Diffusion in the lungs is the passive movement of oxygen from the air you breathe into your blood, and of carbon dioxide from your blood back into the air, across a microscopically thin membrane deep inside the lungs. No pumping or active transport drives this exchange. The gases simply flow from where they are more concentrated to where they are less concentrated, crossing a barrier so thin it would take more than a thousand layers of it to equal the thickness of a sheet of paper. This seemingly simple process is the entire reason your lungs exist, and when it goes wrong, the consequences range from mild breathlessness to life-threatening oxygen deprivation.
The Blood-Gas Barrier
Gas exchange happens in the alveoli, tiny air sacs clustered at the ends of your smallest airways. An adult lung contains hundreds of millions of them, creating a combined surface area roughly the size of a tennis court. Wrapped around each alveolus is a dense mesh of capillaries carrying blood that has just returned from the body, depleted of oxygen and loaded with carbon dioxide. The wall separating the air inside each alveolus from the blood in those capillaries is called the blood-gas barrier, and its design is a study in engineering tradeoffs.
The barrier needs to be as thin as possible so gases can cross it quickly, because the rate of diffusion slows with increasing thickness. At the same time, it has to be strong enough to hold together under the mechanical stress of breathing and the pressure of blood flowing through the capillaries. The average thickness of this membrane is roughly 0.6 micrometers, with the basement membrane providing most of the structural strength.1PubMed. Stress failure of the blood-gas barrier For perspective, a red blood cell is about 8 micrometers across, so the barrier the cell must exchange gases through is more than ten times thinner than the cell itself.
This barrier is not a single homogeneous sheet. It consists of the alveolar epithelial cell lining the air side, the capillary endothelial cell lining the blood side, and a fused basement membrane between them. In some spots the barrier is thicker because of extra collagen or connective tissue providing reinforcement. Gas exchange happens most efficiently across the thinnest regions, and the lung has evolved to maximize how much of the barrier surface falls into that ultra-thin category.
How Gases Cross the Membrane
The driving force behind pulmonary diffusion is a difference in partial pressure, which is just a way of expressing how concentrated a particular gas is in a mixture. When you inhale, the air filling your alveoli has a relatively high concentration of oxygen and a low concentration of carbon dioxide. The blood arriving at the alveolar capillaries has the opposite profile: it has dumped most of its oxygen into the body’s tissues and picked up carbon dioxide as a waste product. Oxygen flows down its pressure gradient from alveolus into blood, and carbon dioxide flows down its own gradient in the opposite direction.
No cellular energy is spent making this happen. It follows the same physics as a drop of food coloring spreading through a glass of water. The gas molecules bounce randomly through the barrier material and end up wherever they are less concentrated. In a healthy lung at rest, oxygen reaches equilibrium with the blood in roughly a quarter to a third of the time the blood spends passing an alveolus. That leaves a comfortable margin of safety, because the blood typically spends about 0.75 seconds flowing through a pulmonary capillary.2PubMed Central. Pushing it to the limit: enhanced diffusing membrane capacity facilitates greater pulmonary diffusing capacity in athletes during exercise
Why Carbon Dioxide Crosses More Easily Than Oxygen
Although both gases cross the same barrier, carbon dioxide diffuses about twenty times faster than oxygen through biological tissue. This is mainly because carbon dioxide is far more soluble in water and in the fluid lining the alveoli. The blood-gas barrier is not dry; it is coated in a thin liquid layer, and any gas must dissolve into that layer before it can traverse the membrane. Carbon dioxide dissolves readily, so even though the partial-pressure difference driving it out of the blood is smaller than the gradient driving oxygen in, carbon dioxide has no trouble leaving.
This difference has a practical consequence. In most lung diseases that impair diffusion, oxygen levels in the blood drop well before carbon dioxide levels rise. A patient can be significantly hypoxic while still eliminating carbon dioxide efficiently, because the carbon dioxide advantage is large enough to compensate for substantial membrane damage. Elevated blood carbon dioxide usually signals either very severe diffusion impairment or a ventilation problem where air is not reaching the alveoli in the first place.
The Role of Surfactant
The liquid lining the alveoli is not plain water. It contains pulmonary surfactant, a complex mixture of lipids and proteins that is best known for reducing surface tension so the alveoli do not collapse. But surfactant also appears to play a direct role in gas diffusion. Research has shown that water layers containing pulmonary surfactant membranes transport oxygen substantially faster than pure water or water saturated with protein-free lipid membranes.3PubMed Central. Pulmonary surfactant layers accelerate O(2) diffusion through the air-water interface The hydrophobic surfactant proteins seem to be key: membranes made from the full surfactant extract, including those proteins, allowed much faster oxygen movement than membranes made from lipids alone.
This means the surfactant layer is not just a structural necessity. It is functionally tuned to help oxygen get across the air-liquid interface and into the barrier tissue as quickly as possible. Premature infants who lack sufficient surfactant struggle not only because their alveoli tend to collapse but also because the diffusion step itself is less efficient.
