Pulmonary alveolus

A pulmonary alveolus (pl. alveoli; from Latin alveolus 'little cavity'), also called 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. Oxygen is exchanged for carbon dioxide at the blood–air barrier between the alveolar air and the pulmonary capillary. 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.

Alveoli are first located in the respiratory bronchioles that mark the beginning of the respiratory zone. They are located sparsely in these bronchioles, line the walls of the alveolar ducts, and are more numerous in the blind-ended alveolar sacs. The acini are the basic units of respiration, with gas exchange taking place in all the alveoli present. The alveolar membrane is the gas exchange surface, surrounded by a network of capillaries. Oxygen is diffused across the membrane into the capillaries and carbon dioxide is released from the capillaries into the alveoli to be breathed out.

Alveoli are particular to mammalian lungs. Different structures are involved in gas exchange in other vertebrates.

The alveoli are first located in the respiratory bronchioles as scattered outpockets, extending from their lumens. The respiratory bronchioles run for considerable lengths and become increasingly alveolated with side branches of alveolar ducts that become deeply lined with alveoli. The ducts number between two and eleven from each bronchiole. Each duct opens into five or six alveolar sacs into which clusters of alveoli open.

Each terminal respiratory unit is called an acinus and consists of the respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli. New alveoli continue to form until the age of eight years.

A 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. Each alveolus is wrapped in a fine mesh of capillaries covering about 70% of its area. The diameter of an alveolus is between 200 and 500 μm.

An alveolus consists of an epithelial layer of simple squamous epithelium (very thin, flattened cells), and an extracellular matrix surrounded by capillaries. The epithelial lining is part of the alveolar membrane, also known as the respiratory membrane, that allows the exchange of gases. The membrane has several layers – a layer of alveolar lining fluid that contains surfactant, the epithelial layer and its basement membrane; a thin interstitial space between the epithelial lining and the capillary membrane; a capillary basement membrane that often fuses with the alveolar basement membrane, and the capillary endothelial membrane. The whole membrane however is only between 0.2 μm at its thinnest part and 0.6 μm at its thickest.

In the alveolar walls there are interconnecting air passages between the alveoli known as the pores of Kohn. The alveolar septum that separates the alveoli in the alveolar sac contains some collagen fibers and elastic fibers. The septa also house the enmeshed capillary network that surrounds each alveolus. The elastic fibres allow the alveoli to stretch when they fill with air during inhalation. They then spring back during exhalation in order to expel the carbon dioxide-rich air.