Using the 40X objective, carefully examine walls of alveoli (Fig. 17-14).

• Identify macrophages (dust cells),
• The flattened type I pneumocytes, and
• The rounded type II pneumocytes (Fig. 17-14).
• Not many nuclei of type I cells will be visible. Why?

Study the electron micrographs of the alveolar wall (Fig. 17-15), the air-blood barrier, across which gas exchange occurs (Fig. 17-13), and the surfactant-producing type II pneumocytes (Fig. 17-16). Also identify and note the location of capillaries around every alveolus.

Specifically, across which cells does gas exchange occur in the lungs and what is the physiological role of the other cells in alveoli?

Make a sketch at high magnification of an alveolus showing the components of the wall separating air and blood.

Where might “dust cells” (alveolar macrophages) be most common?

Now comes the urinary system.