What happens to air in the lungs when the diaphragm relaxes?

When the diaphragm relaxes, it moves upward, reducing the volume of the thoracic cavity. This decrease in volume creates a higher pressure within the lungs compared to the atmospheric pressure outside the body. As a result, air is expelled from the lungs in a process known as expiration. This mechanism is a key part of the respiratory cycle, allowing for the removal of carbon dioxide-rich air from the body.

In contrast, other options do not accurately represent the physiological process that occurs when the diaphragm relaxes. The pumping of air into the blood, mixing air with digestive juices, and filtering air for impurities are not functions of the diaphragm's relaxation but rather pertain to different systems or processes in the body. Understanding this mechanism is crucial for grasping respiratory physiology.