How Doctors Measure Lung Diffusion
The standard clinical test for diffusion is the DLCO test, which measures how well the lungs transfer carbon monoxide from inhaled air into the blood. Carbon monoxide is used because it binds to hemoglobin so aggressively that the blood essentially acts as a bottomless sink for it, making the measurement sensitive to the membrane itself rather than to blood-flow patterns. You breathe in a tiny, safe amount of carbon monoxide mixed with a tracer gas, hold your breath for about ten seconds, and then exhale. The difference between how much carbon monoxide you inhaled and how much you exhaled tells clinicians how efficiently gas is crossing your alveolar membrane.
The total resistance to gas transfer can be split into two components. One is the membrane itself: its thickness, its surface area, how intact the alveolar structure is. The other is the reaction of the gas with hemoglobin once it reaches the red blood cells inside the capillaries.4Comprehensive Physiology. Lung Diffusing Capacities (DL) for Nitric Oxide (NO) and Carbon Monoxide (CO): The Evolving Story This distinction matters because different diseases impair different parts of the process. A disease that thickens the membrane (like pulmonary fibrosis) mainly increases membrane resistance. A condition that reduces blood volume in the capillaries (like pulmonary embolism) mainly reduces the blood-side component.
Hemoglobin levels also affect the result. If you are anemic, your blood has fewer hemoglobin molecules to grab the carbon monoxide, so DLCO will be artificially low even if your membrane is perfectly healthy. Studies in anemic patients have confirmed a clear linear relationship between hemoglobin concentration and DLCO, and clinical guidelines include correction formulas so that a low hemoglobin does not get misread as a diffusion problem.5American Journal of Respiratory and Critical Care Medicine. Adjustment of DLCO for Hemoglobin Concentration In patients with anemia from chronic kidney failure, the correction required was larger than older formulas had predicted, meaning that failing to correct properly can lead to overdiagnosing diffusion impairment.6PubMed. Effect of low hemoglobin levels on the diffusing capacity of the lungs for CO
The Nitric Oxide Alternative
More recently, nitric oxide has emerged as a complementary test gas. Nitric oxide reacts with hemoglobin so fast that the red blood cell step barely slows it down. That means the lung’s diffusing capacity for nitric oxide is almost entirely determined by the membrane component, giving clinicians a cleaner window into membrane health specifically.4Comprehensive Physiology. Lung Diffusing Capacities (DL) for Nitric Oxide (NO) and Carbon Monoxide (CO): The Evolving Story By measuring both DLCO and DLNO in a single breath, it becomes possible to separately estimate membrane conductance and capillary blood volume without needing the older, more cumbersome method of testing at two different oxygen levels.7PubMed. Pulmonary membrane diffusing capacity and capillary blood volume measured during exercise from nitric oxide uptake
What Happens When Diffusion Fails
Two broad categories of lung disease illustrate how diffusion can break down in opposite ways: emphysema and pulmonary fibrosis.
Emphysema
In emphysema, the walls between alveoli break down and the tiny air sacs merge into larger, fewer spaces. The lung does not become denser or thicker; it becomes emptier. The surface area available for gas exchange shrinks, and pores in the alveolar walls enlarge. Both of these changes directly reduce diffusing capacity.8PubMed Central. Breakdown of lung framework and an increase in pores of Kohn as initial events of emphysema and a cause of reduction in diffusing capacity CT-based measurements of lung surface area in patients with emphysema confirm that mild disease initially increases lung volume while reducing the surface-to-volume ratio, and that total surface area and tissue weight drop significantly only in severe disease.9American Journal of Respiratory and Critical Care Medicine. A Quantification of the Lung Surface Area in Emphysema Using Computed Tomography The membrane is not necessarily thicker, but there is simply less of it.
Pulmonary Fibrosis
Fibrosis is the opposite architectural problem. Instead of losing tissue, the lung gains it. Scar tissue thickens the alveolar walls, increasing the distance gases must travel. In idiopathic pulmonary fibrosis, advanced imaging with hyperpolarized xenon gas shows the problem vividly: the barrier tissue absorbs more xenon than normal (because there is more of it), while the signal from red blood cells drops (because less gas makes it all the way through). The ratio of red blood cell signal to barrier signal was about 3.3-fold lower in fibrosis patients than in healthy volunteers.10PubMed Central. Measuring diffusion limitation with a perfusion-limited gas–hyperpolarized 129Xe gas-transfer spectroscopy in patients with idiopathic pulmonary fibrosis Separate xenon MRI work found that barrier uptake in fibrosis patients increased by about 188% compared with healthy controls, and both the barrier and red blood cell signals correlated strongly with standard DLCO measurements.11Thorax. Using hyperpolarized 129Xe MRI to quantify regional gas transfer in idiopathic pulmonary fibrosis The membrane conductance for carbon monoxide was below normal in every fibrosis patient tested in one study, and it tracked closely with the extent of fibrosis seen on imaging.12PubMed. Lung diffusing capacity for nitric oxide as a marker of fibrotic changes in idiopathic interstitial pneumonias
Exercise and the Speed Limit of Diffusion
At rest, diffusion has time to spare. Blood flowing through a pulmonary capillary typically spends about three-quarters of a second in contact with the alveolus, and oxygen equilibrates with the blood in roughly a third of that time. During intense exercise, cardiac output rises dramatically, pushing blood through the capillaries much faster. Transit time can drop toward 0.25 seconds, and at that speed the blood may leave the capillary before oxygen has fully equilibrated.2PubMed Central. Pushing it to the limit: enhanced diffusing membrane capacity facilitates greater pulmonary diffusing capacity in athletes during exercise
Trained athletes compensate partly by having a higher membrane diffusing capacity, meaning their alveolar membranes can move gas faster per unit of time. This is not just a matter of having bigger lungs. The membrane component itself appears to be enhanced, allowing athletes to push harder before diffusion becomes the bottleneck. Even so, elite endurance athletes at maximal effort commonly show a measurable drop in arterial oxygen levels, evidence that they are bumping up against the physical limits of how fast gas can diffuse through tissue.
