which connect them to other alveoli which are adjacent. These pores are generally 7-9µm in diameter and allow for the collateral circulation of air, which aids to prevent the collapse of the alveoli in the event of a proximal blockage (1). It may also be noted, that these pores arise in conjunction with maturity, and are not found in the fetal lung (2).