Physical principles of gas diffusion

Physical principles of gas diffusion Henry’s law.

• Fresh air is a mixture of gases. All the gas molecules are moving freely in all directions but their net diffusion occurs down the concentration gradient and energy for this diffusion is taken from the kinetic energy of gas molecules.

• In respiratory physiology we deal with mixture of nitrogen-79%,oxygen-21%,and carbon dioxide-0.04%.

• Total pressure of this mixture is 760mmHg at sea level.

• The pressure exerted by each gas in this mixture is called as its partial pressure and denoted by PO2, PCO2,PN2, PHe etc.,

• Partial pressure of nitrogen(PN2)at sea level would be 79% of 760mmHg = 600mmHg

• Partial pressure of Oxygen (PO2)at sea level in the atmospheric air would be 21% of 760mmHg =160 mmHg.

• Partial pressure of a gas in atmosphere is directly proportional to its concentratio.

• But partial pressure of a gas dissolved in fluid like plasma is = concentration of dissolved gas/solubility coefficient.

• If a gas is readily soluble in blood like carbon monoxide and carbon dioxide ,it will exert less partial pressure.

• The solubility of oxygen is 0.024 as compared to CO2 which has solubility of 0.57.Therefore oxygen has more partial pressure to remain dissolved in plasma.

Partial pressures of oxygen

• In atmospheric air PO2=(FO2*Patm) 21% of 760=159mmHg (FO2 is fraction of O2 in air)

• In the conducting zone.PiO2=(Patm-PH2O)FO2=(760-47)0.21=150mmHg

• In the alveolar air.• PAO2=(Patm-47)FO2-PaCO2/R• (760-47)0.21-40/0.8=100mmHg(this is

known as gas equation)• Important slide.

Factors affecting the Partial pressure of CO2 in alveolar air(PACO2)

• Normal value is 40mmHg when alveolar ventilation is 350*12=4.2L/min and rate of CO2 excretion is 200ml/min in resting state.

• So PACO2= excretion rate of CO2

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• VA

• PACO2 finally will affect the PaCO2• What happens to PACO2 in hyperventilation and

hypoventilation?

Factors affecting the partial pressure of the alveolar oxygen(PAO2)

• These factors will eventually affect the PaO2• According to the gas eq. PAO2=(Patm-

47)*FO2-PaCO2/R.• Most important factors affecting the PAO2 and thus

PaO2 are partial pressure of oxygen in atm and fraction of oxygen in air.

• At high altitudes PAO2 decreases,WHY?• During exercise as more oxygen is absorbed per min

so more ventilation rate is required to maintain PAO2

Remember

• 300 million alveoli in the two lungs and diameter of an alveolus is 0.2 mm.

• Total SA of respiratory membrane =70 square meters• Blood spread along this area is 70ml out of 450 ml

present in pulmonary circulation (9% of total blood volume)

• Overall thickness of respiratory membrane is 0.2-0.6 micrometers

• Average diameter of pulmonary capillary is 5 micrometers

Factors that affect the rate of gas diffusion through the respiratory

membrane.

Diffusion capacity of the lung.(DLCO)

• Definition: is the volume of gas diffusing through the resp.memb each minute with 1mmHg difference in partial pressure.

• Usually carbon monoxide is used to measure DLCO because its partial pressure can be measured in the alveoli and no need to measure partial pressure in pulm.blood as it is essentially zero.

• Then amount absorbed in 1 min is calculated and divided by partial pressure to get DLCO

Remember. • Diffusion capacity of oxygen is calculated by

DLCO as DLCO*1.23 as oxygen is 1.23 times more diffusable than oxygen.

• Diff capacity of carbon dioxide is 20 times the diff capacity of oxygen.

• DLCO is 17ml/min/mmHg• Of oxygen is 21ml/min/mmHg• Of carbon dioxide is 400ml/min/mmHg• How lung can increase its diffusion capacity?

The red curve is indicating VA/Q ratio

VA/Q=0.8

Effect of ventilation/perfusion ratio on alveolar PO2 and PCO2

Physiologic dead space.

• Both physiologic shunt and physiologic dead space decrease the capability of lung as gase exchange organ.

• They can be produced s in chronic obstructive pulmonary diseases( COPD),as smoker’s emphysema.