GAS DIFFUSION

CHAPTER 7

DR. CARLOS ORTIZ

BIO-208

DIFFUSION

• THE NET MOVEMENT OF A PARTICULAR GAS’S MOLECULES FROM AN AREA OF HIGH PARTIAL PRESSURE TO AN AREA OF LOWER PARTIAL PRESSURE IS CALLED DIFFUSION.

• DIFFUSIION GRADIENTS OF RESPIRATORY GASES• INSPIRED AIR CONTAINS 21% O2, THUS INSPIRED PIO2 IS

ABOUT 160 mmHg.• PO2 IN CONDUCTING AIRWAYS IS LOWER BECAUSE GAS IN

THE LUNG IS 100% SATURATED WITH WATER VAPOR. AT 37ºC THE PH2O IS 47 mmHg. PO2 IN CONDUCTING AIRWAYS IS ABOUT 150 mmHg.

• ALVEOLAR PO2 (PAO2) IS LOWER STILL BECAUSE CO2 DIFFUSES INTO THE ALVEOLI, DILUTING INCOMING O2 AND LOWERING PAO2 TO ABOUT 100mmHg.

• THE DIFFUSION GRADIENT BETWEEN ALVEOLAR GAS AND INCOMING MIXED VENOUS BLOOD IS MUCH LARGER FOR OXYGEN THAN FOR CARBON DIOXIDE.

DIFFUSION

• THE PO2 OF MIXED VENOUS BLOOD (PVO2) IS ABOUT 40 mmHg AS IT ENTERS THE PULMONARY CAPILLARY, AND PVCO2 IS ABOUT 46mmHg

• AT REST THE DIFFUSION GRADIENTS FOR O2 (100-40 = 60mmHg) AND CO2 (46-40=6mmHg) TRANSFER ABOUT 250 ml/min OF O2 INTO THE BLOOD AND 200 ml/min OF CO2 INTO THE ALVEOLI.

• ALVEOLAR AIR EQUATION• THE SUM OF ALL GAS PRESSURES AT ANY POINT IN THE

LUNG MUST EQUAL 760 mmHg AT SEA LEVEL. WHEN INSPIRED GAS ENTERS THE ALVEOLI, PCO2 SUDDENLY RISES FROM 0 TO 40 mmHg, CAUSING A SIMILAR DECREASE IN PO2.

• THE RATIO OF ALVEOLAR CO2 EXCRETION(VCO2) TO ALVEOLAR OXYGEN UPTAKE (VO2) IS CALLED THE RESPIRATORY EXCHANGE RATIO (R) AND ITS VALUE IS NORMALLY ABOUT O.8 ( R=VCO2/VO2=200/250 =0.8).

• WHEN R EQUALS 0.8, OXYGEN LEAVING THE ALVEOLUS IS ONLY 80% REPLACED BY INCOMING CO2.

DIFFUSION

• THE ALVEOLAR AIR EQUATION IS KNOWN AS THE IDEAL ALVEOLAR AIR EQUATION BECAUSE IT ASSUMES THE VENTILATION-BLOOD FLOW RATIOS OF ALL ALVEOLI IN THE LUNG ARE IDENTICAL.

• FOR FIO2 GREATER THAN 60%, A SUFFICIENTLY ACCURATE CLINICAL EQUATION IS

• PAO2 = PIO2 – PaCO2

• A NORMAL PAO2FOR A PERSON BREATHING ROOM AIR, WITH A PACO2 EQUAL TO 40 mmHg AND AN R EQUAL TO 0.8, IS SLIGHTLY OVER 100 mmHg

• PAO2 = FIO2( 760-47) – PaCO2 X 1.2)

• PAO2 = 0.21(760-47) – 40 X 1.2)

• PAO2 = 149.2 – 48

• PAO2 = 101.2

DIFFUSION

• PHYSICAL GAS CHARACTERISTICS AND DIFFUSION• O2 AND CO2 DIFFUSE THROUGH GASEOUS AND LIQUID

PHASES IN THE LUNG; THE ALVEOLAR CAPILLARY MEMBRANE IS A LIQUID BARRIER.

