respiratory system diffusion xinping yue xyue@lsuhsc.edu department of physiology lsuhsc-no

Respiratory System Diffusion

Xinping Yuexyue@lsuhsc.edu

Department of PhysiologyLSUHSC-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) (cm3/sec) (cm2)

1. Diffusion through the gas phase

2. Diffusion through alveolar-capillary membrane (Fick’s Law)

3. Diffusion through the plasma

4. Combine with hemoglobin

The Laws Governing the Behavior of Gases 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 CO2 in the liquid phase is ~24 times that of O2.

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.

Fick’s Law of Diffusion : A x D x (P1 – P2)

TVgas =.

MW of CO2

MW of O2

Diffusion rate for O2

Diffusion rate for CO2

= 1.17

Fick’s Law for DiffusionFick’s Law for Diffusion

Vgas =Vgas =A x D x (P1 – P2)A x D x (P1 – P2)

TT

Vgas = volume of gas diffusing through the tissue barrier per time (ml/min) A = surface area available for diffusion (70 m2 in a healthy adult)

D = diffusion coefficient, or diffusivity, of the particular gas

P1 – P2 = partial pressure difference of the gas across the barrier

T = thickness of the barrier or the diffusion distance (0.2 to 0.5 m)

.

.

D solubilityMW

CO2 is diffuses ~20 times (24/1.17) more rapidly through the alveolar-capillary barrier than O2

Limitation of Gas Transfer

• Diffusion-limited, CO

• Perfusion-limited, N2O

• Perfusion-limited, O2

Limitation of O2 Transfer

• Healthy at rest – perfusion-limited

• Healthy during excise – may reach diffusion limitation

• Abnormal alveolar-capillary barrier – diffusion limited

Limitation of CO2 Transfer

• Healthy at rest – perfusion-limited

• Healthy during excise – may reach diffusion limitation

• Abnormal alveolar-capillary barrier – diffusion limited

Diffusing Capacity

Vx =Vx =A x D x (P1 – P2)A x D x (P1 – P2)

TT

VxVx A x DA x D

TT

.

(P1 – P2)

.

== (DLx)

==VxVx

(PAx – PCx)DLx

.

ml / min / mm Hg

Fick’s equation:

Measurement of Diffusing Capacity

==VCOVCO

(PACO – PCCO)DLCO

==VCOVCO

(PACO – 0)

.

.

==VCOVCO

PACO

.

Conditions that Decrease the Diffusing Capacity

• Thickening of the barrier– Interstitial or alveolar edema

– Interstitial or alveolar fibrosis

• Sarcoidosis

• Scleroderma

• Decreased surface area– Emphysema

– Tumors

– Low cardiac output

– Low pulmonary capillary blood volume

• Decreased uptake by erythrocytes– Anemia

– Low pulmonary capillary blood volume

• Ventilation-perfusion mismatch

Normal Lung IPF Lung

Emphysema