Following an opening segment on the importance of oxygen to the human body, this program studies the structure and function of the respiratory system. Topics include the anatomy of the upper respiratory tract, the lower respiratory tract, and the alveoli; the diffusion of gas molecules through the walls of the alveoli and capillaries, and the effect of high altitude on the diffusion process; and the mechanics of breathing via the bulk flow transport process. In addition, the program describes three factors that help prevent lung collapse and the role of the medulla oblongata in regulating breathing. A viewable/printable instructor’s guide is available online. A Films for the Humanities & Sciences Production. A part of the series The Human Body: How It Works. (20 minutes)