When we breath in the intercostal muscles and the diaphram contract. This means the diaphram flattens & the rib cage & sternum move up and out. This makes the volume of the thorax increase in size which causes a decrease in pressure. Air is then drawn into the lungs
Breathing out occurs when the intercostal muscles & the diaphram rises. This means that the ribs and Sternum move in and down, as a result the volume of the thorax decreases and the pressure increases meaning that the air is forced out of the lungs.