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THE AUSTRALIAN NATIONAL UNIVERSITY

THE AUSTRALIAN NATIONAL UNIVERSITY. Respiratory Partial Pressures and Blood Gasses Christian Stricker Associate Professor for Systems Physiology ANUMS/JCSMR - ANU Christian.Stricker@anu.edu.au http://stricker.jcsmr.anu.edu.au/PP&BG.pptx. Aims. The students should

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THE AUSTRALIAN NATIONAL UNIVERSITY

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  1. THE AUSTRALIAN NATIONAL UNIVERSITY Respiratory Partial Pressures and Blood GassesChristian StrickerAssociate Professor for Systems PhysiologyANUMS/JCSMR - ANUChristian.Stricker@anu.edu.auhttp://stricker.jcsmr.anu.edu.au/PP&BG.pptx

  2. Aims The students should • be familiar with the concepts of atmospheric, barometric and partial pressures; • be cognisant of the approximate composition of air; • know how water vapour affects partial pressures; • be able to describe the O2 cascade from inspired air to blood; • understand physiological principles involved in formulating the alveolar gas equation; • recognise the concept of ‘shunt’; and • be familiar with standard values for blood gases.

  3. Contents • Basic terms and concepts • Partial pressures of N2, O2 and CO2 • Air saturated with water: • Partial pressures at following locations: • Nose • Trachea • Alveolus • Lung capillary / Artery • Blood gas values

  4. Gasses & Pressures [kPa] 1 kPa ≈ 10.2 cm H2O ≈ 7.5 torr 1 kPa = 1000 N / m2 1 torr = 0.1333 kPa

  5. Atmospheric Pressure (Pb) • Patm at sea level = 101.325 kPa = 760 torr. • ≡ barometric pressure (Pb) • “Force per m2 exerted against a surface by weight of air above that surface in the atmosphere.” • = hydrostatic pressure caused by weight of air above measurement area. • A column of air of 1 m2 in cross-section, measured from sea level to the top of the atmosphere has a mass of about 104 kg and a weight of 63·104 N.

  6. Altitude and Pb • Pb drops exponentially with altitude = density of air drops with altitude. • Variable with weather conditions (highs and lows). • At 8’848 m, it is ~⅓ of that at sea level. • Plane cabins are pressurised to about 2’100 m; ~ 80 kPa.

  7. Composition of Air • Air created over a long time period by bacteria/algae. • O2 has been constant over the last 10 million years. • Water content variable, depending on weather (in rain clouds saturated). • Omitted for respiratory conside-rations (small change) as air will become fully saturated in airways. • Noble (Ar, He, etc.) and inert gasses (N2) are not metabo-lically relevant.

  8. 1. Ambient Gas Pressures [kPa] • Since 1 mol of gas takes identical volume (22.4 L) irrespective of type of gas, pressure affects all gases identically: concentrations ∞ volume content (FX) norma-lisedto Pb (barometric pressure) = partial pressure (PX). • In medical physiology, only N2 and O2 are “important”; under normal conditions, CO2 in inspired air is too small. • Partial pressures in ambient air:

  9. Water Vapour Pressure • Upon inhaling, H2O vapour becomes part of air/gas mixture → reduces partial pressures of all inspired gasses (O2, N2, CO2, etc.). • In a gas mixture saturated with H2O, water vapour pressure equals its partial pressure, . • At 37°C, is 6.3 kPa • is only dependent on temperature. • is NOT dependent on ambient pressure. • is the same at sea level as well as on top of Mt. Everest… • At 19’200 m, Pb = 6.3 kPa; therefore = 0 at this altitude (and likewise for any other gas…): Armstrong limit/line. • At 19’200 m, water boils at 37°C.

  10. 2. Tracheal Gas Pressures • In trachea, air gets H2O saturated at 37°C. Therefore, some partial pressure stems from H2O. • Therefore, and are smaller than Pb; i.e. 101.3 - 6.3 kPa = 95 kPa • Partial pressures in trachea: • Due to H2O saturation, drops (21.3 → 20.0 kPa). • What happens in alveoli?

  11. Conventions for Volume Reporting • Measured lung volumes and flows in laboratory: ATPS (ambient temperature & pressure, saturated): not for reporting • Conditions not standardised: ambient T and P; = 6.3 kPa • Reason for saturation: typically ambient T < body T • Standard reporting of lung volumes and flow: BTPS(body temperature & pressure, saturated) • Conditions standardized to 37°C; 101.3 kPa and = 6.3 kPa • Reasons: to have a physiologically meaningful measure in regard to lung volume; allows comparisons between patients. • Standard reporting of gas volumes (in blood): STPD(standard temperature and pressure, dry) • Conditions standardized to 0°C; 101.3 kPa and = 0 kPa • Conversion to BTPS: VBTPS = 1.21 VSTPD.

  12. Gas Transport to and from Periphery • Total gas volume transport is dependent on cardiac output /venous return (~ 5 L/min). • Relationship between O2 uptake and CO2 elimination. • More O2 is taken up than CO2 is breathed off. • Respiratory quotient (at rest, mixed food intake): Rhoades & Pflanzer 2003

  13. Gas Exchange in Alveoli • So far, no gas exchange was considered. • In alveoli, O2 is taken up into blood → ↓. • At same time, CO2 is exchanged → ↑. • For equimolar exchange, ↓ matched with ↑. • As less CO2 produced than O2 consumed, something has to “patch” the drop in partial pressure: dissolved N2 in blood.

