Inspiration is an active process. The medulla cervical plexus phrenic nerve( medulla, cervical plexus, phrenic nerve ) sends and impulse down the cervical plexus medulla phrenic nerve( cervical plexus, medulla, phrenic nerve ) to the phrenic nerve medulla cervical plexus( phrenic nerve, medulla, cervical plexus ), stimulating the diaphragm to contract. Because the phrenic nerve medulla cervical plexus( phrenic nerve, medulla, cervical plexus ) receives impulses through the cervical plexus medulla phrenic nerve( cervical plexus, medulla, phrenic nerve ) at the level of the fourth cervical nerve, spinal injury between the level of the 3rd and 5th cervical nerves will affect ventilatory function.

When the muscles of inspiration contract to create a larger space within the chest, the parietal pleura visceral pleura intrapleural space( parietal pleura, visceral pleura, intrapleural space ) moves outward, increasing negative pressure in the intrapleural space visceral pleura parietal pleura( intrapleural space, visceral pleura, parietal pleura ). Intrapleural pressure drops from a resting pressure of -4 cm H2O to -10 cm H2O. This negative pressure creates pull on the visceral pleura parietal pleura intrapleural space( visceral pleura, parietal pleura, intrapleural space ) and the lungs expand. At this point, pressure inside the lungs is less than atmospheric pressure, so air moves into the lung.

During expiration, the muscles of the thorax relax, compressing the lungs and increasing intrapulmonary pressure. When intrapulmonary pressure becomes greater than atmospheric pressure, air moves out of the lungs.

Alveoli may be ventilated, but have no corresponding blood flow to pick up oxygen, as in the case of pulmonary emboli. This is also known as alveolar dead space, or a high V/Q shunt low V/Q shunt( high V/Q shunt, low V/Q shunt ).

Occurs when adequate blood flow surrounds underventilated alveoli. The clinical effect of low V/Q high V/Q shunt( low V/Q, high V/Q shunt ) is similar to shunting, and may result from similar pathology.