Alveoli are tiny air sacs at the very ends of your airways where your lungs actually do their job: swapping oxygen into your blood and pulling carbon dioxide out. An adult human has roughly 300 million of these sacs, and together they create a gas exchange surface of about 70 to 80 square meters, comparable to half a tennis court packed inside your chest. Every breath you take ends here.
Where Alveoli Sit in Your Lungs
When you inhale, air travels down your windpipe, through progressively smaller tubes called bronchi and bronchioles, and finally reaches clusters of alveoli at the very tips. Picture a bunch of grapes at the end of a stem. Each “grape” is one alveolus, and each cluster is wrapped in a dense net of capillaries, the smallest blood vessels in your body. This arrangement puts air and blood almost directly next to each other, separated by a barrier as thin as 0.2 micrometers in some spots. For perspective, a single red blood cell is about 35 times thicker.
How Gas Exchange Works
Oxygen and carbon dioxide move across the alveolar wall by simple diffusion, driven by pressure differences on either side. Oxygen in a freshly filled alveolus sits at a partial pressure of about 104 mmHg, while the blood arriving from the rest of the body carries oxygen at only about 40 mmHg. That steep gradient pushes oxygen through the barrier and into the blood almost instantly.
Carbon dioxide works in reverse. Blood returning to the lungs carries carbon dioxide at roughly 45 mmHg, while the alveolar air holds it at about 40 mmHg. The difference is smaller, but carbon dioxide diffuses about 20 times more easily than oxygen through tissue, so the exchange is still fast. By the time blood leaves the capillary network around each alveolus, it has been freshly loaded with oxygen and offloaded its carbon dioxide, which you then exhale.
The Cells That Line Each Alveolus
Two main cell types make up the alveolar wall, and they have very different jobs.
Type I cells cover about 70% of each alveolus’s inner surface. They are extremely flat, less than 0.1 micrometers thick, which is what makes the barrier between air and blood so remarkably thin. Their primary role is to let gases pass through with minimal resistance. They also help regulate the balance of fluid and ions inside the air sac.
Type II cells are smaller and rounder, covering only about 7% of the surface, but they perform a critical function: producing surfactant. This substance coats the entire inner lining of the alveolus and prevents the sac from collapsing every time you breathe out. Type II cells also act as repair cells. When Type I cells are damaged, Type II cells can divide and differentiate to replace them.
Why Surfactant Matters
Each alveolus is lined with a thin layer of moisture, and water molecules naturally pull toward each other, creating surface tension. Without something to counteract that tension, the tiny air sacs would collapse like wet plastic bags sticking together. Surfactant is the solution. This lipid-rich film drops the surface tension inside the alveolus from about 70 millinewtons per meter to nearly zero.
That reduction is not a minor detail. It is what keeps millions of microscopic air sacs open and functional throughout your life. Premature babies sometimes lack adequate surfactant because their Type II cells haven’t matured enough to produce it in sufficient quantities. Small amounts of surfactant begin appearing around 22 to 26 weeks of gestation, but full production ramps up closer to term. Without it, newborns can develop severe breathing difficulty.
Built-In Immune Defense
Because your lungs are open to the outside world with every breath, alveoli need their own security system. Alveolar macrophages, a type of immune cell that lives on the surface of the air sacs, serve as the first line of defense. They patrol the alveolar lining, engulfing bacteria, viruses, dust particles, and cell debris before these threats can cause damage or infection. These macrophages also help initiate a broader immune response when a serious pathogen shows up, recruiting additional immune cells to the area.
How Alveoli Develop Over Your Lifetime
Alveoli are not fully formed at birth. In late pregnancy, the fetal lungs transition from simple sac-like structures into more complex air spaces, but true alveoli, with their characteristic thin walls and dense capillary networks, begin forming around 36 weeks of gestation. The most rapid phase of alveolar growth occurs from late fetal life through about age three, during which new walls (called septa) subdivide existing air spaces into smaller, more numerous sacs.
A second, slower phase continues from around age two all the way into young adulthood, potentially up to age 21. During this period, new alveoli still form, though at a lower rate, and the capillary network within alveolar walls matures from a double layer into a more efficient single layer. Roughly 90% of your total gas exchange surface area is created through this process of alveolar formation. This is one reason why childhood lung health, including avoiding smoke exposure, has lasting consequences.
What Happens When Alveoli Are Damaged
Because alveoli are so thin and delicate, they are vulnerable to disease. Two of the most common conditions that affect them work in very different ways.
Emphysema gradually destroys the walls between alveoli, merging many small sacs into fewer, larger ones. This dramatically reduces the surface area available for gas exchange. Cigarette smoke is the primary driver, causing progressive disruption of the cells that maintain alveolar structure. The damage is largely irreversible. People with emphysema feel increasingly short of breath because their lungs simply have less functional tissue to work with, even though the lungs may actually appear larger on imaging due to trapped air.
Pneumonia takes a different approach. Instead of destroying walls, bacterial or viral infections fill the alveoli with fluid, pus, and inflammatory debris. This blocks oxygen from reaching the capillary network, impairing gas exchange from the air side of the barrier. Unlike emphysema, pneumonia is typically treatable and the alveolar structure can recover once the infection clears, though severe cases can leave scarring.
Both conditions ultimately produce the same result for the person experiencing them: less oxygen getting into the blood and more difficulty breathing. The distinction matters because emphysema reflects permanent structural loss, while pneumonia is usually a temporary obstruction that responds to treatment.