Deoxygenated blood picks up oxygen in the lungs, specifically at tiny air sacs called alveoli. There are roughly 300 million of these sacs in your lungs, and each one is wrapped in a mesh of ultra-thin blood vessels called pulmonary capillaries. This is where the actual exchange happens: oxygen passes from the air you breathe into your blood, while carbon dioxide moves the opposite direction to be exhaled.
How Blood Gets to the Lungs
After your body’s tissues have used up the oxygen in your blood, the now-deoxygenated blood returns to the right side of your heart. The right ventricle pumps it out through the pulmonary arteries, which are the only arteries in your body that carry oxygen-poor blood. These arteries branch into smaller and smaller vessels as they spread through both lungs, eventually becoming capillaries so narrow that red blood cells pass through in single file.
The blood arriving at the lungs typically has an oxygen partial pressure of about 40 mmHg, compared to roughly 100 mmHg in the alveolar air. That pressure difference is what drives oxygen from the air side into the blood side, purely through passive diffusion. No energy is required. Oxygen simply moves from where there’s more of it to where there’s less.
The Blood-Air Barrier
For oxygen to reach your red blood cells, it has to cross four layers: a thin coating of fluid called surfactant that lines the inside of each alveolus, the alveolar wall itself (just one cell thick), a shared basement membrane, and the capillary wall (also one cell thick). Together, these layers are incredibly thin, roughly 0.5 micrometers in places. That’s about 1/100th the width of a human hair.
Your lungs compensate for this microscopic scale with enormous surface area. If you could flatten out all the alveoli in both lungs, they’d cover about 118 square meters, roughly the size of a singles tennis court. This massive contact zone between air and blood is what allows your lungs to load oxygen into your bloodstream so efficiently.
How Hemoglobin Loads Oxygen
Most oxygen doesn’t just dissolve in your blood plasma. Instead, it binds to hemoglobin, the protein packed inside red blood cells. Each hemoglobin molecule has four binding sites, one on each of its four subunits. At the center of each binding site sits an iron atom that grabs onto one oxygen molecule.
Hemoglobin arriving at the lungs is in what scientists call the “T state,” a tense, low-affinity shape that doesn’t bind oxygen easily. But once the first oxygen molecule attaches, the protein shifts shape, making it progressively easier for the second, third, and fourth oxygen molecules to bind. This cooperative effect means that in the oxygen-rich environment of the alveoli, hemoglobin quickly loads up to full capacity. The whole process happens fast: red blood cells spend about 0.75 seconds passing through the pulmonary capillaries at rest, and hemoglobin reaches nearly 100% saturation well before the cell exits. Full oxygen loading takes only about a third of the available transit time, leaving a comfortable margin.
Once fully loaded, hemoglobin flips into its “R state,” a relaxed shape that holds oxygen tightly. The oxygenated blood then flows into the pulmonary veins, returns to the left side of your heart, and gets pumped out to the rest of your body. When it reaches tissues where oxygen levels are low, the process reverses: hemoglobin shifts back to the T state and releases its oxygen where cells need it most.
What Changes at High Altitude
The percentage of oxygen in the air stays the same at any altitude (about 21%), but atmospheric pressure drops as you go higher. At 5,500 meters (roughly 18,000 feet), atmospheric pressure is half of what it is at sea level. That means the partial pressure of oxygen in your alveoli is also much lower, which shrinks the pressure gradient that drives oxygen into your blood.
At extreme altitudes, the reduced driving pressure can become too low to fully oxygenate blood during the brief time it passes through the capillaries. On the summit of Everest (8,900 meters), atmospheric pressure drops to just 30% of the sea level value. This is why climbers at extreme altitude experience symptoms of oxygen deprivation even while breathing as hard as they can. Their lungs still work the same way, but the physics of diffusion simply can’t push enough oxygen across the barrier in the time available.
Your body adapts to moderate altitude over days and weeks by producing more red blood cells, increasing the oxygen-carrying capacity of each liter of blood. Breathing rate also increases, which raises the oxygen concentration inside the alveoli and partially restores the diffusion gradient. These adaptations explain why acclimatization matters so much for mountaineers and why ascending too quickly causes altitude sickness.
How Saturation Numbers Reflect This Process
When you clip a pulse oximeter on your finger, you’re measuring the percentage of hemoglobin that’s carrying oxygen. In healthy people at sea level, arterial blood leaving the lungs is typically 95% to 100% saturated. Mixed venous blood returning to the lungs after delivering oxygen to tissues normally sits around 75% saturated. That gap represents the oxygen your body extracted during one trip through the circulation.
If venous saturation drops below 65%, it suggests tissues are straining to pull enough oxygen from the blood, either because not enough oxygen is being loaded at the lungs or because the heart isn’t pumping enough blood to meet demand. Values below 30% to 50% signal that cells have nearly exhausted their ability to extract oxygen and are starting to shift toward less efficient energy production without it.