Pulmonary Ventilation

The parasympathetic and sympathetic nervous system both innervate the lungs. These control

the dilation and constriction of the airways. The parasympathetic nervous system results in

bronchoconstriction, while the sympathetic nervous system causes bronchodilation. The

autonomic nervous system also helps regulate the cough reflex and the regulation of the

oxygen and carbon dioxide levels in the lungs.

The lining of the lungs is called the pleura. There is a right and left pleural lining that cover the

lungs as the visceral pleura. This is the layer that is closest to the lungs and lines the fissures of

the lobes of the lungs. The parietal pleura is a reflection of the visceral pleura and lines the

inner aspect of the thoracic wall. The space between these two linings is called the pleural

cavity.

The pleurae form two major functions. First, they produce pleural fluid, which is secreted by

mesothelial cells (on both layers) and lubricates the space between the two layers. This reduces

friction between the two layers. It also creates “surface tension” that keeps the two layers in

contact with one another. The pleurae also prevent the spread of infection by separating the

different organs in the chest cavity.

PULMONARY VENTILATION

Pulmonary ventilation is also referred to as the “act of breathing” or the movement of air into

and out of the lungs. There are three pressures involved in the act of breathing: the

atmospheric pressure, the alveolar pressure, and the intrapleural pressure (the pressure inside

the pleural cavity). The act of breathing depends on the relationship between these pressures.

Atmospheric pressure is the amount of force exerted by gases in the air around the body. This

is expressed in millimeters of mercury. Air pressure is about 760 mm Hg, which is also called

“one atmosphere.” This is actually the air pressure that exists in the environment at sea level.

When it comes to pulmonary ventilation, a pressure is said to be “negative” if it is less than

atmospheric pressure and “positive” if it is greater than atmospheric pressure. This means that

at one atmosphere or the air pressure outside, the pressure is considered to be “zero.”

The intra-alveolar pressure is the pressure of air within the alveoli, which changes during the

different phases of breathing. This pressure eventually equalizes with the atmospheric pressure

but, in reality, it changes with the breathing effort. The intrapleural pressure is the pressure

within the pleural cavity—between the visceral and the parietal pleurae. This pressure also

changes during the breathing effort. The intrapleural pressure is always lower than (or more

negative than) the intra-alveolar pressure. While it fluctuates, the mean intrapleural pressure is

-4 mm Hg throughout the cycle of breathing. This is what keeps the lungs and the thoracic wall

together.

The transpulmonary pressure is the difference between the intrapleural pressure and the intra-

alveolar pressures, which determines the lung size. The higher the transpulmonary pressure,

the greater the size of the lungs.

Breathing is dependent upon the contraction and relaxation of the muscle fibers of the thoracic

wall and the diaphragm. Lungs are passive transporters of oxygen and do not actually

participate in the breathing process. They only participate in breathing by being adherent to

the chest wall. The main action of breathing is caused by the diaphragm, which contracts to

cause a pressure change inside the chest cavity. The intercostal muscles participate to a lesser

degree.

The size of the alveoli determines how big and how much the lungs can expand. At some point,

the airway resistance is overcome and the air rushes into the alveoli. The resistance increases

with the smaller size of the bronchioles. The surfactant keeps the surface tension of the alveoli

low enough so that the they do not collapse in the act of expiration. The chest wall, in addition,

must be compliant enough to allow the chest to expand during inhalation.

It is natural for air to flow down a pressure gradient from an area of high pressure to an area of

low pressure. It is the differences in the atmospheric pressure and the intra-alveolar pressure

that draws air into the lungs. When the pressure is greater in the alveoli, there is a natural exit

of air from the lungs.

A respiratory cycle is one inspiration (breathing in) and one expiration (breathing out). The

contraction of the diaphragm causes a relative negative pressure in the alveoli, so air rushes in

to make full inspiration; upon relaxation of the diaphragm, the pressure equalizes again as the

air flows out of the lungs. One can force air out of the lungs actively although this is not the