Alveoli are tiny air sacs where oxygen diffuses from air into pulmonary capillary blood and carbon dioxide diffuses from blood into the alveolar space. The respiratory membrane consists of alveolar epithelium, basement membranes, and capillary endothelium—a thin barrier optimized for rapid gas exchange.
Gas exchange is fundamentally a diffusion problem, and diffusion — as you studied via Fick's laws — is driven entirely by concentration gradients. In the lungs, concentration gradients are expressed as partial pressures: the pressure exerted by a single gas in a mixture. In freshly inhaled alveolar air, the partial pressure of oxygen (PO₂) is approximately 100 mmHg. In deoxygenated blood arriving at the pulmonary capillaries, PO₂ is about 40 mmHg. Oxygen moves down this 60 mmHg gradient, crossing the respiratory membrane from alveolus into blood. Carbon dioxide flows the other way: PCO₂ is about 45 mmHg in venous blood and only 40 mmHg in alveolar air, so CO₂ diffuses out of blood and into the alveolus to be exhaled.
The respiratory membrane is the physical barrier gases must cross, and its anatomy is designed to minimize resistance. It consists of the alveolar epithelial cell, the fused basement membranes of the epithelium and capillary endothelium, and the capillary endothelial cell — together only about 0.5 micrometers thick. Fick's law tells you that diffusion rate is proportional to surface area and inversely proportional to membrane thickness. The ~70 m² of alveolar surface area combined with this extraordinarily thin membrane makes the lungs enormously efficient. Conditions that thicken the membrane (pulmonary fibrosis) or reduce surface area (emphysema) directly impair gas exchange by these physical laws.
The efficiency of exchange also depends on ventilation-perfusion matching — how well airflow (ventilation) and blood flow (perfusion) are distributed to the same alveoli. From your study of respiratory anatomy, you know that the lungs have complex branching airways. Ideally, every alveolus that receives fresh air also receives blood flow to pick up that oxygen. When airways are blocked but blood still flows (low V/Q ratio), blood passes without picking up oxygen — a shunt. When alveoli receive air but no blood flow (high V/Q ratio), ventilation is wasted — dead space. Perfect matching produces maximal gas exchange; mismatching reduces the efficiency even if the membrane itself is intact.
Oxygen and carbon dioxide differ in their diffusion characteristics in an important way. CO₂ is about 20 times more soluble in tissue fluid than O₂, which means it diffuses much more readily across the membrane even though its partial pressure gradient is smaller. This is why hypercapnia (excess CO₂) is usually driven by ventilation problems rather than membrane problems — if someone can breathe adequately, CO₂ clears easily. Hypoxemia (low blood oxygen) is more sensitive to membrane thickening and surface area loss, because O₂ has less solubility to compensate. This distinction is clinically important: a patient with pulmonary fibrosis often develops hypoxemia long before hypercapnia appears.