Long description

(a) shows an artificially coloured electron micrograph of an alveolus with purple walls; cavities in the walls show capillaries in cross-section, some of them containing erythrocytes coloured red. Within the alveolus is a large pulmonary macrophage (coloured pale green) with many processes.

(b) is a diagram based on (a) above. The alveolus is shown as a half of a hollow sphere with pale blue epithelial cells lining it. Darker blue macrophages with branching processes are seen on the alveolar walls. Also in the alveolar walls black alveolar pores are shown. Within the wall of the alveolus, comprising the respiratory membrane, capillaries are seen in cross-section, containing erythrocytes (red blood cells).