Air movement into and out of the lungs is a purely mechanical process driven by pressure differences. Air always flows from an area of higher pressure to an area of lower pressure, a fundamental principle of gas dynamics. The body uses the respiratory muscles to change the volume of the chest cavity, which in turn manipulates the air pressure inside the lungs. This manipulation creates the necessary gradients, allowing air to flow spontaneously either into or out of the respiratory system. The atmospheric pressure surrounding the body acts as the constant reference point for these internal pressure changes.
Understanding the Pressure Gradients of Respiration
Breathing depends on the interplay of three primary pressures. Atmospheric Pressure (\(P_{atm}\)) is the force exerted by the air outside the body, standardized at 760 mmHg at sea level. The Intrapulmonic Pressure (\(P_{alv}\)), or intra-alveolar pressure, is the pressure of the air within the alveoli. This pressure fluctuates constantly during the breathing cycle, but it equalizes with atmospheric pressure between breaths.
The third pressure, Intrapleural Pressure (\(P_{pl}\)), is measured within the pleural cavity, the space between the lungs and the chest wall. This pressure is always negative relative to the intrapulmonic pressure, remaining about -4 mmHg below \(P_{alv}\) throughout the breathing cycle. This persistent negative pressure acts like a suction, holding the lungs against the chest wall and preventing them from collapsing due to their natural elastic recoil.
The mechanism that allows these internal pressures to change is described by Boyle’s Law. This law states that the pressure of a gas is inversely proportional to its volume, assuming temperature is constant. Therefore, if the volume of the lungs increases, the pressure inside them must decrease, and if the volume decreases, the internal pressure must rise. This physical relationship is the basis for all air movement in and out of the body.
The Mechanics of Quiet Inspiration
The process of quiet inspiration, or breathing in at rest, is considered an active process because it requires muscle contraction. To bring air into the lungs, the Intrapulmonic Pressure (\(P_{alv}\)) must drop to a value lower than the Atmospheric Pressure (\(P_{atm}\)). The primary inspiratory muscle, the diaphragm, contracts and flattens, moving downward into the abdominal cavity.
Simultaneously, the external intercostal muscles contract, lifting the ribs and sternum upward and outward. These coordinated muscle movements significantly increase the volume of the thoracic cavity. Because the lungs are held tightly to the chest wall by the negative intrapleural pressure, the lungs are stretched and their volume increases. According to Boyle’s Law, this increase in lung volume causes the intrapulmonic pressure to fall to approximately 758 mmHg. The resulting pressure difference of -2 mmHg relative to the atmosphere creates a gradient, causing air to rush into the lungs until \(P_{alv}\) once again equals \(P_{atm}\).
What the 763 mmHg Pressure State Means
The state where Intrapulmonic Pressure (\(P_{alv}\)) is 763 mmHg signifies that the pressure inside the lungs is 3 mmHg higher than the external Atmospheric Pressure (\(P_{atm}\) of 760 mmHg). This positive pressure gradient means that air will immediately flow out of the alveoli and out of the body. Therefore, an intrapulmonic pressure of 763 mmHg occurs during the phase of quiet expiration.
Quiet expiration is typically a passive process that relies on the relaxation of the inspiratory muscles. The diaphragm and external intercostals relax, causing the thoracic cavity to decrease in volume. The elastic properties of the lung tissue and chest wall, which were stretched during inspiration, cause them to recoil back to their resting size. This decrease in lung volume compresses the air inside the alveoli, raising the Intrapulmonic Pressure to a value like 763 mmHg. Air flows out of the lungs down this pressure gradient until the pressures equalize again at 760 mmHg, marking the end of the breath.
Regulation of Respiratory Flow
While the pressure gradient determines the direction of air flow, two mechanical factors govern the efficiency and speed of that flow. Airway resistance is the opposition to the flow of air caused by friction within the respiratory passages. The diameter of the bronchioles is the most significant factor affecting resistance, as smaller airways create more friction and slow the movement of gas. The body can regulate this resistance by constricting or dilating the smooth muscle surrounding the bronchioles.
Another factor is lung compliance, which is a measure of the lung’s ability to stretch and expand in response to a change in pressure. High compliance means the lungs are easily inflated, like a thin balloon, while low compliance indicates a stiff lung that requires more force to expand. Compliance is largely determined by the elasticity of the lung tissue and the surface tension within the alveoli. These two factors influence how effectively the small pressure differences, such as the 3 mmHg positive pressure during expiration, can move the required volume of air.