Altitude and Long-Term Adaptation
At high altitude, the air contains less oxygen per breath, which shrinks the partial-pressure gradient driving diffusion. Your body compensates in several ways. In the short term, you breathe faster and your heart pumps harder. Over weeks of acclimatization, more targeted changes kick in. A study at 5,400 meters found that after three weeks, participants showed increases in hemoglobin, alveolar volume, and membrane diffusing capacity, while capillary blood volume did not change.13PubMed. High-altitude exposure of three weeks duration increases lung diffusing capacity in humans The improvement in membrane diffusion was disproportionately large, suggesting the barrier itself was becoming more permeable to gas, possibly mediated by changes in sympathetic nervous system tone.
Populations that have lived at high altitude for generations go further. A large study comparing over 10,000 participants at different elevations found that high-altitude residents had significantly higher diffusing capacity than their low-altitude counterparts, even after statistical matching. Tibetans showed the most pronounced advantages, with DLCO gains that exceeded what could be explained by lung volume alone.14PubMed Central. Comparative analysis of pulmonary function in diverse ethnic adults living at low and high altitudes Within the altitude range of 2,500 to 4,500 meters, DLCO correlated positively with elevation, and this correlation persisted even above 4,500 meters. Chronic high-altitude adaptation involves broader physiological remodeling as well, including molecular pathways that enhance capillary recruitment and reduce inflammatory damage to the lungs.15PubMed Central. Chronic hypoxia adaptation at high altitude: a perspective on Its potential role in mortality in viral pneumonia-associated ARDS and implications for personalized critical care
Aging and the Gradual Decline
Even in perfectly healthy people, diffusion capacity drifts downward with age. The lung’s supporting framework loosens over the decades, and the alveoli gradually dilate into larger, less efficient spaces, sometimes described as “senile emphysema.” This increases the volume of air that sits in areas where gas exchange is poor (alveolar dead space), which lowers arterial oxygen levels even though carbon dioxide elimination stays intact.16PubMed Central. Effect of aging on respiratory system physiology and immunology The decline is gradual enough that most people never notice it at rest, but it contributes to why older adults become breathless more easily during exertion. It also means that the margin of safety for diffusion, the excess capacity that keeps oxygen levels stable when something goes mildly wrong, shrinks year by year.
Environmental Insults to the Barrier
The blood-gas barrier sits at the interface between your body and whatever you breathe, making it vulnerable to airborne insults. Ultrafine particles, the smallest category of air pollution, are small enough to penetrate deep into the alveoli and interact with the barrier directly. In controlled exposure experiments, healthy subjects exposed to ultrafine carbon particles at relatively modest concentrations showed a measurable drop in DLCO about 21 hours after exposure, along with signs of small-airway dysfunction, even without detectable airway inflammation.17PubMed. Pulmonary function, diffusing capacity, and inflammation in healthy and asthmatic subjects exposed to ultrafine particles The effect was subtle but real, suggesting that the barrier’s efficiency is sensitive to particulate exposure even in the absence of an obvious immune response.
Cigarette smoke is the most familiar chronic insult, driving both the emphysematous destruction of alveolar walls and, over time, inflammatory thickening of the remaining membrane. Occupational dust exposures, certain chemotherapy drugs, and radiation to the chest can all damage the barrier as well. In each case, the core problem maps back to the same physics: either the surface area shrinks, or the barrier thickens, or both, and diffusion slows.
How Bird Lungs Outperform Ours
If the human blood-gas barrier seems impressively thin at 0.6 micrometers, birds have taken the concept considerably further. The avian blood-gas barrier is about 2.5 times thinner than the mammalian version and far more uniform in thickness, meaning a larger fraction of the barrier surface is optimized for gas exchange rather than serving a structural role.18PubMed Central. Comparative physiology of the pulmonary blood-gas barrier: the unique avian solution Birds solve the thinness-versus-strength dilemma differently from mammals. Their pulmonary capillaries are mechanically supported from the outside by surrounding tissue, rather than relying solely on the internal structure of the capillary wall for strength. This external bracing allows the barrier itself to be thinner without risking rupture under the high cardiac outputs birds sustain during flight. It is one reason birds can fly at altitudes where mammals would be incapacitated by hypoxia: the gas-exchange membrane is simply more efficient at moving oxygen per unit of time.