9.1.7—Gas exchange between alveolar air and blood

Efficient gas exchange depends on a maintained difference in oxygen and carbon-dioxide partial pressure across the alveolus–capillary interface. The interface combines a thin, moist barrier with a large surface and close blood supply, so the gradient can produce useful net diffusion.

• Gradient at the interface: alveolar air has the higher oxygen partial pressure, while arriving capillary blood has the higher carbon-dioxide partial pressure; these differences set the opposite directions of gas movement.
• Ventilation maintains the air side: fresh air replaces gas in the alveoli, helping prevent the alveolar partial pressures from moving too close to the incoming blood values.
• Blood flow maintains the blood side: capillaries bring deoxygenated blood to the alveoli and carry oxygenated blood away, preserving contact with blood that can still take up oxygen.
• Thin, moist surface improves transfer: the one-cell-thick squamous lining and moist interface give gases a short path through which to diffuse.
• Large surface and capillary contact scale the exchange: many alveoli provide extensive area, while the surrounding capillary network keeps that area supplied with moving blood.

The system is passive diffusion across an exchange surface, not ATP-driven membrane transport. Area, ventilation and blood flow support exchange only by helping preserve usable gradients and contact with the thin barrier; they are not separate active pumps. This final card synthesises the mechanism without repeating the airway route, the full diffusion sequence or a quantitative diffusion equation. No image generated or bound.