Exchange of Gases in Alveoli: How Lungs Work Simply

Exchange of gases in alveoli is the diffusion of oxygen from the air sacs into the blood and carbon dioxide from the blood into the air sacs. It is the central event of respiration, happening across millions of tiny, thin-walled alveoli in both lungs.

This exchange matters because it is the only place where the body can load oxygen and unload carbon dioxide. If the alveolar wall thickens with fluid or scar tissue, diffusion slows, and a person feels breathless even while breathing hard.

Key Concepts

Diffusion is the movement of molecules from higher to lower partial pressure without energy use. Partial pressure is the pressure contributed by one gas in a mixture. In alveoli, oxygen partial pressure (pO2) is about 104 mmHg, while in incoming venous blood it is about 40 mmHg, so oxygen moves into blood. Carbon dioxide partial pressure (pCO2) is about 45 mmHg in venous blood and about 40 mmHg in alveoli, so carbon dioxide moves out. The diffusion membrane has three layers: alveolar epithelium, basement substance and capillary endothelium, together less than 1 micrometre thick. A fluid called surfactant keeps alveoli from collapsing.

• Ventilation: bringing fresh air to alveoli.
• Perfusion: blood flow through alveolar capillaries.
• Diffusion capacity: depends on surface area, membrane thickness and pressure difference.

Formulas

The driving force follows Fick principle in simple form: rate of diffusion is proportional to (surface area multiplied by pressure difference) divided by membrane thickness. Symbolically, rate ∝ (A × ΔP) / T, where A is alveolar surface area in square metres, ΔP is the partial pressure difference in mmHg, and T is membrane thickness. Doubling surface area doubles diffusion, while doubling thickness halves it. Oxygen diffuses about as fast as its gradient allows; carbon dioxide diffuses about 20 times more readily through tissue than oxygen because it is far more soluble. This is why oxygen problems appear earlier than carbon dioxide retention in many lung diseases.

Worked Example

At rest, venous blood arrives with pO2 of 40 mmHg and alveolar pO2 is 104 mmHg, giving a difference of 64 mmHg driving oxygen inward. Blood spends only about 0.75 seconds in an alveolar capillary, yet oxygen loading is normally complete within 0.25 seconds, leaving a safety margin. During heavy exercise, blood moves faster and contact time may fall to 0.25 seconds, but loading still finishes in time in healthy lungs. In pneumonia, fluid thickens the membrane from under 1 micrometre to several micrometres, perhaps halving or thirding diffusion by the formula above, so blood leaves with pO2 well below normal and the patient becomes hypoxic.

Common Misconceptions

• Misconception: Oxygen and carbon dioxide are actively pumped across alveoli. Correction: Both move only by passive diffusion down partial pressure gradients.
• Misconception: We use all oxygen we inhale. Correction: Inhaled air has about 21 percent oxygen, exhaled air still has about 16 percent; only a fraction is absorbed.
• Misconception: Carbon dioxide leaves mainly through the skin. Correction: Essentially all carbon dioxide is eliminated through alveoli.

Real-World Applications

Pulse oximeters estimate blood oxygen saturation to detect poor alveolar exchange. Oxygen therapy raises alveolar pO2 to increase the diffusion gradient in pneumonia and COVID-19. Premature babies receive surfactant to prevent alveolar collapse. High-altitude training works because lower atmospheric pressure reduces alveolar pO2 and the body adapts by making more red cells.

Gas exchange in alveoli is pure physics serving biology: a huge, thin, moist surface and maintained pressure gradients let oxygen rush in and carbon dioxide rush out with every breath.