Alveoli are tiny air sacs at the very end of your airways where your lungs do their real job: swapping oxygen into your blood and pulling carbon dioxide out. The average adult has roughly 480 million of them, clustered at the tips of the smallest breathing tubes like bunches of grapes. Despite each one being microscopic, together they create an enormous surface for gas exchange, about 140 square meters, roughly the floor area of a singles tennis court packed inside your chest.
Where Alveoli Sit in the Lungs
When you breathe in, air travels down the windpipe and into two main bronchi, one for each lung. Those bronchi branch again and again into progressively thinner tubes called bronchioles. At the very end of the smallest bronchioles, you find clusters of alveoli. Each cluster looks a bit like a tiny bunch of grapes attached to a stem, with each “grape” being a single alveolus. A single alveolus is incredibly small, and the walls between neighboring air sacs (called septa) are only about 2 micrometers thick, roughly 50 times thinner than a sheet of paper.
How Gas Exchange Works
Every alveolus is wrapped in a dense mesh of capillaries, the smallest blood vessels in your body. When you inhale, oxygen-rich air fills the alveoli. Because the oxygen concentration inside the air sac is higher than in the blood arriving from the body, oxygen naturally moves across the thin wall and into the capillaries. At the same time, carbon dioxide, a waste product your cells have dumped into the bloodstream, moves in the opposite direction, from the blood into the alveolus, so you can exhale it.
This entire process runs on diffusion, the same principle that makes a drop of food coloring spread through a glass of water. Two features make it remarkably efficient. First, the barrier gases must cross is extremely thin: just a layer of surfactant fluid, the alveolar wall, a shared basement membrane, and the capillary wall. Second, the total surface area is enormous. Your lungs don’t need to use the entire capillary network at rest. Parts of it sit idle and can be recruited during exercise when your body demands more oxygen.
The Two Cell Types That Line Each Alveolus
The inner wall of an alveolus is built from two specialized cell types, each with a distinct role. Type I cells are flat and extremely thin, covering the vast majority of the alveolar surface. Their thinness is the reason gases can cross so quickly. Type II cells are smaller and rounder. Their primary job is producing surfactant, a slippery coating that lines the inside of each air sac.
Why Surfactant Matters
Surfactant is not a single substance but a mixture of fats (mostly phospholipids), cholesterol, and specific proteins. It solves a critical physics problem. The inside of each alveolus is moist, and water molecules naturally pull toward each other, creating surface tension. Without something to counteract that tension, the tiny air sacs would collapse in on themselves every time you exhaled, and reinflating them would take enormous effort.
Surfactant works by forming a film across the inner surface that lowers surface tension dramatically. During exhalation, as the alveoli shrink, the film compresses and drives surface tension to near zero. This keeps the sacs from collapsing. During inhalation, the film spreads back out and allows the alveoli to expand easily. The film behaves almost like a solid under compression, resisting the forces that would otherwise squeeze the air sac shut. Premature babies sometimes lack enough surfactant, which is why they can struggle to breathe immediately after birth.
How Alveoli Develop
Alveoli don’t fully form before birth. During the later weeks of pregnancy (roughly weeks 24 to 38), the fetal lung is in what’s called the saccular stage: the tiniest airways widen into simple sac-like spaces, but true alveoli haven’t appeared yet. These primitive sacs have thick walls containing a double layer of capillaries separated by connective tissue.
Around week 36, a process called alveolarization begins. New walls, called secondary septa, start rising up from the existing sac surfaces, dividing the simple spaces into smaller, more numerous alveoli. This bulk phase of alveolar formation continues until roughly age 3. But the lungs keep adding alveoli well beyond that, through a “continued alveolarization” phase that lasts into young adulthood, essentially as long as the lungs are still growing. At the same time, the thick double-layered capillary networks in each wall gradually thin and merge into a single, more efficient capillary sheet. This maturation process runs in parallel with the creation of new alveoli.
What Happens When Alveoli Are Damaged
Because gas exchange depends on thin walls and open air spaces, anything that thickens those walls or floods the sacs with fluid can seriously impair breathing.
- Emphysema destroys the walls between alveoli, merging many small sacs into fewer, larger ones. This dramatically reduces the total surface area available for gas exchange, leaving you chronically short of breath. Smoking is the most common cause.
- Pneumonia fills alveoli with fluid and immune cells as the body fights infection. Oxygen has a much harder time diffusing through liquid than through a thin, dry membrane, which is why pneumonia causes low blood oxygen levels.
- Acute respiratory distress syndrome (ARDS) occurs when the barrier between the alveoli and capillaries breaks down, usually from severe infection, trauma, or inflammation. Fluid floods the air sacs, causing dangerous drops in blood oxygen and a buildup of carbon dioxide. Clearing that fluid is difficult; most ARDS patients have impaired fluid clearance, which is linked to worse outcomes.
How Doctors Measure Alveolar Function
A test called lung diffusion testing measures how well gases cross from your alveoli into your blood. You breathe in a small, harmless amount of carbon monoxide and then the test measures how much of it your blood absorbs. A normal result falls between 75% and 140% of the predicted value for your age, sex, and height. Results between 60% and 75% suggest mildly reduced function, while results below 40% indicate severe impairment. Doctors use this test to help diagnose and monitor conditions like emphysema, pulmonary fibrosis, and other diseases that affect the alveolar surface.