Alveoli are tiny air sacs in your lungs where oxygen enters your bloodstream and carbon dioxide leaves it. An adult lung contains roughly 480 million of these sacs, creating a combined surface area of about 118 square meters, roughly the size of a singles tennis court. That enormous surface, packed inside your chest, is what makes breathing efficient enough to keep every cell in your body supplied with oxygen.
How Gas Exchange Works
Every breath you take pulls air down through your airways until it reaches the alveoli, which sit in clusters at the very ends of the smallest bronchial tubes. Each alveolus is wrapped in a mesh of capillaries, the tiniest blood vessels in your body. The walls of the alveoli and the walls of these capillaries share a membrane, and this shared barrier is astonishingly thin: in some places less than 0.5 micrometers, or about one-hundredth the width of a human hair.
Oxygen and carbon dioxide move across this membrane through diffusion, the same process that lets a drop of food coloring spread through a glass of water. Oxygen concentrations are higher in the air you just inhaled than in the blood arriving from the body, so oxygen naturally crosses into the blood. Carbon dioxide works in reverse: it’s more concentrated in the returning blood, so it crosses into the alveoli and gets exhaled. No pumps or energy are needed for this exchange. The thinness of the membrane and the massive surface area make it happen fast enough to oxygenate your entire blood supply in the few seconds it takes to flow through the lungs.
Why Alveoli Don’t Collapse
Alveoli face a physics problem. They’re essentially tiny wet bubbles, and the water molecules lining their inner surface create surface tension that constantly tries to pull them shut, the same force that makes soap bubbles shrink and pop. Without a solution, your smallest alveoli would collapse every time you exhaled.
The solution is surfactant, a slippery coating produced by specialized cells in the alveolar walls. Surfactant dramatically reduces surface tension, bringing it close to zero at the end of each exhale. This keeps alveoli open and stable so they can reinflate easily with the next breath. Surfactant also plays a role in immune defense, helping to suppress harmful inflammatory responses inside the lung.
Premature babies sometimes lack enough surfactant because the cells that produce it aren’t fully mature until late in pregnancy. This is one reason premature infants can struggle to breathe and often need medical support until their lungs catch up.
Two Cell Types, Two Jobs
The alveolar walls contain two main cell types, each doing something different. Type I cells are extremely flat and thin, forming most of the surface area. Their job is purely structural: they create the ultra-thin barrier that gases pass through. Think of them as the walls of the exchange window.
Type II cells are smaller and rounder, tucked into the corners of alveoli. These are the cells that manufacture surfactant. They also act as stem cells for the alveolar lining. When Type I cells are damaged, Type II cells can divide and transform into new Type I cells, giving the alveoli a limited ability to repair themselves.
How Alveoli Develop and Grow
Babies are born with functioning alveoli, but not nearly as many as adults. The number of alveoli increases exponentially during the first two years of life, and growth continues, at a slower pace, well into adolescence. By adulthood, the gas exchange surface has expanded roughly 20-fold from birth without any matching increase in chest size. The lungs accomplish this by packing more and more tiny sacs into the same volume, like subdividing rooms inside a house to create more wall space.
Research from the American Journal of Physiology has shown that this growth period is longer than scientists originally thought. Rather than finishing in early childhood, new alveoli keep forming through the teenage years, which means lung development in children and adolescents may be more sensitive to environmental factors like air pollution and cigarette smoke than previously assumed.
What Happens When Alveoli Are Damaged
Because gas exchange depends on having millions of intact, thin-walled air sacs, anything that destroys alveoli directly reduces your ability to get oxygen. Two of the most common conditions that affect alveoli work in very different ways.
Emphysema
In emphysema, the inner walls between neighboring alveoli break down and rupture. Instead of a cluster of many small sacs, you’re left with fewer, much larger air spaces called bullae, which can grow to fill half a lung. This drastically reduces total surface area for gas exchange. The damage also destroys the elastic quality of normal alveoli. Healthy sacs stretch when you inhale and snap back to push air out when you exhale. Damaged sacs lose that recoil, trapping stale air inside the lungs and making it difficult to fully exhale. The result is a persistent feeling of breathlessness that worsens over time. Smoking is the leading cause.
Pneumonia
Pneumonia attacks alveoli from the inside. Infection causes the air sacs to fill with fluid, pus, or both. The membrane that gases normally cross freely becomes waterlogged and swollen, slowing or blocking oxygen transfer. Unlike emphysema, the structural damage from pneumonia is often temporary. Once the infection clears, the fluid drains and the alveoli can return to normal function, though severe cases can leave scarring that permanently reduces capacity.
Why Surface Area Matters So Much
The fundamental design principle of alveoli is maximizing contact between air and blood. Your lungs achieve this not by being large, but by being intricately subdivided. Each alveolus is only about 0.2 millimeters across, but multiplied by hundreds of millions, they create a surface area roughly 40 times larger than your skin. Pair that with a barrier thin enough for gases to cross in a fraction of a second, and you have a system efficient enough to oxygenate about 5 liters of blood per minute at rest, and several times that during intense exercise.
This is also why lung damage is so consequential. You can’t feel individual alveoli being destroyed, and your lungs have enough reserve capacity that significant damage can accumulate before symptoms appear. By the time someone with emphysema notices they’re short of breath during routine activities, a substantial portion of their alveolar surface area has already been lost.