Diffusion Across Alveolar-Capillary Membrane

Diffusion across the alveolar-capillary membrane is the passive movement of oxygen and carbon dioxide between alveolar gas and capillary blood, driven by partial-pressure gradients across the thin barrier that separates them. The barrier's enormous surface area and minimal thickness make this transfer remarkably rapid.

Definition

Diffusion across the alveolar-capillary membrane is the passive transfer of gases between alveolar air and pulmonary capillary blood down their partial-pressure gradients, with a rate set by the gradient, the barrier area and thickness, and the gas's diffusion properties.

Scope

This topic covers the structure of the alveolar-capillary barrier, Fick's law of diffusion as applied to the lung, the distinction between diffusion-limited and perfusion-limited gas transfer, the concept of diffusing capacity, and how the membrane and blood components each contribute. It is reference physiology and not clinical guidance.

Core questions

• What structural features of the alveolar-capillary barrier favour rapid diffusion?
• How does Fick's law describe the rate of gas transfer in the lung?
• What distinguishes a perfusion-limited from a diffusion-limited gas?
• What is diffusing capacity, and what membrane and blood factors determine it?

Key concepts

• Alveolar-capillary barrier structure
• Fick's law of diffusion
• Perfusion-limited versus diffusion-limited transfer
• Diffusing capacity (transfer factor)
• Membrane and blood (red-cell) resistances
• Effect of barrier thickening on transfer

Key theories

Fick's law applied to the lung

The rate of gas transfer is proportional to the surface area and the partial-pressure difference and inversely proportional to barrier thickness, scaled by a gas-specific diffusion constant; this framework explains why the thin, vast alveolar surface is so effective.

Membrane and blood components of diffusing capacity

Roughton and Forster partitioned the resistance to gas uptake into a membrane component and a blood (red-cell reaction with hemoglobin) component, showing that diffusing capacity reflects both the barrier and the rate of chemical combination in the blood.

Mechanisms

The alveolar-capillary barrier comprises alveolar epithelium, a fused basement membrane region, and capillary endothelium, presenting a very large total surface and a very short diffusion path. Under Fick's law, transfer rate rises with surface area and the partial-pressure gradient and falls with barrier thickness. For oxygen and carbon dioxide in health, blood equilibrates with alveolar gas well within the capillary transit time, so transfer is perfusion-limited (set by blood flow) rather than diffusion-limited; with thickened membranes or shortened transit, diffusion limitation can emerge. Diffusing capacity quantifies the lung's conductance for a gas and, as Roughton and Forster showed, reflects both a membrane resistance and a blood-side resistance arising from the rate at which the gas reacts with hemoglobin in the red cell.

Clinical relevance

Diffusing capacity for carbon monoxide is a standard pulmonary function measurement used to characterize the gas-transfer properties of the lung, and the perfusion- versus diffusion-limited distinction explains why diffusion impairment matters most during exercise or at altitude. This entry describes the physiology for reference and does not provide diagnostic criteria or treatment advice.

Evidence & guidelines

The concepts are established physiology, grounded in the classic partition of diffusing capacity into membrane and blood components, in morphometric study of the gas-exchange surface, and in standard textbooks. The topic is descriptive physiology rather than guideline-governed practice.

History

Quantitative study of pulmonary diffusion advanced in the mid-twentieth century when Roughton and Forster separated the membrane and blood contributions to gas uptake, and Weibel's later morphometric work established the structural basis — surface area and barrier thickness — for the lung's high diffusing capacity. These ideas remain the foundation of clinical transfer-factor measurement.

Key figures

• Francis Roughton
• Robert Forster
• Ewald Weibel
• John B. West

Seminal works

• roughton-forster-1957
• weibel-1973

Frequently asked questions

What does it mean that oxygen transfer is normally perfusion-limited?

It means that under resting conditions the blood fully equilibrates with alveolar oxygen long before it leaves the capillary, so the amount of oxygen taken up is set by how much blood flows past rather than by the speed of diffusion.

Why is diffusing capacity usually measured with carbon monoxide?

Carbon monoxide binds hemoglobin so avidly that its capillary partial pressure stays near zero, making its uptake depend on the diffusion properties of the lung rather than on blood flow, which is what diffusing capacity aims to measure.