## Gas pressures

According to Fick's first law of diffusion, the external gas exchange between lungs and blood depends on the partial pressures of inspired gases and on the partial pressures of gases dissolved in the blood. In this respect it is only of indirect importance that O2 and CO2 are also transported actively bound to haemoglobin and that CO2 is, in addition, transported as HCO3- ion in the blood. At the site of gas exchange only the physically dissolved gas molecules will play a role in the diffusion process.

When breathing air, in addition to O2 and CO2, N2 and water vapour play a role: N2 diffuses between lungs and blood like the other gases. Because N2 is an "inert" gas, in that it does not react chemically in the human body, there is no difference in partial pressure between lungs and blood as long as there is no change in total atmospheric pressure.

Inspired air may contain varying amounts of water vapour which behaves like any other gas and also produces a partial pressure pH2O. After humidification in the airways the breathing gas in the lungs is saturated with H2O at 37 °C (ie 100% of relative humidity) and the pH2O is 47 mmHg.

At this point we start using mmHg and leave IS units because in many European countries it is still common to use mm Hg for blood gases.

Conversion is as follows:

750mm Hg = 1 bar = 100kPa and 760mm Hg = 1.013 bar = 101.3kPa.

Total atmospheric pressure must be the same for inspired gas, alveolar gas and exspired gas. The gases other than water vapour must share the remaining pressure. For example:

inspired gas: Ptot 760mm Hg - pH2O 6mm Hg = 754mm Hg alveolar gas: Ptot 760mm Hg - pH2O 47mm Hg = 713mm Hg

So, each inspired gas other than H2O have to share 754mm Hg and 713mm Hg respectively according to their gas fraction Fgas.

Figure 1.3-4. Gas partial pressures breathing air at 101.3kPa (760mm Hg) (Graphic by J.

Mair4)

Figure 1.3-4. Gas partial pressures breathing air at 101.3kPa (760mm Hg) (Graphic by J.

Mair4)

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