Inspiration is brought about by contraction of diaphragm muscles and some intercostals muscles. The diaphragm moves downward and the intercostals muscles. Move the lateral walls of thorax outward and upward. The volume of thorax increases and the air pressure is decreased. So, air is drawn into the lungs.
Expiration is brought about when the contracted muscles of diaphragm and intercostals muscles relax, the diaphragm moves upward and the lateral walls, move inward and downward. This decreases the volume of thorax and increases the air pressure. SO, air is sent out of lungs.