Gas exchange in the lungs happens through simple diffusion: oxygen and carbon dioxide move across a thin membrane driven entirely by differences in pressure, with no energy required. Oxygen passes from the air in your lungs into your blood, while carbon dioxide moves the opposite direction, from blood into your lungs to be exhaled. The entire process takes less than a second and happens continuously across a surface area estimated between 70 and 140 square meters, roughly the size of a tennis court packed inside your chest.
What Drives the Exchange: Pressure Gradients
Gases naturally move from areas of higher pressure to areas of lower pressure, the same way a balloon deflates into a room. In your lungs, this principle does all the work. The air sitting in your alveoli (the tiny air sacs at the end of your airways) has an oxygen pressure of about 100 mmHg. The deoxygenated blood arriving from your body’s tissues has an oxygen pressure of only 40 mmHg. That 60 mmHg difference pushes oxygen out of the air and into the blood until the blood matches the alveolar pressure at 100 mmHg.
Carbon dioxide works the same way, just in reverse and with a much smaller gradient. Blood returning to the lungs carries carbon dioxide at a pressure of about 45 mmHg, while the air in the alveoli sits at 40 mmHg. That modest 5 mmHg gap is all it takes because carbon dioxide dissolves about 20 times more readily than oxygen, so it crosses the membrane easily despite the smaller pressure difference.
The Membrane Where It Happens
Your lungs contain roughly 300 million alveoli, each wrapped in a dense net of capillaries so narrow that red blood cells pass through in single file. The barrier between air and blood, called the respiratory membrane, is built from just a few layers of cells: a thin lining of alveolar cells on the air side, a layer of endothelial cells forming the capillary wall, and a small amount of supporting tissue between them. The total thickness is about 10 micrometers, roughly one-tenth the width of a human hair.
This design is optimized for diffusion in every way that matters. The membrane is extremely thin, minimizing the distance gases have to travel. The surface area is enormous, giving oxygen and carbon dioxide as much room as possible to cross. And the blood flows slowly enough through these capillaries that it has time to fully equilibrate with the air before moving on. Four factors govern how quickly gas crosses: the surface area of the membrane, the pressure difference between the two sides, how soluble the gas is, and how thin the barrier is. Anything that damages the alveoli, thickens the membrane, or reduces surface area (as emphysema does) slows the exchange.
How the Body Keeps It Efficient
Having millions of alveoli only helps if blood actually flows past the ones that are filled with fresh air. Your body matches airflow to blood flow through a process called ventilation-perfusion matching, the single most important factor in how efficiently your lungs exchange gas. When a section of lung isn’t receiving much air (say, because of mucus or a collapsed airway), the blood vessels in that area constrict, redirecting blood toward better-ventilated alveoli. This ensures that blood doesn’t pass through the lungs without picking up oxygen.
At the extremes, this matching can fail. If blood flows past alveoli that receive no air at all, it acts like a shunt, and deoxygenated blood returns to the body unchanged. If air fills alveoli with no blood flow, that ventilation is wasted, functioning as dead space. Healthy lungs keep these mismatches to a minimum.
How Oxygen Travels Through Your Blood
Once oxygen crosses into the capillaries, nearly all of it binds to hemoglobin inside red blood cells. Hemoglobin is remarkably efficient at loading up in the lungs, where oxygen pressure is high, and then releasing its cargo when it reaches tissues where oxygen pressure is low. This loading and unloading isn’t random. It responds to local conditions in a way that automatically prioritizes the tissues that need oxygen most.
When cells are working hard, such as exercising muscle, they produce more carbon dioxide and lactic acid, making the surrounding blood more acidic. The local temperature also rises from the heat of metabolism. All three of these signals cause hemoglobin to release oxygen more readily. This is called the Bohr effect: acidic, warm, carbon dioxide-rich environments essentially loosen hemoglobin’s grip on oxygen, delivering more of it exactly where demand is highest. During intense exercise, this effect can dramatically increase oxygen delivery to working muscles without any conscious effort on your part.
How Carbon Dioxide Gets Back to the Lungs
Carbon dioxide’s return trip is more chemically complex than oxygen’s journey. It travels in three different forms. About 80% is converted into bicarbonate ions through a chemical reaction inside red blood cells. Water and carbon dioxide combine to form carbonic acid, which quickly splits into a hydrogen ion and a bicarbonate ion. This bicarbonate dissolves into the plasma and rides the bloodstream back to the lungs, where the entire reaction reverses: bicarbonate recombines with hydrogen ions, re-forms carbon dioxide gas, and crosses back into the alveoli to be exhaled.
The remaining carbon dioxide splits between two simpler routes. About 10% stays dissolved directly in the plasma, much like carbon dioxide dissolved in a can of soda. The final 10% binds to hemoglobin itself (at a different site than where oxygen attaches), hitching a ride on the same protein that carries oxygen. Once blood reaches the lungs and oxygen pressure rises, hemoglobin preferentially grabs oxygen and releases carbon dioxide, completing the cycle.
Gas Exchange in Your Tissues
The exchange that happens in your lungs is only half the picture. The same diffusion process occurs in reverse at every tissue in your body. Oxygen-rich blood arriving through arteries has an oxygen pressure of about 100 mmHg. The cells surrounding those capillaries have been consuming oxygen for fuel, dropping local oxygen pressure to around 40 mmHg. Oxygen diffuses down that gradient from blood into tissue, while carbon dioxide produced by cellular metabolism diffuses from tissue into the blood. This tissue-level exchange is sometimes called internal respiration, as opposed to the external respiration happening in the lungs.
The two sites of exchange form a continuous loop. Your lungs load oxygen and dump carbon dioxide. Your tissues dump oxygen and load carbon dioxide. The blood circulating between them is just the delivery system, and the pressure gradients at each end of the loop do all the actual moving. No pumps, no active transport, no energy expenditure beyond what the heart uses to keep blood flowing. The elegance of the system is that it’s entirely passive: gases simply follow the laws of physics, flowing from where there’s more to where there’s less.