Mechanics of Breathing - Physiology

The external intercostal muscles and diaphragm contract.

The rib cage moves up and out and the diaphragm flattens.

The volume of the thoracic cavity increases therefore the pressure of the thoracic cavity decreases becuase volume and pressure are directly poportional. 

The pressure decreases because the air pressure in the lungs is lower than the atmospheric pressure outside so air rushes into the lungs because of diffusion. 

The external intercostal muscles and the diaphragm relax.

The diaphragm returns to a dome shape and the rib cage moves down and in. 

The volume of the thoracic cavity decreases so the pressure increases.

Because the air pressure in the lungs is higher than the atmospheric pressure outside the air diffuses out of the lungs. 

Inspiration:

The sternocliedomastoid contarcts to help increase the volume of the thoracic cavity.

The volume increases much more than at rest    and     the pressure decraeses much more than at rest. 

Expiration:

The internal intercostal muscles contract.

The rectus abdominus contracts in order to aid expiration.

Rib cage moves down and in more rapidly.

Volume of the thoracic cavity decreases more than at rest.

Pressure od the thoracic cavity increases more than at rest.