Why does air enter the lungs upon inhalation?

During inhalation, the diaphragm (the primary respiratory muscle) contracts and moves downward, while the intercostal muscles (muscles between the ribs) also contract, pulling the rib cage upward and outward. These combined movements increase the volume of the thoracic cavity, causing the pressure inside the lungs to decrease relative to the atmospheric pressure outside. As a result, air flows into the lungs through the airways (trachea, bronchi, and bronchioles) to equalize the pressure. This process ensures that oxygen-rich air reaches the delicate air sacs (alveoli) in the lungs, where gas exchange takes place during respiration.