Gas ExchanGEGas ExchanGE- Dr. Chintan

Physics of Gas Physics 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

Physics of Gas Physics 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

Physics of Gas Physics 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.

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

Gas DiffusionGas Diffusion• diffusion coefficient of the gas

Respiratory UnitRespiratory Unit

Respiratory Respiratory 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

Respiratory Respiratory membranemembrane

Respiratory Respiratory 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.

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.

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

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.

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

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, thereby increasing the surface area of the blood

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

Measurement of Diffusing Measurement 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

Measurement of Diffusing Measurement 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.

THANQ…