inhalation

Meaning: The diaphragm contracts and flattens, and the external intercostal muscles lift the ribs up and out. Thoracic volume rises, intrapleural pressure becomes more negative, and the lungs expand with the chest wall. Alveolar pressure falls below atmospheric pressure, so air flows down its pressure gradient into the lungs. This is called negative pressure breathing. In psychology it appears only indirectly, for example as the route by which inhaled drugs reach the brain quickly.

Equation: P₁V₁ = P₂V₂, where P is gas pressure in pascals (Pa) and V is gas volume in cubic meters (m³). This is Boyle's law, which holds at constant temperature and amount of gas.

On the MCAT: Passages describe a bell jar lung model, a spirometry tracing, or a pressure-versus-time graph. You predict that when thoracic volume increases, pressure decreases, and air moves in. Questions may also contrast this with a positive pressure ventilator, which pushes air in. The medulla oblongata sets the breathing rhythm, and a rise in blood CO₂ (lower pH) increases the rate.

Common confusion: Students think the lungs pull air in on their own, or that falling pressure causes the volume change. The muscles change the volume first, and pressure follows. Lungs contain no skeletal muscle and follow the chest wall because of pleural fluid cohesion. Quiet exhalation is passive elastic recoil, but forced exhalation recruits the internal intercostals and abdominal muscles.