Gas Exchange Gas Exchange 11

Gas lawGas law

• Total pressure of a mixture of gases=sum of Total pressure of a mixture of gases=sum of individual gases pressures (Dalton’s law), individual gases pressures (Dalton’s law), Atmospheric pressure=760 mm Hg , Atmospheric pressure=760 mm Hg , OO22=160mm Hg, N=160mm Hg, N22=600mm Hg=600mm Hg

• Gas moves along pressure and conc. gradientGas moves along pressure and conc. gradient• Gas dissolution in fluid depends upon gas Gas dissolution in fluid depends upon gas

partial pressure and its solubility in the fluidpartial pressure and its solubility in the fluid• Volume of the container inversely proportional Volume of the container inversely proportional

to partial pressure of the gas (Boyle’s law) to partial pressure of the gas (Boyle’s law)

DiffusionDiffusion

• Random movement of molecules of Random movement of molecules of gas by their own kinetic energygas by their own kinetic energy

• Net diffusion from higher conc. to Net diffusion from higher conc. to lower conc.lower conc.

• Molecules try to equilibrate in all Molecules try to equilibrate in all empty placesempty places

• Rate of diffusion is directly related Rate of diffusion is directly related to partial pressure of a gasto partial pressure of a gas

Partial pressurePartial pressure

• The pressure exerted by the gas The pressure exerted by the gas molecules on a surfacemolecules on a surface

• Partial pressure depends upon the conc. Partial pressure depends upon the conc. of gasof gas

• 21% O21% O2 2 in air, POin air, PO2 2 is 160mm Hg in is 160mm Hg in atmospheric pressure of 760mm Hgatmospheric pressure of 760mm Hg

• NN22 in air 79%,P Nitrogen is 600mm Hg in air 79%,P Nitrogen is 600mm Hg or PO or PO22=Fgas×Patm=0.21×760=160mm =Fgas×Patm=0.21×760=160mm Hg Fgas= fractional conc. of gasHg Fgas= fractional conc. of gas

• PP= conc. of dissolved gas/solubility PP= conc. of dissolved gas/solubility coefficientcoefficient

• When molecules are attracted When molecules are attracted towards water molecules, far more towards water molecules, far more of them can be dissolved without of them can be dissolved without building excess PP within solution.building excess PP within solution.

• CO2 is 20 times as soluble as oxygenCO2 is 20 times as soluble as oxygen

Pressure of gases dissolved Pressure of gases dissolved in water and tissuesin water and tissues

• Pp in fluid develops same way as in airPp in fluid develops same way as in air• Pp in fluid determined by Pp in fluid determined by

conc. of dissolved gas/solubility conc. of dissolved gas/solubility coefficientcoefficient

• S. coefficients of different gases S. coefficients of different gases OO22=0.024 , CO=0.024 , CO22=0.57 , CO=0.018 , =0.57 , CO=0.018 , NN22=0.012 , He=0.008=0.012 , He=0.008

• Water solubility of COWater solubility of CO22 20 times more 20 times more than that of Othan that of O22

Rate of diffusionRate of diffusion

• Quantification of gas diffusion expressed by Quantification of gas diffusion expressed by

D=D=ΔΔ P×A×S/d×√MW P×A×S/d×√MW

ΔΔ P=Partial pressure difference, A=cross- P=Partial pressure difference, A=cross-sectional area , S=solubility of gas, d= sectional area , S=solubility of gas, d= distance , distance ,

√ √MW=molecular weightMW=molecular weight• Two structural, A and d and two gas factors Two structural, A and d and two gas factors ΔΔP and P and

solubilitysolubility• Fifth factor is temp. which remains constantFifth factor is temp. which remains constant• Diffusion coefficient=S/ √MW, relative rates at Diffusion coefficient=S/ √MW, relative rates at

which different gases at same Pp levels will diffuse.which different gases at same Pp levels will diffuse.

• Two gases at same Pp, rate of diffusion Two gases at same Pp, rate of diffusion proportional to diffusion coeff.proportional to diffusion coeff.

• If diffusion coefficient of OIf diffusion coefficient of O22 taken as 1, taken as 1, the diffusion coefficient of COthe diffusion coefficient of CO22=20.3 , =20.3 , CO=0.81 , N CO=0.81 , N22=0.53 , H=0.95=0.53 , H=0.95

• As both OAs both O22and COand CO22 highly soluble in highly soluble in lipids so can cross cell membrane, for lipids so can cross cell membrane, for dissolution in fluid water solubility dissolution in fluid water solubility importantimportant

Gas ExchangeGas Exchange

• Resp. unit – Resp. bronchioles, Resp. unit – Resp. bronchioles, alveolar ducts, atria & alveolialveolar ducts, atria & alveoli

• Resp. membrane (Alveolar & Resp. membrane (Alveolar & capillary walls) (0.6capillary walls) (0.6μμm) m)

• Extremely thin with – high flow of Extremely thin with – high flow of bloodblood

• Sheet of bloodSheet of blood• From inward outwardsFrom inward outwards

• A layer of surfactantA layer of surfactant• Alveolar epitheliumAlveolar epithelium• Basement of alveolar epitheliumBasement of alveolar epithelium• Very thin interstitial spaceVery thin interstitial space• Capillary basement membrane Capillary basement membrane • Capillary endothelium Capillary endothelium • Surface Area 70 – 100 square metersSurface Area 70 – 100 square meters• Quantity of blood in pulmonary Quantity of blood in pulmonary

capillaries – 60- 140mlcapillaries – 60- 140ml

Diffusion of Gases Across Diffusion of Gases Across Resp. MembraneResp. Membrane

• Follows gas/ficks lawFollows gas/ficks law• Partial partial gradientPartial partial gradient• Surface AreaSurface Area• Thickness of Resp. membranes Thickness of Resp. membranes • DistanceDistance• Diffusion coefficient _________ Diffusion coefficient _________

