Physical principles of gas diffusion Physical principles of

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Physical principles of gas diffusion

Physical principles of gas diffusion

Physical principles of gas diffusion Henry’s law.

Physical principles of gas diffusion Henry’s law.

 • Fresh air is a mixture of gases. All the gas molecules are

• 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

• In respiratory physiology we deal with mixture of nitrogen-79%, oxygen-21%, and carbon dioxide-0. 04%. • Total pressure of this mixture is 760 mm. Hg at sea level. • The pressure exerted by each gas in this mixture is called as its partial pressure and denoted by PO 2, PCO 2, PN 2, PHe etc. ,

 • Partial pressure of nitrogen(PN 2)at sea level would be 79% of 760

• Partial pressure of nitrogen(PN 2)at sea level would be 79% of 760 mm. Hg = 600 mm. Hg • Partial pressure of Oxygen (PO 2)at sea level in the atmospheric air would be 21% of 760 mm. Hg =160 mm. Hg. • 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

• 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 CO 2 which has solubility of 0. 57. Therefore oxygen has more partial pressure to remain dissolved in plasma.

Partial pressures of oxygen • In atmospheric air PO 2=(FO 2*Patm) 21% of 760=159

Partial pressures of oxygen • In atmospheric air PO 2=(FO 2*Patm) 21% of 760=159 mm. Hg (FO 2 is fraction of O 2 in air) • In the conducting zone. Pi. O 2=(Patm. PH 2 O)FO 2=(760 -47)0. 21=150 mm. Hg • In the alveolar air. • PAO 2=(Patm-47)FO 2 -Pa. CO 2/R • (760 -47)0. 21 -40/0. 8=100 mm. Hg(this is known as gas equation) • Important slide.

Factors affecting the Partial pressure of CO 2 in alveolar air(PACO 2) • Normal

Factors affecting the Partial pressure of CO 2 in alveolar air(PACO 2) • Normal value is 40 mm. Hg when alveolar ventilation is 350*12=4. 2 L/min and rate of CO 2 excretion is 200 ml/min in resting state. • So PACO 2= excretion rate of CO 2 • --------------------- • VA • PACO 2 finally will affect the Pa. CO 2 • What happens to PACO 2 in hyperventilation and hypoventilation?

Factors affecting the partial pressure of the alveolar oxygen(PAO 2) • These factors will

Factors affecting the partial pressure of the alveolar oxygen(PAO 2) • These factors will eventually affect the Pa. O 2 • According to the gas eq. PAO 2=(Patm-47)*FO 2 -Pa. CO 2/R. • Most important factors affecting the PAO 2 and thus Pa. O 2 are partial pressure of oxygen in atm and fraction of oxygen in air. • At high altitudes PAO 2 decreases, WHY? • During exercise as more oxygen is absorbed per min so more ventilation rate is required to maintain PAO 2

Remember • 300 million alveoli in the two lungs and diameter of an alveolus

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 70 ml 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.

• 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

Diffusion capacity of the lung. (DLCO) • Definition: is the volume of gas diffusing through the resp. memb each minute with 1 mm. Hg 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

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

The red curve is indicating VA/Q ratio VA/Q=0. 8

The red curve is indicating VA/Q ratio VA/Q=0. 8

Effect of ventilation/perfusion ratio on alveolar PO 2 and PCO 2

Effect of ventilation/perfusion ratio on alveolar PO 2 and PCO 2

Physiologic dead space.

Physiologic dead space.

 • Both physiologic shunt and physiologic dead space decrease the capability of lung

• 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.