The lungs bring oxygen into your body and remove carbon dioxide with every breath. They do this through a process called gas exchange, where fresh air meets your bloodstream across a membrane thinner than a sheet of plastic wrap. But the lungs do more than just swap gases. They also defend against airborne threats, help regulate your blood’s acidity, and move air through a branching network of passages engineered for maximum efficiency.
How Air Reaches Your Lungs
Your lungs don’t pull air in on their own. They rely on the diaphragm, a dome-shaped muscle sitting beneath them, and the small muscles between your ribs. When you inhale, the diaphragm contracts and pulls downward while the rib muscles contract and pull upward. This expands the chest cavity, drops the pressure inside, and air rushes in to fill the space. Exhaling is mostly passive: the diaphragm relaxes, the chest cavity shrinks, and air is pushed back out.
A healthy adult’s lungs can hold about 6 liters of air at maximum capacity. You never actually empty them completely, though. Even after the hardest exhale you can manage, roughly 1.2 liters of air stays behind. The maximum you can forcibly exhale after a full breath, called forced vital capacity, is about 4.8 liters, or 80 percent of total capacity.
The Branching Airway System
Air enters through your nose or mouth, passes down the trachea (windpipe), and hits a fork at about the level of your mid-chest. The trachea splits into two primary bronchi, one for each lung. These bronchi branch again and again into progressively smaller tubes, like an upside-down tree, until they end in tiny air sacs called alveoli. Each lung contains roughly 300 million of them.
Your right and left lungs aren’t identical. The right lung is shorter and broader, divided into three lobes, and holds a greater volume. The left lung is longer and narrower, with only two lobes, because it has to make room for your heart. There’s actually a small indentation on its inner surface, called the cardiac notch, where the heart sits.
Gas Exchange: The Core Function
Everything in the lung’s design points toward one goal: getting oxygen into the blood and carbon dioxide out. This happens in the alveoli, where air sits separated from your bloodstream by an extremely thin barrier. Those 300 million alveoli create a combined surface area of about 75 square meters, roughly the size of half a tennis court, all folded up inside your chest.
The transfer works through diffusion, the same process that lets a drop of food coloring spread through a glass of water. Gases move from areas of higher concentration to lower concentration. Air arriving in the alveoli carries oxygen at a partial pressure of about 100 to 104 mmHg. Blood entering the lung’s capillaries carries oxygen at only 40 mmHg. That steep difference drives oxygen across the membrane and into the blood. Carbon dioxide moves the opposite direction: it’s slightly higher in the blood (46 mmHg) than in the alveoli, so it crosses over to be exhaled.
This transfer happens remarkably fast. Blood picks up enough oxygen to match the alveolar level after traveling only one-fifth to one-third of the way through the capillary network. The speed depends mostly on how easily gases can pass through the membrane itself. When lung disease thickens that barrier, through scarring or fluid buildup, gas exchange slows and breathing becomes less efficient.
How the Lungs Protect Themselves
Every breath carries potential threats: bacteria, viruses, dust, pollen, and other particles. The lungs have a layered defense system to handle them, starting with mucus. A thin layer of mucus lines the airways, and its molecular structure is surprisingly sophisticated. Mucus contains large sugar-coated proteins called mucins that carry an enormous variety of carbohydrate chains on their surface. This diversity lets them bind to virtually any particle that lands on the airway lining, trapping it before it can reach deeper tissue.
Beneath the mucus sits a thin layer of watery fluid that bathes the cilia, tiny hair-like structures covering the airway cells. These cilia beat rapidly, between 8 and 15 times per second, pushing the mucus layer upward toward the throat. The watery layer underneath is critical: it gives the cilia a low-resistance environment to move in while keeping the sticky mucus from pinning them down. The mucus rides on top, carrying trapped particles up and out of the lungs. You either swallow this material without noticing or cough it up. This entire system, mucus plus cilia working together, acts as the lung’s primary built-in cleaning mechanism.
Beyond Breathing: Other Roles
Gas exchange and defense are the headline functions, but the lungs contribute to several other processes. By controlling how much carbon dioxide leaves the body, they help regulate blood pH. Carbon dioxide dissolved in blood forms an acid, so exhaling more of it makes the blood slightly less acidic. When your body detects that blood is becoming too acidic, your breathing rate increases to blow off more carbon dioxide. When it’s too alkaline, breathing slows.
The lungs also serve as a blood filter of sorts. The entire output of the right side of your heart passes through the lung’s capillary network before reaching the rest of the body. Small blood clots or air bubbles that form in veins can get trapped in these narrow vessels, preventing them from reaching the brain or other organs. The lungs also play a role in converting certain inactive compounds in the blood into their active forms as blood passes through the pulmonary circulation.
What Reduces Lung Function
Anything that disrupts the steps described above can impair how well your lungs work. Conditions like asthma narrow the airways, making it harder for air to reach the alveoli. Emphysema destroys the alveolar walls, reducing the surface area available for gas exchange. Pulmonary fibrosis thickens the barrier between air and blood, slowing oxygen transfer. Pneumonia fills alveoli with fluid, blocking gas exchange in the affected areas entirely.
Age also plays a role. Lung capacity gradually declines after your mid-twenties as the chest wall stiffens and the diaphragm weakens slightly. The alveoli lose some of their shape over time, reducing surface area. This is a normal part of aging, but it means the margin for error shrinks. Habits that damage the lungs, particularly smoking, accelerate these changes dramatically and can push normal age-related decline into the range of disease.