During exhalation, the relaxation of respiratory muscles leads to what effect on the thorax and air pressure?

• A. Decreased air pressure and expanded thorax
• B. Increased air pressure and compressed thorax
• C. No change in air pressure or thorax
• D. Increased air pressure and expanded thorax

Answer: (B)

During exhalation, the process begins with the relaxation of respiratory muscles, particularly the diaphragm and intercostal muscles. As these muscles relax, the diaphragm moves upwards, and the ribcage descends, leading to a decrease in the volume of the thoracic cavity. This decrease in volume results in an increase in air pressure within the thoracic cavity compared to the atmospheric pressure outside the body.

Simultaneously, the compression of the thorax occurs as the lungs are pushed inward, helping to expel air from the lungs. This dynamic is crucial for efficient breathing, allowing carbon dioxide-rich air to be pushed out of the lungs and facilitating gas exchange with fresh air during the subsequent inhalation.

Thus, the correct understanding involves recognizing that the relaxation of the muscles during exhalation compresses the thorax and leads to increased air pressure. This relationship is essential in understanding the mechanics of breathing and the physiological principles governing respiratory function.