• LIGHT GASES DIFFUSE MORE RAPIDLY THAN HEAVIER GASES, AND HIGHLY SOLUBE GASES DIFFUSE THROUGH LIQUIDS MORE RAPIDLY THAN LESS SOLUBLE GASES.

• SPECIFICALLY, THE GAS DIFFUSION RATE IS INVERSELY PROPORTIONAL TO THE SQUARE ROOT OF ITS GRAM MOLECULAR WEIGHT ( GRAHAM’S LAW). THEREFORE BECAUSE O2 IS A LIGHTER MOLECULE, IT DIFFUSES THROUGH A GASEOUS MEDIUM 1.7 TIMES FASTER THAN CO2.

• CONVERSELY, CO2 IS MUCH MORE SOLUBLE IN WATER THAT O2. HENRY’S LAW STATES THAT THE AMOUNT OF GAS DISSOLVING IN A LIQUID IS DIRECTLY PROPORTIONAL TO THE GAS PARTIAL PRESSURE. CO2 IS ABOUT 24 TIMES MORE SOLUBLE THAN O2.

DIFFUSION

• PHYSICAL GAS CHARACTERISTICS AND DIFFUSION

• COMBINING GRAHAM’S AND HENRY’S LAW, CO2 DIFFUSES ACROSS THE A-C MEMBRANE ABOUT 20 TIMES FASTER THAN O2.

• FOR THIS REASON, ALVEOLAR CAPILLARY MEMBRANE DEFECTS LIMIT OXYGEN DIFFUSION LONG BEFORE THEY LIMIT CO2 DIFFUSION.

• IN PRACTICAL CLINICAL SENSE, THE A-C MEMBRANE NEVER LIMITS OUTWARD DIFFUSION OF CO2 FROM BLOOD TO ALVEOLI.

LIMITATIONS OF OXYGEN DIFFUSION

• FACTORS INFLUENCING THE RATE OF O2 TRANSFER ACROSS THE A-C MEMBRANE INCLUDE

• 1- THE PARTIAL PRESSURE GRADIENT ACROSS THE MEMBRANE• 2- THE DIFFUSION PATH LENGTH THE DISTANCE FOR DIFFUSION INCLUDES THE ENTIRE PATH FROM ALVEOLAR GAS TO THE HEMOGLOBIN IN THE RBC. THE TOTAL DIFFUSION PATH IS QUITE SHORT (<0.1µ) AND INCLUDES THE 1- THE SURFACTANT LAYER 2- ALVEOLAR EPITHELIUM 3- THE BASEMENT MEMBRANE OF THE ALVEOLAR EPITHELIUM 4-INTERSTITIAL SPACE 5- THE BASEMENT MEMBRANE OF THE CAPILLARY ENDOTHELIUM 6- CAPILLARY ENDOTHELIUM.7-PLASMA 8- RBC MEMBRANE 9- INTERCELLULAR FLUID BATHING THE Hb MOLECULE.• 3- THE MEMBRANE SURFACE AREA.

MEASURING DIFFUSION CAPACITY

• DIFFUSION LIMITED TEST GAS SUCH AS CO IS USED TO MEASURE THE DIFFUSION CAPACITY OF THE LUNG. THE PRINCIPLE OF THE SINGLE-BREATH CO DIFFUSION TEST IS TO MEASURE THE AMOUNT OF CO THAT DIFFUSES INTO THE PULMONARY CAPILLARIES DURING A 10-SECONDS BREATH HOLDING PERIOD AFTER FIRST INHALING A KNOWN CONCENTRATION OF CO.

• NORMAL VALUES

• MEAN DLCO OF 26.4ml/min/mmHg

• CLINICAL USE OF DLCO

• THE DLCO TEST ASSESSES THE EXTENT TO WHICH THE A-C MEMBRANE IS A BARRIER TO GAS DIFFUSION. DLCO TEST CAN HELP CLARIFY THE MECHANISM OF ARTERIAL HYPOXEMIA; IF DLCO IS NORMAL, DIFFUSION IMPAIRMENT CANNOT BE A CONTRIBUTING FACTOR.