  14. 3. Alveolar Gas Pressures • In alveoli, as CO2 is exchanged, O2 is taken up. • Under “normal” conditions corresponds to 5.3 kPa; i.e. is reduced by this amount: • Holds if metabolism produces same CO2 volume as O2 is utilised / burnt; i.e. for glucose… • Correction needed for how CO2 is made from O2: respiratory quotient (“normal” metabolism) i.e. • Difference of 1.3 kPa from dissolved N2 → ↑.

  15. Determinants of Gas Exchange • Structural elements: • Film on alveolar walls: watery solute. • Cell membrane: lipids. • Blood plasma: watery solute. • Gas exchange ( ) via diffusion • scales with • membrane surface area (A) and thickness (a), • difference in partial pressure ( ) and • diffusion capacity of the lung DL (CO to det.), • which is dependent on solubility, • directly ~ to difference in partial pressure; • indirectly ~ to temperature (T). • Solubility of CO2, O2 and N2 in water depends on temperature (T). • In fever, less is dissolved in body fluids. • In hypothermia much more (avalanche). • CO2 solubility at 37°C is ~23 x better than that for O2, which is ~ 2 x better than that for N2.

  16. Diffusion from Alveolus to EC • Diffusion over many different media. • All steps “resist” free diffusion: ↓. • Membrane diffusion rate (DM) limited by sum of D0+ … + D10 (in series). • Binding of O2 to haemoglobin takes time and also “resists” free diffusion (DH). • Normally, DM ≈ DH such that • Under normal conditions, O2exchange is perfusion limited. • Blood spends sufficient time in pulm. capillary to fully equilibrate with ; • BUT can become diffusion limited • in pathology (interstitial fibrosis) or • under strenuous exercise / at high altitude. • Similar for CO2, just faster…

  17. 4. Arterio-Venous Difference • O2 concentration at end of lung capillary: 13.4 kPa (a’). • Practically not possible to measure easily. • O2 concentration in aorta: 12.0 kPa (a). • Practically taken from a peripheral artery (femoral/brachial). • O2 difference is result of venous admixture (heart) • Called shunt. • O2 concentration in right atrium: 5.3 kPa ( ). • Average concentration as venous blood is mixed with different O2 extraction rates in various parts of body. • Arterio-venous difference (a - ): 6.7 kPa. • drops by ~ 60%: extraction from blood. • A large amount of O2 remains “bound” in blood (partial extraction). • In particular vascular beds, this difference can be much larger (heart muscle; leg muscles in a marathon runner…).

  18. Arterial Blood Gas Values • Values for different analytes are given incl. reference ranges. • not to be known by heart! • Arterial values vary considerably. • Link to acid-base control via CO2 (see lecture series by K. Saliba).

  19. Review of Changes in • Axis along bottom indicates distance from nose. • At each step, ↓. Note notation.

  20. Overview of Gas Pressures • Under resting conditions and with a “normal” metabolism. • Values in arteries/veins can be measured directly (blood gas analysis). • Without diffusion barriers, can be determined from blood gas. • ↑ in alveoli because R is 0.8; i.e. insufficient CO2 is produced. As a consequence ↑. • ↑ in arteries due to venous admixture into arterial blood (shunt). • Total pressure in veins < arteries because ↓ is > ↑.

  21. Take-Home Messages • Pb drops with altitude; it is ~⅓ of normal on Mt. Everest. • For purpose here, air consists of 79% N2 and 21% O2. • Water vapour pressure is 6.3 kPa at all pressures. • Reporting of lung and gas volumes in BTPS & STPD, resp. • In alveolus, O2 is exchanged for CO2 at a relative volume described by respiratory quotient R = 0.8. • Alveolar gas equation describes . • Gas exchange is via diffusion of dissolved gas, governed by gas solubility ( » > ). • Under normal conditions, blood is sufficiently long in alveolar capillary to fully saturate ( ). • Arterial O2 value is smaller due to shunt.

  22. MCQ A 25 year-old medical student ascends the summit of Mt. Blanc in France (4810 m). Assuming standard barometric pressure at this altitude (Pb = 55.4 kPa), a normal metabolism and a CO2 concentration of 4.2 kPa, which of the following values best describes the predicted alveolar partial pressure for O2 on Mt. Blanc? • 8.3 kPa • 7.5 kPa • 6.2 kPa. • 5.1 kPa • 4.7 kPa

  23. That’s it folks…

  24. MCQ A 25 year-old medical student ascends the summit of Mt. Blanc in France (4810 m). Assuming standard barometric pressure at this altitude (Pb = 55.4 kPa), a normal metabolism and a CO2 concentration of 4.2 kPa, which of the following values best describes the predicted alveolar partial pressure for O2 on Mt. Blanc? • 8.3 kPa • 7.5 kPa • 6.2 kPa. • 5.1 kPa • 4.7 kPa

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