Respiration, or the uptake of O₂ and removal of CO₂ from the body as a whole, is the primary goal of the lung. At rest, a normal human breathes 12–15 times a minute. With each breath containing ~500 mL of air, this translates to 6–8 L of air that is inspired and expired every. Once the air reaches the depths of the lung in the alveoli, simple diffusion allows O₂ to enter the blood in the pulmonary capillaries and CO₂ to enter the alveoli, from where it can be expired. Using some basic math, on average, 250 mL of O₂ enters the body per minute and 200 mL of CO₂ is excreted. In addition to the O₂ that enters the respiratory system, inspired air also contains a variety of particulates that must be properly filtered and/or removed to maintain lung health. Finally, although humans have a certain amount of control over breathing, most of the minute to minute function, including the fine adjustments necessary for proper lung function, are accomplished independent of voluntary control. The goal of this section is to review basic concepts that underlie important aspects of the control and outcome of breathing as well as to highlight other important functions in respiratory physiology.