Diffusion of Gas Exchange in Lungs
Dr. K. Ambareesha PhD,.
Assistant Professor Physiology, GMC, Sec,bad

Learning Objectives
1. Introduction
2. Factors on which gas exchange depends
3. Alveolo capillary membrane
a. Factors affectng diffusion of gses
b. Clincal significance
4. Partial pressure gradient
5. Laws governing diffusion
a. Henrs law
b. Ficks law
6. Difusion coefficient
7. Capillary blood flow , its measuremt and factors affecting it

Diffusion of gases manly depends on:
1. Alvelo capillary membrane
2. Partial pressure gradient
3. Pulmonary capillary blood flow

Introduction

I. Alveo - capillary membrane

Factors affecting Diffusion
1. Difference in partial pressure
2. Diffusing capacity
3. Surface area
4. Solubility of gas
5. Thickness
6. Molecular weight

DαPG
DαS
DαA
Dα1/MW
Dα1/D
DαGas solubility/√MV
Pressure Gradient
Solubility of a gas in fluid
Medium
Total surface area
Molecular weight of gas
Thickness of membrane
Diffusion coefficient
FACTORS AFFECTING DIFFUSION

Clinical significance
Alveo capillary obstruction syndrome
Types:
1. Thickening of membrane Ex: interstitial fibrosis
2. Increased distance between alveo - capillary membrane Ex:
interstitial edema in heart failure

Fibrosis of the lung

SURFACE AREA OF MEMBRANE IS ALTERD

SURFACE AREA OF MEMBRANE IS ALSO
ALTERD

II. Partial pressure gradient
Diffusion Gradient:
Diffusion of a gas depends upon the difference of partial pressure
of individual gas across the alveolo - capillary membrane of that
particular gas.

Laws Governing Diffusion
Amount of the gas
dissolved in a liquid at
a given temperature
PP and solubility of
the gas
Henr’s Law:
α

Fick’s Law
Volume of the gas
diffusing/min across
the membrane
α
Membrane surface area
Diffusion coefficient
PP difference of a gas
Thickness (T)

DIFFUSION OF OXYGEN

DIFFUSION OF OXYGEN AT CELLULAR
LEVEL

DIFFUSION OF CO2 AT CELLULAR
LEVEL

DIFFUSION OF CO2 AT ALVEOLI

Partial pressure of different gases

Diffusion coeffiient
DC α its solubility
√ Its MW

Capillary blood flow
1.Transient time of 0.75 sec
2.Different for different gases

Diffusing capacity
Is a measure of rate of gas transfer into
the lungs per partial pressure gradient

Measurement
1.CO is used to determine
2.Its uptake limited by diffusion
3.Ability of CO for Hb is 210 times greater than oxygen
4.Most common techniqu is simple breath test

Factors affecting DC
1.Hb and capillary blood volume affects it
2.It is not only depends on diffusion across the membrane and
also on hematocrit
3.A decrease in either hematocrit or capillary blood volume
decreases the DC

THNK YOU