The measurement of partial pressure of oxygen and carbon dioxide in the blood is one of the key physiological chemical measurements. The partial pressure of a dissolved gas is defined as the contribution of this dissolved gas to the total pressure of all dissolved gases in the blood. Hence, the partial pressure of a gas is proportional to the quantity of that gas in the blood.
The effectiveness of both the respiratory system (inspiration and expiration by the lungs) and cardiovascular system (circulation of deoxygenated blood to the lungs and oxygenated blood to the heart for further circulation) can be found out by the partial pressure of oxygen and carbon dioxide.
The partial pressure of Oxygen (pO2) can be measured in vitro (test tube) and in vivo (blood vessel). A typical method of measurement pO2 is illustrated in figure (a). A fine wire of platinum is embedded in glass for insulation purposes with only its tip exposed. The glass wire platinum wire is placed in an electrolyte into which oxygen is allowed to diffuse. If a voltage of about 0.7 V is applied between the platinum wire and the reference electrode in such a way that the platinum wire is at negative polarity, then the oxygen starts reducing at the platinum cathode. This results into an oxidation-reduction current flowing which is proportional to the partial pressure of the diffused oxygen.
The measurement of the partial pressure of carbon dioxide (pCO2) is based on the linear relationship between the logarithm of the pCO2 and the pH of a solution. Therefore, the measurement of pCO2 is accomplished by surrounding a pH electrode with a suitable membrane which is permeable to carbon dioxide.
Related: Key Considerations in Respiratory Measurements & Instrumentation Involved
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