Breathing in and out
The lungs sit in the chest, protected by the rib cage. Intercostal muscles lie between the ribs, and the floor of the chest is the diaphragm. Changing the volume of the chest moves air in and out.
Inhaling
The intercostal muscles contract: the ribs lift and the chest moves forward.
The diaphragm muscles contract and it flattens.
The chest's volume increases, so the lungs' volume increases and air flows in through the nose.
Exhaling
The intercostal muscles relax: the chest and ribs move down to where they were.
The diaphragm muscles relax and it domes upwards again.
The chest's volume decreases, so the lungs' volume decreases and air leaves through the trachea and nose.
Figure to be added