Inhalation occurs when which of the following events takes place?

• A. Contraction of the diaphragm and intercostal muscles causes a decrease in intrathoracic pressure.
• B. Relaxation of the diaphragm increases intrathoracic pressure.
• C. The chest wall becomes rigid and prevents air entry.
• D. Oxygen diffuses into the bloodstream.

Answer: (A)

Inhalation happens when the muscles used for breathing contract, expanding the chest cavity. When the diaphragm contracts, it moves downward and, along with the intercostal muscles lifting the ribs, increases the volume of the thoracic cavity. This expansion lowers intrathoracic pressure below the surrounding atmospheric pressure, drawing air into the lungs. That active drop in pressure is what pulls air in.

Relaxing the diaphragm would decrease chest space or increase intrathoracic pressure, which pushes air out rather than in, so it’s associated with exhalation, not inhalation. If the chest wall were rigid and blocked air entry, inhalation could not occur. Oxygen diffusing into the bloodstream is a downstream step that happens after air reaches the alveoli, not the mechanical event of drawing air in.