What physiological process occurs during exhalation?

During exhalation, the primary physiological process involves the expulsion of carbon dioxide from the body. This occurs as a result of the respiratory system's function to exchange gases. When we breathe in, oxygen enters the lungs and binds to hemoglobin in red blood cells. Throughout this process, the body metabolizes oxygen to produce energy, with carbon dioxide being generated as a waste product.

When it's time to exhale, the diaphragm and intercostal muscles relax, causing the thoracic cavity's volume to decrease, which in turn increases the pressure within the lungs. This pressure differential forces the air, which now contains a higher concentration of carbon dioxide, out of the lungs. Thus, exhalation is crucial for maintaining a balance in blood pH and ensuring that excess carbon dioxide is removed from the body. This is why the expulsion of carbon dioxide is the correct answer, as it reflects the essential function of breathing in maintaining homeostasis.