The lungs sit inside the thoracic cavity, the large space within your chest that is protected by your ribcage. More specifically, each lung occupies its own pleural cavity, a separate compartment lined with a thin double-layered membrane. Understanding the difference between these two terms, and how the space around your lungs actually works, helps explain everything from normal breathing to why a “collapsed lung” happens.
The Thoracic Cavity
The thoracic cavity is the entire enclosed space inside your chest. It holds five organs: the heart, lungs, esophagus, trachea, and thymus. Twelve pairs of ribs wrap around the sides and back, forming the walls. The top opens at the base of your neck through a small passage called the thoracic inlet, which connects your chest to your neck. The floor is your diaphragm, a dome-shaped muscle that separates the chest from the abdomen below.
Inside the thoracic cavity, the space is divided into three main compartments. The two pleural cavities sit on the left and right sides, each housing one lung. Between them is the mediastinum, a central column that contains the heart (wrapped in its own protective sac called the pericardium), the esophagus, the trachea, the thymus, and major blood vessels like the aorta and the superior vena cava. The trachea travels down through the mediastinum before splitting into two branches, one entering each pleural cavity to deliver air to the corresponding lung.
The Pleural Cavities and Their Membranes
Each lung is individually wrapped in a two-layered membrane called the pleura. The inner layer clings directly to the surface of the lung. The outer layer lines the inside of the chest wall. Between these two layers is a razor-thin gap called the pleural space.
That tiny space is not empty. It contains a small amount of fluid, roughly 0.1 to 0.2 milliliters per kilogram of body weight. For an average adult, that works out to less than a tablespoon. This fluid acts as a lubricant so the two pleural layers can slide smoothly against each other every time you breathe in and out. Without it, the constant expansion and contraction of your lungs would create painful friction against the chest wall.
How Negative Pressure Keeps Lungs Inflated
The pleural space does more than reduce friction. It also maintains a slight vacuum that keeps your lungs expanded. The pressure inside the pleural space stays at roughly negative 4 mmHg throughout the breathing cycle, meaning it is slightly lower than the pressure inside the lungs themselves. This difference in pressure pulls the lung outward against the chest wall, preventing it from collapsing inward like a deflated balloon.
When you inhale, your diaphragm contracts and moves downward while your rib muscles pull the chest wall outward. This expands the pleural cavity, drops the pressure even further, and draws air into the lungs. When you exhale, the diaphragm relaxes, the cavity shrinks, pressure rises, and air flows back out. The entire cycle depends on the pleural space maintaining that sealed, low-pressure environment.
What Happens When the Pleural Space Is Disrupted
Because the lungs rely on negative pressure to stay inflated, anything that breaks the seal of the pleural space can cause serious problems. In a pneumothorax (collapsed lung), air leaks into the pleural space from either a lung injury or a wound in the chest wall. That extra air pushes on the outside of the lung, overpowering the vacuum that normally holds it open, and the lung partially or fully collapses.
A pleural effusion is a different kind of disruption. Instead of air, excess fluid builds up in the pleural space. This can happen with infections, heart failure, or other conditions. The fluid compresses the lung and limits how much it can expand, making it harder to take a full breath. In both cases, the core issue is the same: the carefully balanced environment of the pleural cavity has been disturbed, and the lung can no longer inflate properly.
Left vs. Right Pleural Cavity
The two pleural cavities are not mirror images of each other. The right lung has three lobes and is slightly larger. The left lung has only two lobes and is a bit smaller because it shares space with the heart, which sits slightly left of center in the mediastinum. This is why the left pleural cavity is narrower than the right. Despite the size difference, both cavities function identically, each maintaining its own sealed pleural space with its own fluid layer and its own pressure balance, independent of the other side.