5. Explain the mechanism of ventilation of the lungs in terms of volume and pressure changes caused by the internal and external intercostal muscles, the diaphragm and abdominal muscles

https://sites.google.com/site/niosorgin/college/degree-course/bachelor-of-dental-surgery-bds/syllabus-for-bds-programmes/bds-entrance-exam/mechanics-of-breathing-or-micturition 

When we inhale (breath in), the external intercostal muscles contracts and moves the rib cage up and out. As abdominal muscles relax, the diaphragm contracts - moves down - and becomes flat, allowing more air inside. The muscle activities increase the volume of the thorax and its pressure drops. At last, air flows from the outside into the body/lungs until the pressure inside the lungs rises back.

For exhaling (breath out), the opposite happens: the internal intercostal muscles contracts and moves the rib cage down and in. Abdominal muscles contract and the diaphragm moves up. The muscle activities decrease the volume of the thorax and the pressure rises. Than, air flows out from the lungs to the environment until the pressure inside the lungs falls.