How to make an alveolus.

A lveoli are the hallmark of lung structure. In 1959, when I embarked on studies of lung structure that ‘‘should interest physiologists’’, the first question I was asked was ‘‘how many alveoli are there in the human lung?’’ Along with Prof. D. Gomez, I counted alveoli by devising a now obsolete method [1], only to realise that what is of real functional importance is the design of the wall of these small chambers, such as their surface area and their provision with capillaries [2]. However, the fascination with the notion of alveoli has remained, and a new unbiased method for counting them has now shown that the human lung is made of ,480 million of these small bubbles [3], all openly connected to the terminal branches of the airway tree that constitute the pulmonary acinus.