Cycle of Respiration

This occur 12 to 15 times per minute and consists of three phase:

1. Inspiration
2. expiration
3. pause

Inspitation :

When the capacity of thoracic cavity is increased by simultaneously contraction of the intercostal muscles and the diaphragm, the parietal pleura moves with the walls of the thorax and diaphragm. The reduces the pressure in pleural cavity to a level considerably lower than atmospheric pressure. The visceral pleura follows the parietal pleura pulling the lungs with it. this stretches the lungs and the pressure within the alveoli and in the air passage falls, drawing air  into the lungs in an attempt to equalize the atmospheric and alveolar air pressures.

The process of inspiration is ACTIVE, as it requires expenditure of energy for muscle contraction. The negative pressure created in the thoracic cavity aids venous return to the Heart and is known as the respiratory pump.

Expiration:

Relaxation of intercostal muscle and the the diaphragm results in downward and inward movement of the ribs cage and elastic recoil of the lungs. As this occurs, pressure inside the lungs exceeds that in the atmosphere and therefore air is expelled from the Respiratory tract. The lungs still contain some air and are prevented from complete collapse by intact pleura. This process is passive as it does not required expenditure of energy.

After expiration, there is pause before the next cycle begins.