1. Gas ExchanGEGas ExchanGE- Dr. Chintan

2. Physics of GasPhysics of Gas
DiffusionDiffusion• net diffusion of the gas will occur from the high
concentration area toward the low-concentration area
• the pressure is directly proportional to the concentration
of the gas molecules
• The rate of diffusion of gases is directly proportional to the
pressure caused by gas, which is called the partial
pressure
• total pressure of air (79 % nitrogen and 21 % oxygen) at
sea level averages 760 mm Hg – PO2 →160 mmHg, PCO2,
PN2 → 600 mmHg

3. Physics of GasPhysics of Gas
DiffusionDiffusion• The partial pressure of a gas in a solution is determined not only
by its concentration but also by the solubility coefficient
• carbon dioxide, are physically or chemically attracted to water
molecules, whereas others are repelled.
• When molecules are attracted, far more of them can be
dissolved without building up excess partial pressure within the
solution.
• Henry’s law:
• Partial pressure
= Concentration of dissolved gas / Solubility
coefficient

4. Physics of GasPhysics of Gas
DiffusionDiffusion• If the partial pressure is greater in the gas phase in
the alveoli, as is normally for oxygen,
• then more molecules will diffuse into the blood than in
the other direction.
• if the partial pressure of the gas is greater in the
dissolved state in the blood, is normally for carbon
dioxide,
• then net diffusion will occur toward the gas phase in the
alveoli.

5. Gas DiffusionGas Diffusion
• Pressure difference plus
• (1) the solubility of the gas in the fluid,
• (2) the cross-sectional area of the fluid,
• (3) the distance through which the gas must diffuse,
• (4) the molecular weight of the gas,
• (5) the temperature of the fluid

6. Gas DiffusionGas Diffusion• diffusion coefficient of the gas

7. Respiratory UnitRespiratory Unit

8. RespiratoryRespiratory
membranemembrane• 1. A layer of fluid lining the alveolus and containing
surfactant that reduces the surface tension of the alveolar
fluid
• 2. The alveolar epithelium composed of thin epithelial cells
• 3. An epithelial basement membrane
• 4. A thin interstitial space between the alveolar epithelium
and the capillary membrane
• 5. A capillary basement membrane that in many places
fuses with the alveolar epithelial membrane
• 6. The capillary endothelial membrane

9. RespiratoryRespiratory
membranemembrane

10. RespiratoryRespiratory
membranemembrane• thickness of the respiratory membrane averages
about 0.6 micrometer
• Rate of Gas Diffusion
• (1) the thickness of the membrane,
• (2) the surface area of the membrane,
• (3) the diffusion coefficient of the gas in the substance
of the membrane,
• (4) the partial pressure difference of the gas between
the two sides of the membrane.

11. Factors - 1Factors - 1• the rate of diffusion through the membrane is inversely
proportional to the thickness of the membrane
• The thickness of the respiratory membrane
occasionally Increases as a result of edema fluid in the
interstitial space of the membrane and in the alveoli
• some pulmonary diseases cause fibrosis of the lungs,
which can increase the thickness of some portions of
the respiratory membrane.

12. Factors - 2Factors - 2• surface area of the respiratory membrane
• removal of an entire lung decreases the total surface
area to one half normal.
• in emphysema, many of the alveoli join, with
dissolution of many alveolar walls.
• During competitive sports and strenuous exercise, even
the slightest decrease in surface area of the lungs can
be a serious impairment to respiratory exchange of
gases

13. Factors - 3Factors - 3• The diffusion coefficient for transfer of each gas
through the respiratory membrane
• depends on the gas’s solubility in the membrane and,
inversely, on the square root of the gas’s molecular
weight.
• for a given pressure difference, carbon dioxide
diffuses about 20 times as rapidly as oxygen.
• Oxygen diffuses about twice as rapidly as nitrogen.

14. Factors - 4Factors - 4• The pressure difference across the respiratory
membrane
• difference between the partial pressure of the gas in
the alveoli and the partial pressure of the gas in the
pulmonary capillary blood
• Oxygen - net diffusion from the alveoli into the blood
• carbon dioxide - net diffusion from the blood into the
alveoli

15. Diffusing CapacityDiffusing Capacity• Def. - volume of a gas that will diffuse through the
membrane each minute for a partial pressure difference of
1 mmHg
• diffusing capacity for oxygen under resting conditions
averages 21 ml/min/mm Hg
• During strenuous exercise, the diffusing capacity for
oxygen increases in young men to a maximum of about
65 ml/min/mm Hg
• opening up of many previously dormant pulmonary
capillaries or extra dilation of already open capillaries,

16. Diffusing CapacityDiffusing Capacity• diffusing capacity for carbon dioxide under resting
conditions of about 400 to 450 ml/min/ mm Hg
• during exercise of about 1200 to 1300 ml/min/mm Hg

17. Measurement of DiffusingMeasurement of Diffusing
CapacityCapacity• The oxygen diffusing capacity can be calculated from
measurements of
• (1) alveolar Po2, (2) Po2 in the pulmonary capillary
blood, and (3) the rate of oxygen uptake by the blood
• measuring the Po2 in the pulmonary capillary blood is so
difficult
• carbon monoxide diffusing capacity
• diffusing capacity = rate of CO uptake / pressure
difference of CO across the respiratory membrane

18. Measurement of DiffusingMeasurement of Diffusing
CapacityCapacity• A small amount of CO is breathed into the alveoli, and the partial
pressure of the CO in the alveoli is measured from appropriate
alveolar air samples
• The CO pressure in the blood is essentially zero, because Hb
combines with this gas so rapidly that its pressure never has time
to build up
• To convert CO diffusing capacity to oxygen diffusing capacity, the
value is multiplied by a factor of 1.23 because the diffusion
coefficient for oxygen is 1.23 times that for CO
• the average diffusing capacity for CO in young men at rest is
17 ml/min/mm Hg, and the diffusing capacity for oxygen is 1.23
times this, or 21 ml/min/mm Hg.

19. THANQ…