Final answer:
Air enters the lungs due to negative pressure generated by contraction of the diaphragm and the external intercostal muscles during inhalation. This increase in thoracic cavity volume decreases lung pressure below atmospheric levels, drawing air into the lungs.
Step-by-step explanation:
Air flows into the lungs because of negative pressure created by diaphragm contraction. During inhalation, as the diaphragm contracts and moves downward, it increases the volume of the thoracic cavity. The external intercostal muscles also contract, lifting the rib cage and further expanding the chest. This expansion leads to a decrease in pressure inside the lungs, making it lower than the atmospheric pressure, which causes air to rush in to equalize the pressure.
The relaxation of these muscles during expiration results in the opposite effect; the recoil of the lung tissue raises the pressure in the lungs above that of the atmosphere, pushing the air out. The contraction and relaxation of these muscles thus play a crucial role in the process of pulmonary ventilation, facilitating the movement of air in and out of the lungs.