Respiratory System Diffusion Xinping Yue xyuelsuhsc edu Department

Respiratory System Diffusion Xinping Yue xyue@lsuhsc. edu Department of Physiology LSUHSC-NO

Definition of Diffusion of a Gas Net movement of the molecules of a gas from a region of higher concentration to a region of lower concentration. Or Net movement of the molecules of a gas from a region in which it exerts a higher partial pressure to an area in which it exerts a lower partial pressure.

Linear Velocity = Flow / Cross sectional Area (cm/sec) (cm 3/sec) (cm 2)

1. Diffusion through the gas phase 2. Diffusion through alveolarcapillary membrane (Fick’s Law) 3. Diffusion through the plasma 4. Combine with hemoglobin

The Laws Governing the Behavior of Gases Ø Henry’s Law : The amount of a gas absorbed by a liquid with which it does not combine chemically, is directly proportional to the partial pressure of the gas to which the liquid is exposed and its solubility in the liquid. The solubility of CO 2 in the liquid phase is ~24 times that of O 2. Ø Graham’s Law : The rate of diffusion of a gas (in the gas phase) is inversely proportional to the square root of its molecular weight. Diffusion rate for O 2 Diffusion rate for CO 2 MW of O 2 Ø Fick’s Law of Diffusion : . Vgas = A x D x (P 1 – P 2) T = 1. 17

Fick’s Law for Diffusion. Vgas = A x D x (P 1 – P 2) T . Vgas = volume of gas diffusing through the tissue barrier per time (ml/min) A = surface area available for diffusion (70 m 2 in a healthy adult) D = diffusion coefficient, or diffusivity, of the particular gas CO 2 is diffuses ~20 times (24/1. 17) more solubility D rapidly through the alveolar-capillary MW barrier than O 2 P 1 – P 2 = partial pressure difference of the gas across the barrier T = thickness of the barrier or the diffusion distance (0. 2 to 0. 5 m)

Limitation of Gas Transfer • Diffusion-limited, CO • Perfusion-limited, N 2 O • Perfusion-limited, O 2

Limitation of O 2 Transfer • Healthy at rest – perfusion-limited • Healthy during excise – may reach diffusion limitation • Abnormal alveolar-capillary barrier – diffusion limited

Limitation of CO 2 Transfer • Healthy at rest – perfusion-limited • Healthy during excise – may reach diffusion limitation • Abnormal alveolar-capillary barrier – diffusion limited

Diffusing Capacity DLx = . Vx ml / min / mm Hg (PAx – PCx) . Fick’s equation: Vx =. Vx (P 1 – P 2) A x D x (P 1 – P 2) T = Ax. D T (DLx)

Measurement of Diffusing Capacity DLCO = = = . VCO (PACO – PCCO). VCO (PACO – 0). VCO PACO

Conditions that Decrease the Diffusing Capacity • Thickening of the barrier • Decreased surface area • Decreased uptake by erythrocytes • Ventilation-perfusion mismatch – Interstitial or alveolar edema – Interstitial or alveolar fibrosis • Sarcoidosis • Scleroderma – Emphysema – Tumors – Low cardiac output – Low pulmonary capillary blood volume – Anemia – Low pulmonary capillary blood volume

Normal Lung IPF Lung

Emphysema