Intrathoracic pressure is the pressure within the chest cavity, the enclosed space that holds the lungs, heart, and major blood vessels. This pressure is a fundamental physical force that drives the mechanical process of breathing and influences the circulatory system. The chest cavity acts like a sealed container: any change in its volume directly alters the pressure of the air and fluids inside it, following basic physics principles. The interplay of muscles and elastic tissues works to control this internal pressure, ensuring continuous air exchange and regulating blood flow throughout the body.
The Resting State of Intrathoracic Pressure
Intrathoracic pressure remains below the surrounding atmospheric pressure even when a person is resting between breaths. This negative pressure is a consequence of opposing forces exerted by structures within the chest. The lungs have a natural tendency to recoil and collapse inward. Simultaneously, the chest wall tends to spring outward, expanding the thoracic cavity.
These two opposing forces pull against a thin layer of fluid in the pleural space, which acts like a seal between the lungs and the chest wall. This creates a continuous vacuum effect within this space. At the end of a quiet exhalation, the intrathoracic pressure typically sits around -4 to -5 centimeters of water (cmH2O). This negative value keeps the lungs gently expanded and against the chest wall, ready for the next breath.
How Pressure Drives Air Movement
Breathing is a dynamic process entirely dependent on the precise manipulation of intrathoracic pressure to create a pressure gradient. This process is governed by Boyle’s Law, which states that the pressure of a gas is inversely proportional to its volume. If the volume of the chest cavity increases, the pressure inside must decrease.
Inspiration is an active process that begins with the contraction of the diaphragm and the external intercostal muscles. The diaphragm moves downward, and the ribs move up and out, significantly increasing the volume of the thoracic cavity. This volume increase causes the intrathoracic pressure to drop sharply, often reaching -6 cmH2O or lower during a quiet breath.
This greater negative pressure pulls the lungs open, which lowers the pressure inside the lungs (alveolar pressure) below the atmospheric pressure outside the body. Air naturally moves from a region of higher pressure to a region of lower pressure, forcing air to rush into the lungs until the pressure gradient is equalized.
Expiration is typically a passive process that occurs when the inspiratory muscles relax. Relaxation allows the chest wall to recoil inward and the elastic lungs to spring back to their smaller, resting volume. The resulting decrease in thoracic volume raises the intrathoracic pressure, making it less negative, and raising the pressure within the lungs above atmospheric pressure. This positive pressure gradient then pushes the air out of the lungs. During a forced exhalation, accessory muscles contract to further decrease the thoracic volume, which can temporarily push the intrathoracic pressure into positive values.
Intrathoracic Pressure and Blood Flow
Beyond its role in respiration, intrathoracic pressure regulates blood flow through a mechanism often referred to as the “respiratory pump.” The pressure changes that draw air into the lungs also assist in returning venous blood to the heart. During inspiration, the drop in intrathoracic pressure lowers the pressure within the large veins inside the chest cavity.
This creates a pressure gradient where the pressure in the veins of the abdomen and lower body is higher than the pressure in the thoracic veins. Blood is effectively drawn from the abdominal cavity and extremities into the central veins and the right atrium of the heart, increasing venous return. The valves within the veins prevent this blood from flowing backward.
Conversely, during exhalation, the increase in intrathoracic pressure compresses the central veins, temporarily impeding the flow of blood back to the heart. This cyclical change in pressure aids the cardiovascular system, helping the heart to fill with blood efficiently. Actions that dramatically increase intrathoracic pressure, such as straining or a forceful cough, can temporarily reduce the return of blood to the heart.