According to Boyle's law, as volume decreases, pressure , and as volume increases, pressure . Air always moves from an area of high pressure to an area of pressure. During , the pressure in the alveoli is less than the atmospheric pressure. During , the atmospheric pressure is less than the alveolar pressure. When the diaphragm muscle contracts, the pressure in the alveoli . As the intercostal muscles relax and the thoracic cavity becomes smaller, the pressure in the alveoli .