The diaphragm divides the abdomen from the chest. In breathing, its contraction lowers the floor of the chest and causes an increase in chest volume. At the same time some of the intercostal muscles, between the ribs, contract and lift the ribs upwards and outwards causing a further increase in chest volume. These increases in volume decrease the air pressure in the lungs, and air at atmospheric pressure is drawn into the lungs as we inhale.
