The normal lung as seen by surface stereomicroscopy1 is made up of a series of saccules separated from one another by a thin, transparent, interalveolar wall. The bulk of this wall is composed of large and small capillaries so arranged as to allow for the largest possible exposure of blood to the alveolar gases. Under normal conditions air enters the alveoli through the terminal bronchiole, and with each expiration a part or the whole of this air is expelled back into the bronchial tree and thence outward.
When inflammation occurs in the alveoli, it has its origin in the vascular tree, in the inner wall of the alveolus or, by extension, in the pleural surface. Regardless of origin the steps of inflammation in the alveolus are practically the same as those in inflammation in any other part of the body. In other words, one would expect hyperemia followed by swelling
JOANNIDES M, STEINMAN FH. THE MECHANISM OF PNEUMONIA. Arch Intern Med (Chic). 1931;47(1):24-27. doi:10.1001/archinte.1931.00140190033003