CS 2014

Respiratory Partial Pressures and Blood Gasses

Christian StrickerAssociate Professor for Systems Physiology

ANUMS/JCSMR - ANU

Christian.Stricker@anu.edu.auhttp://stricker.jcsmr.anu.edu.au/PP&BG.pptx

THE AUSTRALIAN NATIONAL UNIVERSITY

CS 2014

CS 2014

AimsThe 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.

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Contents

• Basic terms and concepts• Partial pressures of N2, O2 and CO2

• Air saturated with water: • Partial pressures at following locations:

1. Nose2. Trachea3. Alveolus4. Lung capillary / Artery

• Blood gas values

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Gasses & Pressures [kPa]

1 kPa ≈ 10.2 cm H2O ≈ 7.5 torr

1 kPa = 1000 N / m2

1 torr = 0.1333 kPa

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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.

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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.

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Composition of Air

Gas Vol %

N2 78.03

O2 20.99

CO2 0.04

H2O~

0.50Ar 0.94

• 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.

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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-lised to 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:

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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.

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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?

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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.

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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

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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.

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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 → ↑.

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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.

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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, O2 exchange 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…

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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…).

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Arterial Blood Gas Values

Analyte Reference Range

9.3 – 13.3 kPa

4.7 – 6.0 kPa

pH 7.35 – 7.45

HCO3- 22 – 26 mmol/L

Total CO2 25 – 30 mmol/L

• 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).

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Review of Changes in

• Axis along bottom indicates distance from nose.• At each step, ↓. Note notation.

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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 > ↑.

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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.

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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?

A. 8.3 kPa

B. 7.5 kPa

C. 6.2 kPa.

D. 5.1 kPa

E. 4.7 kPa

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That’s it folks…

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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?

A. 8.3 kPa

B. 7.5 kPa

C. 6.2 kPa.

D. 5.1 kPa

E. 4.7 kPa