Alveoli are tiny air sacs at the very ends of your airways where your lungs do their actual job: swapping oxygen into your blood and pulling carbon dioxide out. An average adult has roughly 480 million of them, each about 200 micrometers across (about the width of two or three human hairs). Despite their microscopic size, they collectively create a massive surface for gas exchange, estimated at 50 to 75 square meters in an adult.
Where Alveoli Sit in Your Lungs
When you breathe in, air travels down your windpipe and into two main bronchi, one for each lung. Those bronchi branch into smaller and smaller passages, like a tree splitting into ever-thinner limbs. The smallest branches, called bronchioles, finally dead-end into clusters of alveoli. Picture tiny bunches of grapes at the tips of each branch, and you have a reasonable mental image of how they’re arranged.
How Gas Exchange Works
Each alveolus is wrapped in a dense net of capillaries, the body’s smallest blood vessels. The barrier between air inside the alveolus and blood in the capillary is extraordinarily thin, as little as 0.2 micrometers in places. That’s roughly 500 times thinner than a sheet of paper. Oxygen and carbon dioxide cross this barrier passively, driven by differences in gas concentration on either side.
Oxygen is at a higher concentration in the air you just inhaled than in the blood arriving from the body, so it naturally moves from the alveolus into the bloodstream. Carbon dioxide works in reverse: it’s more concentrated in the returning blood, so it drifts into the alveolus and gets exhaled. No pumping or active transport is needed. The process depends entirely on having a huge surface area, an extremely thin barrier, and a constant flow of fresh air and blood on opposite sides of that barrier.
The Two Types of Cells That Line Them
The inner wall of each alveolus is covered by two distinct cell types that work together to keep the system running.
Type I cells are extremely flat and thin. They make up only about half the total cell count, yet because they’re so spread out, they cover roughly 95% of the alveolar surface. Their thinness is the reason gas can cross so quickly. Think of them as the working surface of the lung.
Type II cells are smaller and rounder. Their main job is producing surfactant, a slippery, lipid-rich film that coats the entire inner surface of every alveolus. Surfactant dramatically lowers surface tension at the air-liquid boundary, dropping it from about 70 millinewtons per meter to nearly zero. Without it, the water lining the alveolus would create enough inward pull to collapse the air sac every time you exhale. Type II cells also serve as a repair crew: when Type I cells are damaged, Type II cells can transform into Type I cells to replace them.
Why Surfactant Matters So Much
Surface tension is the same force that lets a water droplet hold its round shape. Inside a tiny, wet air sac, that force is powerful enough to pull the walls inward and collapse the space entirely. Surfactant counteracts this by forming a thin film across the entire alveolar surface, essentially breaking the grip of surface tension. This keeps alveoli open at the end of each breath and maintains the surface area your lungs need to oxygenate your blood.
The importance of surfactant becomes obvious in premature infants. Alveoli first appear around 29 weeks of gestation, and surfactant production ramps up in the final weeks of pregnancy. Babies born very early often lack enough surfactant to keep their alveoli inflated, leading to serious breathing difficulty. At birth, a full-term baby has about 150 million alveoli, roughly half the adult number. The rest develop during the first years of life, and lung surface area at birth is only about 3 to 5 square meters, around one-twentieth of an adult’s.
Built-In Immune Defense
Because your alveoli are directly exposed to whatever you inhale, they need protection beyond just a thin wall. Alveolar macrophages, sometimes called “dust cells,” patrol the alveolar surface and act as the lung’s first line of immune defense. These cells detect bacteria, viruses, dust particles, and pollutants using specialized receptors on their surface. Once they identify something foreign, they engulf and digest it through a process called phagocytosis.
During this cleanup, alveolar macrophages also release chemical signals that recruit additional immune cells if the threat is large enough. Under an electron microscope, these macrophages are visibly packed with engulfed debris, including carbon particles, bacteria, and dust, which is how they earned the nickname. In a healthy lung, this system quietly clears inhaled contaminants without you ever noticing.
What Happens When Alveoli Are Damaged
Several conditions directly target the alveoli, and the consequences tend to be serious because damage reduces the surface area available for gas exchange.
Emphysema, one of the main forms of chronic obstructive pulmonary disease (COPD), is the clearest example. Chronic exposure to cigarette smoke triggers a cycle of oxidative stress, inflammation, and cell death that progressively destroys alveolar walls. As walls break down, small alveoli merge into larger, simplified air spaces. This dramatically shrinks the total surface area and the capillary network embedded within it. The result is less efficient oxygen transfer, which is why people with advanced emphysema feel breathless doing things that once required no effort. The damage is irreversible because the body cannot rebuild the intricate alveolar architecture once it’s gone.
Pneumonia affects alveoli differently. Instead of destroying walls, infection fills the air sacs with fluid and immune cells, physically blocking gas exchange. This is usually temporary if treated, because the alveolar structure itself remains intact.
Acute respiratory distress syndrome (ARDS) involves widespread inflammation that damages the thin barrier between air and blood. Fluid leaks into the alveoli, surfactant production drops, and large sections of the lung can collapse or flood. This is the mechanism behind the severe breathing failure seen in some critical illnesses and infections.
Why Size and Number Matter
The design of alveoli reflects a basic engineering tradeoff: many tiny sacs provide far more total surface area than a few large ones would. An adult’s 480 million alveoli, packed into two lungs that weigh only about a kilogram combined, create enough exchange surface to cover roughly half a tennis court. That surface area, paired with a barrier thin enough for gases to cross in a fraction of a second, allows the lungs to fully oxygenate blood in the time it takes to pass through the capillary network, typically less than a second at rest.
This is also why alveolar diseases are so impactful. Losing even a modest percentage of that surface area noticeably reduces the body’s ability to deliver oxygen to tissues, particularly during physical activity when oxygen demand spikes.