Intrathoracic pressure is the pressure within the chest cavity, or thorax. This pressure is specifically measured within the pleural space, the thin, fluid-filled gap that separates the lungs from the inner chest wall. The regulation and fluctuation of this pressure are necessary for the mechanical process of breathing. Intrathoracic pressure also plays a role in the body’s circulatory function, affecting how blood moves back to the heart. Understanding this pressure helps grasp how the respiratory and cardiovascular systems work together.
Defining the Pleural Space and Pressure Gradient
The anatomical structure responsible for intrathoracic pressure is the pleural space, a potential cavity lying between two membranes called the pleura. The visceral pleura adheres directly to the surface of the lungs, while the parietal pleura lines the inside of the chest wall. A small amount of pleural fluid lubricates this space, allowing the two layers to slide smoothly past each other during respiration. This fluid also creates a surface tension that causes the pleura to adhere to one another.
The lung tissue has a natural tendency to recoil inward, while the chest wall naturally attempts to spring outward, expanding the thoracic cavity. These opposing elastic forces pull on the pleural membranes, resulting in a pressure within the pleural space that is typically negative, or sub-atmospheric. This baseline negative pressure, often around -5 cm H₂O at rest, keeps the lungs inflated and closely apposed to the chest wall.
Dynamic Changes During the Breathing Cycle
The pressure within the thorax changes continuously with every breath, driving pulmonary ventilation. During inspiration, the diaphragm contracts and moves downward, while the external intercostal muscles pull the rib cage upward and outward. This coordinated muscle action increases the volume of the thoracic cavity, drawing the parietal pleura outward.
Because the pleural space is a closed system, the increase in volume causes the intrathoracic pressure to become significantly more negative, dropping to about -8 cm H₂O during quiet breathing. This more negative pressure exerts a greater pull on the lungs, causing them to expand. As the lungs expand, the pressure inside the air sacs, or alveoli, drops below atmospheric pressure, creating a pressure gradient that draws air into the lungs.
Expiration is typically a passive process driven by the relaxation of the respiratory muscles. When the diaphragm and intercostal muscles relax, the chest wall and the elastic lung tissue recoil to their resting size, decreasing the volume of the thoracic cavity. This decrease in volume makes the intrathoracic pressure less negative, returning it to the resting value of around -5 cm H₂O. The compression causes the alveolar pressure to momentarily rise above atmospheric pressure, which forces the air out of the lungs.
Influence on Blood Circulation
Beyond its respiratory function, intrathoracic pressure influences the movement of blood back to the heart, a process known as venous return. The large veins, such as the vena cava, pass through the thoracic cavity and are subject to the pressure changes within the chest. During inspiration, the decrease in intrathoracic pressure creates a suction effect on the right atrium of the heart, lowering the pressure inside it.
This lower pressure in the chest creates a pressure gradient between the veins in the abdomen, where pressure is relatively higher, and the veins in the thorax. This gradient draws deoxygenated blood from the lower body and the extremities into the right side of the heart. The opposite occurs during expiration, where the slight rise in intrathoracic pressure can momentarily impede venous return.
Clinical Scenarios Where Pressure Changes
Clinical and physiological events can alter the normal dynamics of intrathoracic pressure. A pneumothorax occurs when air enters the pleural space, often due to a puncture in the lung or chest wall. This air collection eliminates the negative pressure gradient, allowing the lung’s natural elastic recoil to cause it to partially or fully collapse.
A deliberate action that drastically increases intrathoracic pressure is the Valsalva maneuver, involving forceful exhalation against a closed windpipe. This action can raise the pressure in the chest to very high positive levels, compressing the large veins and transiently reducing the return of blood to the heart. Conversely, mechanical ventilation delivers air to the lungs using positive pressure, which artificially reverses the normal dynamics. Mechanical ventilation causes intrathoracic pressure to become positive during the air delivery phase, which can potentially impair venous return and cardiac function.