When blood reaches the lungs, it swaps carbon dioxide for oxygen. Deoxygenated blood arrives with an oxygen pressure of about 40 mmHg, and by the time it leaves, that number has jumped to 100 mmHg. The entire exchange happens in tiny air sacs called alveoli, where blood and air come close enough for gases to pass between them. This process transforms dark, oxygen-depleted blood into the bright red, oxygen-rich blood your body needs to survive.
How Blood Travels Through the Lungs
Blood enters the lungs through the pulmonary artery, which exits the right side of the heart. This artery is surprisingly short, only about 5 centimeters, before it splits into left and right branches. From there, it divides again and again into smaller arteries, then arterioles, and finally into an enormous web of capillaries wrapped around the alveoli.
These capillaries are where the real work happens. After gas exchange, blood flows into small venules that merge into larger veins, eventually forming the pulmonary veins that carry freshly oxygenated blood back into the left side of the heart. From there, it’s pumped out to the rest of your body.
The Gas Exchange Itself
Gas exchange works by simple diffusion: gases naturally move from areas of higher concentration to areas of lower concentration. Oxygen is more concentrated in the air inside the alveoli (about 100 to 104 mmHg) than in the incoming blood (40 mmHg), so it flows into the blood. Carbon dioxide works in reverse. It’s slightly more concentrated in the blood (45 to 46 mmHg) than in the alveoli (40 mmHg), so it passes out of the blood and into the air you’re about to exhale.
To make this transfer, gases must cross a remarkably thin barrier: a layer of surfactant coating the alveolus, the alveolar wall itself, a shared basement membrane, and the capillary wall. Despite those four layers, the barrier is so thin that gas exchange reaches completion about one-third of the way through the capillary. A red blood cell spends roughly 0.75 seconds passing through the gas exchange zone at rest, but it only needs about 0.25 seconds to fully load up on oxygen. That three-fold safety margin means your lungs have significant reserve capacity, which is why you can exercise hard before breathing becomes a limiting factor.
How Oxygen Loads Onto Red Blood Cells
Oxygen doesn’t just dissolve in blood. The vast majority binds to hemoglobin, the protein packed inside red blood cells. Each hemoglobin molecule contains four iron atoms, and each iron atom can grab one molecule of oxygen, for a total of four oxygen molecules per hemoglobin.
The process is cooperative, meaning it gets easier as it goes. Hemoglobin starts in a “tense” shape that resists oxygen. When the first oxygen molecule binds, it breaks some of the internal bonds holding that tight shape together, shifting hemoglobin into a more “relaxed” form. Each successive oxygen molecule binds more easily than the last. This cooperative loading is what allows hemoglobin to pick up oxygen so efficiently in the lungs, where oxygen levels are high, and release it later in tissues where oxygen levels are low.
By the time blood leaves the lungs, hemoglobin is typically 95% to 100% saturated with oxygen. That’s the number a pulse oximeter reads when you clip one on your finger.
How Carbon Dioxide Gets Released
Removing carbon dioxide is more chemically involved than picking up oxygen. Your body transports carbon dioxide in three forms: dissolved in the blood plasma, bound directly to hemoglobin, and (the largest share) converted into bicarbonate. All three forms must be reversed in the lungs so carbon dioxide can be exhaled as a gas.
The bicarbonate conversion is the most interesting step. In your tissues, carbon dioxide reacts with water to form carbonic acid, which quickly splits into hydrogen ions and bicarbonate. This reaction is sped up by an enzyme inside red blood cells. In the lungs, the entire reaction runs backward: bicarbonate recombines with hydrogen ions to form carbonic acid, which then breaks apart into carbon dioxide and water. The carbon dioxide diffuses across the alveolar membrane and leaves with your next breath.
Oxygen itself helps push this process along. As hemoglobin picks up oxygen in the lungs, its shape changes in a way that reduces its grip on carbon dioxide. So the very act of loading oxygen helps unload carbon dioxide at the same time.
Blood pH Shifts During the Exchange
Carbon dioxide is acidic. When it dissolves in blood, it produces hydrogen ions that lower your blood’s pH. Deoxygenated blood arriving at the lungs is therefore slightly more acidic than the oxygenated blood that leaves. As carbon dioxide is exhaled, the concentration of hydrogen ions drops and blood pH rises back toward its normal range. This is one of the main ways your body regulates its acid-base balance on a moment-to-moment basis. Breathing faster removes more carbon dioxide and raises pH; breathing slower retains carbon dioxide and lowers it.
Why Surfactant Matters
The alveoli are tiny, and their small size creates a problem: surface tension from the thin layer of fluid lining them would normally cause them to collapse, especially during exhalation when they shrink. Pulmonary surfactant, a mixture produced by specialized cells in the alveolar walls, coats the inside surface and dramatically reduces that tension. During exhalation, when alveoli are at their smallest and most vulnerable, surfactant can reduce surface tension to near zero.
Without adequate surfactant, the alveoli collapse, the barrier between air and blood breaks down, and fluid leaks into the lungs. This is one reason premature babies often struggle to breathe: their lungs haven’t yet produced enough surfactant to keep the alveoli open.
What Can Go Wrong
Efficient gas exchange depends on a good match between airflow and blood flow. Your lungs receive about 300 million alveoli worth of surface area for this exchange, and both air and blood need to reach the same regions for the system to work. When airflow is blocked (by mucus, fluid, or a collapsed airway) but blood still flows to that area, the blood passes through without picking up oxygen. When blood flow is blocked (by a clot, for example) but air still reaches the alveoli, that ventilation is wasted.
Anything that thickens the membrane between air and blood also slows diffusion. Conditions that cause scarring or inflammation in the lungs increase the distance gases must travel, which can prevent the exchange from completing during the brief time blood spends in the capillaries. At rest, the three-fold time cushion usually compensates. During exercise, when blood moves faster through the capillaries, even mild thickening can cause oxygen levels to drop.