Alveoli look like tiny, hollow grape-like clusters at the very ends of your airways. Each one is a small pocket of air wrapped in an incredibly thin wall, and together they form bunches called alveolar sacs that branch off shared ducts, much like grapes hanging from a stem. A healthy adult lung contains roughly 480 million of these structures, packed so densely that their combined surface area stretches to about 118 square meters, roughly the size of a singles tennis court.
Shape, Size, and Arrangement
Individual alveoli are tiny, rounded pouches, each roughly 200 to 300 micrometers across (about the width of a few strands of hair). They aren’t perfectly spherical. Instead, they share walls with neighboring alveoli, giving them a slightly honeycomb-like geometry when viewed in cross-section under a microscope. The shared walls keep the structure compact, maximizing surface area in a limited space.
Several alveoli open into a common space called an alveolar sac, and these sacs cluster together at the ends of small passageways called alveolar ducts. The overall visual effect is strikingly similar to a bunch of grapes, though unlike grapes, each “fruit” is an open pocket rather than a solid sphere. This branching, clustered architecture is what allows your lungs to fit such an enormous gas-exchange surface inside your chest.
What the Walls Look Like Up Close
Under a microscope, alveolar walls are remarkably thin and delicate. Two main cell types make up the lining. The first, called Type I cells, are extremely flat, almost like plastic wrap stretched across a surface. Despite making up only about half the total cell count, their flattened shape means they cover 95% of the alveolar surface. This thinness is the whole point: oxygen and carbon dioxide pass through these cells by simple diffusion, so the thinner the barrier, the faster the exchange.
Scattered among the flat cells are Type II cells, which are rounder and more cube-shaped. These cells produce surfactant, a thin oily film that coats the inside of each alveolus. Type II cells also serve as a repair crew. When Type I cells are damaged, Type II cells can transform into new Type I cells to patch the lining.
Woven through the walls is a mesh of two kinds of fibers. Collagen fibers provide structural rigidity, like the framing of a house, while elastic fibers allow the walls to stretch and snap back with each breath. This combination lets alveoli inflate and deflate thousands of times a day without tearing.
The Capillary Web
If you could peel away the outer layer of an alveolus, you’d see a dense net of capillaries wrapped around it, so tightly packed that some researchers describe it as a “sheet” of blood rather than individual tubes. Each alveolus connects to at least a couple of tiny arterioles feeding blood into this web. The capillary walls are also extremely thin, so the total barrier between air inside the alveolus and blood in the capillary can be as little as half a micrometer, far thinner than a single sheet of paper. That razor-thin gap is what makes rapid oxygen pickup and carbon dioxide release possible.
The Surfactant Film
The inner surface of every alveolus is coated with a thin layer of liquid topped by a film of surfactant. You can think of each alveolus as a tiny air bubble lined with this coating. Surfactant is mostly made of specialized fat molecules that pack together tightly, forming an almost solid-like film at the air-liquid boundary.
This film solves a critical physics problem. Small bubbles naturally want to collapse because surface tension pulls their walls inward. The smaller the bubble, the greater the inward pressure. Surfactant dramatically lowers that surface tension, which means your breathing muscles don’t have to work nearly as hard to keep millions of tiny alveoli inflated. During exhalation, as alveoli shrink, the surfactant molecules crowd closer together and reduce surface tension even further, preventing complete collapse. On the next inhale, the alveoli spring open again with relatively little effort.
How Disease Changes Their Appearance
In a healthy lung, alveoli appear uniformly sized under a microscope, forming a fine, lace-like pattern of small, evenly spaced air pockets separated by thin walls. Disease can dramatically alter this picture.
In emphysema, commonly caused by long-term smoking, the walls between alveoli break down and neighboring air sacs merge into larger, irregular spaces. Under a microscope, an emphysematous lung looks like the fine lace has been torn into ragged holes. The remaining spaces are much bigger than normal alveoli, but the total surface area is significantly reduced because so many walls have been destroyed. This damage is permanent and irreversible, which is why people with advanced emphysema struggle to get enough oxygen even at rest.
In pneumonia, the alveoli themselves may still be structurally intact, but they fill with fluid, mucus, or immune cells. Instead of appearing as clear, open air pockets, affected alveoli look opaque and congested. The fluid thickens the barrier between air and blood, making gas exchange sluggish even though the architecture hasn’t been physically demolished the way it is in emphysema.
Alveoli by the Numbers
- Total count: approximately 480 million across both lungs (ranging from about 274 million to 790 million depending on body size)
- Combined surface area: around 118 square meters
- Individual diameter: roughly 200 to 300 micrometers
- Wall thickness at the thinnest point: about 0.5 micrometers, thin enough that oxygen crosses in a fraction of a second
This combination of enormous quantity, grape-cluster packing, paper-thin walls, and a built-in surfactant system gives alveoli their distinctive look and makes them one of the most efficiently designed structures in the human body.