Your lungs sit inside the thoracic cavity, also called the chest cavity. More specifically, each lung occupies its own subdivision: the right lung sits in the right pleural cavity, and the left lung sits in the left pleural cavity. These two pleural cavities, along with a central compartment called the mediastinum, make up the three main sections of the thoracic cavity.
How the Thoracic Cavity Is Organized
The thoracic cavity is the entire enclosed space within your chest. It holds your lungs, heart, major blood vessels, and the esophagus. Rather than one open chamber, it’s divided into three distinct compartments arranged side by side.
The two pleural cavities sit on either side and each contain one lung. Between them, right in the center of your chest, is the mediastinum. This middle compartment holds your heart, the major blood vessels entering and leaving it, the windpipe, and the esophagus. The mediastinum essentially acts as a wall that separates your left and right lungs from each other. The lateral borders of the mediastinum are the pleural sacs surrounding each lung.
The Pleura: Your Lungs’ Protective Wrapping
Each lung is enclosed by a double-layered membrane called the pleura. The inner layer, called the visceral pleura, directly covers the surface of each lung along with its blood vessels, airways, and nerves. The outer layer, the parietal pleura, attaches to the inside of your chest wall. Between these two layers is a thin gap called the pleural space.
This space isn’t 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, letting the two pleural layers glide smoothly against each other every time you breathe in and out. Without it, the friction of thousands of breaths per day would cause irritation and pain.
How This Cavity Helps You Breathe
The pleural space does more than just cushion your lungs. It plays a critical role in the mechanics of breathing. The pressure inside the pleural space is slightly lower than the pressure inside your lungs and lower than atmospheric pressure. This negative pressure is what keeps your lungs inflated against the chest wall instead of collapsing inward.
When you inhale, your diaphragm contracts and your rib cage expands, increasing the volume of the thoracic cavity. This pulls the chest wall outward, which lowers the pressure in the pleural space even further. That drop in pressure causes your lungs to expand, drawing air in. When you exhale, the process reverses: the thoracic cavity shrinks, pleural pressure rises slightly, and air is pushed back out. The entire system depends on the pleural space maintaining that pressure difference. If air or fluid disrupts it, breathing can become difficult or even dangerous.
What Happens When the Pleural Space Is Disrupted
Because the pleural cavity’s pressure balance is so essential to breathing, problems in this space can be serious. The two most common issues are fluid buildup and air leaks.
Pleural effusion occurs when excess fluid accumulates in the pleural space. This can happen because of inflammation, infection, tumors, or lung injury. It can also result from conditions that increase pressure in blood vessels, such as heart, kidney, or liver failure, which pushes fluid into the pleural cavity. A large effusion compresses the lung and makes it harder to expand fully, causing shortness of breath.
A pneumothorax happens when air leaks into the pleural space. That trapped air disrupts the negative pressure that keeps the lung inflated, causing part or all of the lung to collapse. A pneumothorax can occur spontaneously without any clear cause, or it can result from a chest injury. In the most severe form, called a tension pneumothorax, the air buildup becomes so large that it interferes with blood flow through the chest and causes blood pressure to drop, which is a medical emergency.
The Thoracic Cavity’s Boundaries
The thoracic cavity is a well-protected space. The rib cage forms the walls on the front, back, and sides. At the top, the cavity opens narrowly into the base of the neck, where the windpipe and major blood vessels pass through. At the bottom, a large dome-shaped muscle called the diaphragm seals off the thoracic cavity from the abdominal cavity below. The diaphragm is the primary muscle of breathing, and its contraction is what expands the thoracic cavity with each inhale.
This bony and muscular enclosure serves a dual purpose. It protects the lungs and heart from physical trauma while also providing the rigid, expandable framework that makes the pressure changes of breathing possible. Your lungs can’t move on their own. They rely entirely on the thoracic cavity changing shape around them.