Intrathoracic pressure refers to the pressure within the chest cavity, a dynamic force in bodily functions. This pressure is typically negative, meaning it is lower than the atmospheric pressure outside the body. This sub-atmospheric pressure facilitates crucial physiological processes, particularly breathing and blood circulation.
Anatomy of the Thoracic Cavity
The thoracic cavity, or chest cavity, is enclosed by the rib cage, vertebral column, and sternum. It houses the lungs, heart, major blood vessels, and esophagus.
Each lung is surrounded by a double-layered membrane called the pleura. The outer parietal pleura lines the inner chest wall, while the inner visceral pleura covers the lung surface. Between these layers is the pleural space, containing a small amount of serous fluid. This fluid acts as a lubricant, allowing the pleural membranes to slide smoothly during breathing.
Intrathoracic pressure is measured within this space. The lungs naturally recoil inward, while the chest wall has an outward elastic pull. These opposing forces maintain the negative pressure in this space.
Intrathoracic Pressure and Respiration
Changes in intrathoracic pressure are fundamental for breathing, driving the movement of air into and out of the lungs. During inspiration, the diaphragm contracts and moves downward, while intercostal muscles pull the rib cage upward and outward. This coordinated muscle action increases the volume of the thoracic cavity. As the chest expands, intrathoracic pressure becomes more negative, dropping to approximately -6 to -8 cm H2O during a normal breath. This reduction creates a pressure gradient, causing air to flow into the lungs until the pressure equalizes.
During expiration, the diaphragm and intercostal muscles relax. This decreases the volume of the thoracic cavity, allowing the lungs to recoil. Intrathoracic pressure becomes less negative, or even slightly positive, reaching about -2 to -5 cm H2O during quiet exhalation. This pressure increase pushes air out of the lungs.
Intrathoracic Pressure and Blood Circulation
Intrathoracic pressure significantly influences blood circulation, particularly the return of blood to the heart. The negative pressure within the chest cavity acts like a “suction pump,” drawing deoxygenated blood from the body’s veins into the right atrium. This pressure gradient is crucial because large veins, such as the vena cava, pass through the thoracic cavity.
During inspiration, when intrathoracic pressure becomes more negative, this “suction” effect is enhanced, promoting increased venous return to the heart. The cyclical changes in intrathoracic pressure throughout the breathing cycle contribute to the efficiency of cardiac output by facilitating the filling of the heart’s chambers.
Common Variations in Intrathoracic Pressure
Intrathoracic pressure can fluctuate significantly during common activities, temporarily deviating from its typical negative state. Actions like coughing, sneezing, or straining (e.g., during a bowel movement or lifting heavy objects) can dramatically increase intrathoracic pressure. This often occurs when a forceful exhalation is performed against a closed airway, a maneuver known as the Valsalva maneuver.
During these actions, strong contractions of the abdominal and chest muscles create substantial positive pressure within the chest cavity. A forceful cough, for instance, can momentarily raise intrathoracic pressure to very high positive values. External factors, such as positive pressure ventilation in medical settings, also introduce positive pressure into the chest, which can impact circulatory dynamics differently than normal breathing.