Review of Medical Physiology - William F. Ganong 2002

Respiration
Pulmonary Functions
Properties of Gases

It is known that gas pressure is proportional to its Temperature and the number of moles per volume:

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where P is pressure; n is the number of moles; R is the gas constant; T is absolute temperature; V is volume.

Partial Pressure

Unlike liquids, gases tend to fill the entire volume available to them. The volume occupied by a given quantity of gas molecules at a specific temperature and pressure (assuming ideal behavior) is independent of the gas composition. Therefore, the pressure exerted by an individual gas in a gas mixture (partial pressure P) depends on that gas's fractional contribution to the total pressure of the mixture.

In dry air, O₂ accounts for 20.98%; CO₂ for 0.04%; N₂ for 78.06%; and other inert gases (such as argon and helium) make up 0.92%. Barometric pressure P_B at sea level is 760 mmHg (1 atmosphere). The partial pressure of O₂ in dry air is 0.21 × 760, or 160 mmHg at sea level. The partial pressure of N₂ and other inert gases is 0.79 × 760 mmHg, and P_CO2 is 0.0004 × 760, or 0.3 mmHg. The percentage of Water vapor in the air depends largely on climate, whereas the partial pressures of individual gases vary only slightly. Inspired air becomes saturated with water vapor as it passes into the Lungs. Since P_H2O at a body temperature of 37°C is 47 mmHg, the partial pressures of the gases in the air reaching the lungs are P_O2 = 149 mmHg, P_CO2 = 0.3 mmHg (including other inert gases), and P_N2 = 564 mmHg (including other gases).

Gases diffuse from regions of high pressure to regions of low pressure, with The rate of diffusion depending on the concentration gradient and The Nature of the barrier between these regions. When a gas mixture comes into contact with a liquid, each gas in the mixture diffuses according to its partial pressure and solubility. The partial pressure of a gas in a liquid is the pressure at which the gaseous phase is in equilibrium with the liquid, and it depends on the concentration of that gas's molecules in the liquid.

Methods FOR QUANTITATIVE Measurement of Respiration

Respiratory movements can be assessed by recording changes in chest dimensions or by using spirometers (see Table 17-1), which also make it possible to measure expired or inspired gas volumes. Because gas volumes depend on temperature, pressure, and water vapor saturation, it is essential to correct these measurements to standard conditions. The four most important standards and their Abbreviations are listed in Table 34-1. Modern gas analysis tools allow for rapid and reliable determination of gas mixture composition and gas content in Body Fluids. For example, O₂ and CO₂ detectors—sensitive to minute changes in O₂ and CO₂ levels—can be placed directly into the respiratory tract, Blood Vessels, or Tissues. This enables continuous measurement of P_O2 and P_CO2. Long-term monitoring of oxygenation is performed non-invasively using an oxyhemometer, which is typically attached to the ear. The absorption of light passing through tissue is directly proportional to the oxyhemoglobin content of that tissue; furthermore, the oxyhemometer automatically accounts for the Light absorption of the tissue itself and other components by measuring only arterial blood absorption. Flame absorption spectroscopy allows for the rapid measurement of CO₂, carbon monoxide (CO), and various anesthetic gases. In addition, gases can be quantified using gas Chromatography and mass spectrometry.

Table 34-1. Standard reference conditions for gas volume measurements