A 48-year-old coal miner complains of shortness of breath and a productive cough. He has smoked one to two packs of cigarettes per day since he was 16 years old. Pulmonary function studies are ordered, including an esophageal balloon study to measure intrapleural pressures. Normally, intrapleural pressure is negative throughout a tidal inspiration and expiration because of which of the following?

a. The lungs have the tendency to recoil outward throughout a tidal breath.
b. The chest wall has the tendency to recoil inward throughout a tidal breath.
c. The lungs and chest wall recoil away from each other throughout a tidal breath.
d. The lungs and chest wall recoil in the same direction throughout a tidal breath.
e. A small volume of air leaves the pleural space during a tidal breath.