At high magnification, you can identify the cellular components of the alveolus. In HA20 the arrow indicates a type II alveolar cell, which looks foamy because of the weakly stained granules (surfactant) in their cytoplasm. M indicates an alveolar macrophage, which crawl in the air space along the surface of the alveolus. Most macrophages leave the lung, being washed up the conducting system to the pharynx as part of serous and mucous secretions. Some of them remain in the septal walls for years and their contents (largely carbon particles) give the lungs the dark discoloration you may see in the lungs of your cadaver. The capillaries in the thin portions of the alveolar septa (arrowheads) are shared by adjacent alveoli.