= Solubility of gas/ = Solubility of gas/ √√ Mol. weightMol. weight

Composition of atmospheric Composition of atmospheric and alveolar air and alveolar air

• Atmospheric POAtmospheric PO22=160mm Hg got diluted by water =160mm Hg got diluted by water vapors=47mm Hg , POvapors=47mm Hg , PO22 becomes 150mmHg becomes 150mmHg

• In alveoli POIn alveoli PO22=100mm Hg more diluted as mixes =100mm Hg more diluted as mixes up with residual volumeup with residual volume

• POPO22and PCOand PCO22 in alveolar air and at pulmonary in alveolar air and at pulmonary capillary end samecapillary end same

• Slight change in systemic arterial POSlight change in systemic arterial PO22 to 95mm Hg to 95mm Hg because of mixing up of bronchial venous bloodbecause of mixing up of bronchial venous blood

• The difference b/w alveolar and systemic arterial The difference b/w alveolar and systemic arterial blood can increase in specific pulmonary system blood can increase in specific pulmonary system problems problems

Diffusion CapacityDiffusion Capacity

• Defined as volume of gas diffusing Defined as volume of gas diffusing through membrane each minute for a through membrane each minute for a pressure difference of 1mmHgpressure difference of 1mmHg

• Diffusing capacity of ODiffusing capacity of O22 = = 20-30 ml/mint/mmHg and 20-30 ml/mint/mmHg and during exercise 65ml/mint/mmHgduring exercise 65ml/mint/mmHg

• COCO22 diffusing capacity 20 times more – diffusing capacity 20 times more – 400 – 600ml/mint/mmHg and 400 – 600ml/mint/mmHg and during exercise during exercise 1200-1300ml/mint/mmHg1200-1300ml/mint/mmHg

Ventilation – Perfusion Ventilation – Perfusion RatioRatio

• At rest base of lung more perfused with blood At rest base of lung more perfused with blood than the apexthan the apex

• Capillaries with low pressure divert blood to Capillaries with low pressure divert blood to capillaries with high pressure (automatic capillaries with high pressure (automatic control) control)

• Matching of alveolar ventilation with the blood Matching of alveolar ventilation with the blood perfusion – called ventilation – perfusion ratioperfusion – called ventilation – perfusion ratio

• Expressed as VExpressed as VAA/Q /Q VVAA = Alveolar = Alveolar Ventilation/mint Ventilation/mint

Q = Vol. of pulmonary blood/mintQ = Vol. of pulmonary blood/mint=4.2L/min / 5.5 L = 0.8 = Normal=4.2L/min / 5.5 L = 0.8 = Normal

• If VIf VAA zero though Q adequate – ratio zero though Q adequate – ratio zerozero

• If VIf VAA adequate & Q is zero – ratio infinity adequate & Q is zero – ratio infinity • Both case – no gas exchange Both case – no gas exchange • At rest in apex of lungs – At rest in apex of lungs – ↑↑ ventilation ventilation

but but ↓ ↓ Q – so ratio Q – so ratio ↑↑ (2.5 times (2.5 times ↑↑ than than normal), dead spacenormal), dead space

• At rest in base of lung - ↓ ventilation but At rest in base of lung - ↓ ventilation but ↑ Q – so ratio ↓ (0.6 times ↓ than ↑ Q – so ratio ↓ (0.6 times ↓ than normal), shunted bloodnormal), shunted blood

• So VSo VAA / Q ratio → efficiency of gas / Q ratio → efficiency of gas exchange at lung levelexchange at lung level

• Locally ratio maintained by regulating Locally ratio maintained by regulating the diameters of bronchioles & the diameters of bronchioles & arterioles – Bronchioles by PCOarterioles – Bronchioles by PCO22 & & arterioles by POarterioles by PO22

• If 0/Q = 0 – Alveolar air equilibrates If 0/Q = 0 – Alveolar air equilibrates with venous blood & in both POwith venous blood & in both PO22 & & PCOPCO2 2 will be 40 & 46mmHg will be 40 & 46mmHg respectively respectively

• If VIf VAA / 0 = / 0 =~ ~ - Alveolar air equilibrates - Alveolar air equilibrates with humidified inspired air & POwith humidified inspired air & PO22 & & PCOPCO22 will be 150 & 0.3mmHg will be 150 & 0.3mmHg

Wasted Wasted VentilationVentilation

• In case physiological dead space – In case physiological dead space – No gas exchange No gas exchange → wasted → wasted ventilationventilation

Shunted BloodShunted Blood

• In case VIn case VAA / Q ratio subnormal (< V / Q ratio subnormal (< VAA), ), the blood not fully oxygenated, that the blood not fully oxygenated, that not oxygenated blood called shunted not oxygenated blood called shunted bloodblood

• 2% bronchial blood drained into 2% bronchial blood drained into pulmonary veins – also shunted blood pulmonary veins – also shunted blood

• Total quantity of shunted blood per Total quantity of shunted blood per minute – physiological shunt minute – physiological shunt