The epithelial lining is part of the alveolar membrane, also known as the respiratory membrane, that allows the exchange of gases. Microanatomy Īn alveolus consists of an epithelial layer of simple squamous epithelium (very thin, flattened cells), and an extracellular matrix surrounded by capillaries. The diameter of an alveolus is between 200 and 500 μm. Each alveolus is wrapped in a fine mesh of capillaries covering about 70% of its area. Ī typical pair of human lungs contains about 480 million alveoli, providing a total surface area for gas exchange of between 70 and 80 square metres. New alveoli continue to form until the age of eight years. Each duct opens into five or six alveolar sacs into which clusters of alveoli open.Įach terminal respiratory unit is called an acinus and consists of the respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli. The ducts number between two and eleven from each bronchiole. The respiratory bronchioles run for considerable lengths and become increasingly alveolated with side branches of alveolar ducts that become deeply lined with alveoli. The alveoli are first located in the respiratory bronchioles as scattered outpockets, extending from their lumens. Structure Bronchial anatomy showing terminal bronchioles (BT) leading to respiratory bronchioles (BR) and alveolar ducts (DA) that open into alveolar sacs containing out pockets of alveoli (A) separated by alveolar septa (AS). Different structures are involved in gas exchange in other vertebrates. Īlveoli are particular to mammalian lungs. Oxygen is diffused across the membrane into the capillaries and carbon dioxide is released from the capillaries into the alveoli to be breathed out. The alveolar membrane is the gas exchange surface, surrounded by a network of capillaries. The acini are the basic units of respiration, with gas exchange taking place in all the alveoli present. They are located sparsely in these bronchioles, line the walls of the alveolar ducts, and are more numerous in the blind-ended alveolar sacs. Īlveoli are first located in the respiratory bronchioles that mark the beginning of the respiratory zone. Alveoli make up the functional tissue of the mammalian lungs known as the lung parenchyma, which takes up 90 percent of the total lung volume.
Oxygen is exchanged for carbon dioxide at the blood–air barrier between the alveolar air and the pulmonary capillary. A pulmonary alveolus (plural: alveoli, from Latin alveolus, "little cavity"), also known as an air sac or air space, is one of millions of hollow, distensible cup-shaped cavities in the lungs where pulmonary gas exchange takes place.
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