Medicine
Pulmonary Gas Exchange and Oxygen Transport
Quick fact
The lungs have a surface area roughly the size of a tennis court, packed into your chest, allowing oxygen to diffuse into your blood in less than one second.
Why this is interesting
Every breath you take fills your lungs with air, but how does that air actually get into your bloodstream? The answer is a microscopic journey across a membrane thinner than tissue paper.
Read the full explanation
Understanding Pulmonary Gas Exchange and Oxygen Transport
When you inhale, fresh air reaches tiny air sacs called alveoli. Each alveolus is wrapped in a net of tiny blood vessels called pulmonary capillaries. Oxygen from the air moves across the thin alveolar-capillary membrane into the blood, while carbon dioxide moves in the opposite direction, from blood to air. This exchange is driven by differences in gas pressure—a simple rule: gases move from high pressure to low pressure. Oxygen pressure is higher in the alveoli than in the deoxygenated blood, so oxygen enters the blood. Carbon dioxide pressure is higher in the blood than in the alveoli, so carbon dioxide leaves the blood. Once in the blood, most oxygen attaches to hemoglobin inside red blood cells, forming oxyhemoglobin. The blood then travels to the heart, which pumps it to the body.
A deeper explanation
The efficiency of pulmonary gas exchange depends on two key factors: a thin diffusion barrier and a large surface area. The alveolar-capillary membrane is only about 0.5 micrometers thick, and the combined alveolar surface area is about 70 square meters. Oxygen diffuses in according to Fick's law—the rate of diffusion is proportional to the pressure gradient and surface area, and inversely proportional to membrane thickness. After entering the blood, oxygen binds reversibly to hemoglobin. Hemoglobin's quaternary structure allows cooperative binding: after one oxygen molecule binds, the affinity for the next oxygen increases, so oxygen loading is efficient at high alveolar pressures. At the tissues, lower oxygen pressure and lower pH promote oxygen release. Carbon dioxide, meanwhile, is carried in three forms: dissolved in plasma, as bicarbonate ions, and bound to hemoglobin. This gas exchange process is intimately tied to ventilation and perfusion—alveoli must be well-ventilated (receiving fresh air) and well-perfused (receiving blood). When this matching fails, blood oxygen falls, a condition called hypoxia. Understanding these mechanisms is critical for treating respiratory diseases, anesthesia, and oxygen therapy.