Pulmonary ventilation, commonly referred to as breathing, is the process of air flowing in
and out of the lungs during inspiration and expiration. The air movements are governed
by the principles of gas laws. Basically: - air flows from higher to lower pressure;
- pressure within a cavity increases when its volume decreases, and vice versa;
- volume of a given amount of gas increases with increased temperature.
At rest, in between breaths, the pressure inside the lungs, or intrapulmonary pressure, equals
the pressure outside the body, or atmospheric pressure. When discussing respiratory pressures,
this is generally referred to as a relative pressure of zero. This is because what matters
is the difference between the two pressures, not their absolute values. Thus, a negative pressure
is a pressure below atmospheric, while a positive pressure is above atmospheric.
The lungs are covered in a double-layer membrane, which forms a thin space surrounding the lungs,
called the pleural cavity. The pressure within the pleural cavity, or intrapleural pressure,
is normally negative. This negative pressure acts like a suction to keep the lungs inflated.
If this becomes zero, such as in the case of pneumothorax, when the chest wall is punctured
and the pleural cavity has the same pressure as the outside air, the lung would collapse.
Pulmonary ventilation is achieved by rhythmically changing the volume of the thoracic cavity. During
inspiration, the diaphragm and the external intercostal muscles contract, expanding the
thoracic cavity and the lungs. This increase in volume results in a decrease in pressure,
causing outside air to flow in. Another factor that helps to inflate the lungs is the warming
of the inhaled air. This effect is most notable on a cool day, when the temperature outside is
significantly lower, the inhaled air increases in volume as it warms up inside the body and inflates
the lungs, further facilitating inhalation. While inspiration requires muscular contraction
and hence energy expenditure, expiration during quiet breathing is a passive process. As the
diaphragm returns to its original position and the muscles relax, thoracic and lung volumes decrease
and pressures increase, pushing air out of the lungs. Quiet expiration relies therefore on
the elasticity of the lungs and rib cage - their ability to spring back to the original dimensions.
Conditions that reduce pulmonary elasticity, such as emphysema, can cause difficulty exhaling.
Deep breathing requires more forceful contractions of the diaphragm, intercostal muscles,
and involves additional muscles to produce larger changes in the thoracic volume. Deep expiration,
unlike quiet expiration, is an active process. Another factor that affects ventilation is the
resistance to airflow, which exists within the lung tissues and in the airways. Lung compliance
refers to the ease with which the lungs expand. Healthy lungs normally have high compliance, low
resistance, like a thin balloon, easy to inflate. Lung compliance is reduced when the lungs become
“stiff”, in conditions that cause scarring of lung tissues, or fibrosis. In this case the lung
turns into a thick balloon, harder to inflate. Diseases that narrow the airways, such as asthma,
increase resistance, making it harder to breathe. The airways may also dilate or
constrict in response to various factors. For example, parasympathetic stimulation
and histamine typically narrow the bronchioles, increase resistance and decrease airflow; while
epinephrine, a hormone released during exercises, dilates bronchioles and thereby increases airflow.