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% O 2, THUS INSPIRED P I O 2 IS ABOUT 160 mmHg. PO 2 IN CONDUCTING AIRWAYS IS LOWER BECAUSE GAS IN THE LUNG IS 100% SATURATED WITH WATER VAPOR. AT 37ºC THE P H2O IS 47 mmHg. PO 2 IN CONDUCTING AIRWAYS IS ABOUT 150 mmHg. ALVEOLAR PO 2 (P A O 2 ) IS LOWER STILL BECAUSE CO 2 DIFFUSES INTO THE ALVEOLI, DILUTING INCOMING O 2 AND LOWERING P A O 2 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 PO 2 OF MIXED VENOUS BLOOD (P V O 2 ) IS ABOUT 40 mmHg AS IT ENTERS THE PULMONARY CAPILLARY, AND P V CO 2 IS ABOUT 46mmHg AT REST THE DIFFUSION GRADIENTS FOR O 2 ( = 60mmHg) AND CO 2 (46-40=6mmHg) TRANSFER ABOUT 250 ml/min OF O 2 INTO THE BLOOD AND 200 ml/min OF CO 2 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, PCO 2 SUDDENLY RISES FROM 0 TO 40 mmHg, CAUSING A SIMILAR DECREASE IN PO 2. THE RATIO OF ALVEOLAR CO 2 EXCRETION(VCO 2 ) TO ALVEOLAR OXYGEN UPTAKE (VO 2 ) IS CALLED THE RESPIRATORY EXCHANGE RATIO (R) AND ITS VALUE IS NORMALLY ABOUT O.8 ( R=VCO 2 /VO 2 =200/250 =0.8). WHEN R EQUALS 0.8, OXYGEN LEAVING THE ALVEOLUS IS ONLY 80% REPLACED BY INCOMING CO 2.

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 FIO 2 GREATER THAN 60%, A SUFFICIENTLY ACCURATE CLINICAL EQUATION IS P A O 2 = PIO 2 – PaCO 2 A NORMAL P A O 2 FOR A PERSON BREATHING ROOM AIR, WITH A P A CO 2 EQUAL TO 40 mmHg AND AN R EQUAL TO 0.8, IS SLIGHTLY OVER 100 mmHg P A O 2 = FIO 2 ( ) – PaCO 2 X 1.2) P A O 2 = 0.21(760-47) – 40 X 1.2) P A O 2 = – 48 P A O 2 = 101.2

DIFFUSION PHYSICAL GAS CHARACTERISTICS AND DIFFUSION O 2 AND CO 2 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 O 2 IS A LIGHTER MOLECULE, IT DIFFUSES THROUGH A GASEOUS MEDIUM 1.7 TIMES FASTER THAN CO 2. CONVERSELY, CO 2 IS MUCH MORE SOLUBLE IN WATER THAT O 2. HENRY’S LAW STATES THAT THE AMOUNT OF GAS DISSOLVING IN A LIQUID IS DIRECTLY PROPORTIONAL TO THE GAS PARTIAL PRESSURE. CO 2 IS ABOUT 24 TIMES MORE SOLUBLE THAN O 2.

DIFFUSION PHYSICAL GAS CHARACTERISTICS AND DIFFUSION COMBINING GRAHAM’S AND HENRY’S LAW, CO 2 DIFFUSES ACROSS THE A-C MEMBRANE ABOUT 20 TIMES FASTER THAN O 2. FOR THIS REASON, ALVEOLAR CAPILLARY MEMBRANE DEFECTS LIMIT OXYGEN DIFFUSION LONG BEFORE THEY LIMIT CO 2 DIFFUSION. IN PRACTICAL CLINICAL SENSE, THE A-C MEMBRANE NEVER LIMITS OUTWARD DIFFUSION OF CO 2 FROM BLOOD TO ALVEOLI.

LIMITATIONS OF OXYGEN DIFFUSION FACTORS INFLUENCING THE RATE OF O 2 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 D L CO OF 26.4ml/min/mmHg CLINICAL USE OF D L CO THE D L CO TEST ASSESSES THE EXTENT TO WHICH THE A-C MEMBRANE IS A BARRIER TO GAS DIFFUSION. D L CO TEST CAN HELP CLARIFY THE MECHANISM OF ARTERIAL HYPOXEMIA; IF D L CO IS NORMAL, DIFFUSION IMPAIRMENT CANNOT BE A CONTRIBUTING FACTOR.