alveolus

Alveolus is a Latin diminutive of alveus, a word meaning a channel, trough, or the hollow of a ship's hull, itself from alvus, denoting the belly or any enclosed cavity. The diminutive suffix -ulus made it a small hollow: the pit in a gaming board, the cell of a beehive, the socket of a tooth. Pliny the Elder used alveolus in his Natural History of 77 CE for a shallow tray, and Marcus Varro had used it a century earlier for hive cells in his treatise on farming. The word named any shallow depression that held something else inside it.

Roman physicians applied alveus and its derivatives to body cavities, but the specific anatomical assignment to the lung's air sacs came late. Marcello Malpighi of Bologna made the discovery that mattered in 1661, when he examined frog lung under a microscope and described small membranous sacs filled with air. He wrote his findings in Latin letters to the Royal Society in London, calling the structures vesiculae (little bladders) and utriculi, reaching for vocabulary rather than settling on alveolus. The term for pulmonary air sacs consolidated over the following century as histology developed its own standard language.

William Hewson, the English anatomist who worked with William Hunter in London during the 1770s, described the pulmonary alveoli with enough precision to distinguish them as functional units. By then, the connection between the lung's surface area and respiratory efficiency was becoming clear to physiologists. The 1895 Basle Nomina Anatomica codified alveolus as the standard anatomical term for both the lung's air sacs and the tooth sockets of the jaw: two entirely different structures sharing one Latin word for hollow. Dentists and pulmonologists had unknowingly inherited the same term.

The human lung contains between 300 and 500 million alveoli, each roughly 0.2 millimeters in diameter. Together they form a gas-exchange surface of about 70 square meters, the size of a singles tennis court. The wall separating alveolar air from the blood in the surrounding capillaries is 0.2 micrometers thick, the thinnest functional tissue in the body. Emphysema destroys these walls, merging small alveoli into large useless spaces, and the surface area shrinks as oxygen delivery falls.