Question

Describe the events of quiet (tidal) ventilation, including how the contraction and relaxation of muscles changes the shape of the chest cage, how that changes the relative pressures, and how the air moves in response to that.

Answer 1

During quiet ventilation, following events occur-

During resting state, the intra pleural pressure is negative as compared to atmospheric pressure. During inspiration, the inspiratory muscle contract. These muscles include diaphragm, sternocleidomastoid, scalene and external intercostal muscles. Contaction of these muscle cause increase in volume of thoracic cavity. This increase volume cause more decrease in intra pleural pressure. This pressure difference cause movement of air inside the lungs.

Now during expiration, the inspiratory muscles relax. This cause restoration of thoracic cavity back to orignal position. Due to this , the volume of thoracic cavity decrease and pressure increases ( it comes back to normal as in resting state). This increases in pressure cause movement of air outside lungs and expiration occurs