Carbon dioxide in blood

When the partial pressure of C02 is 40 mmHg, 100 ml of blood at 37 °C contains 50cm3 of C02, 44cm3 of which as bicarbonate ions HC03-, 3cm3 as physically dissolved C02, and the remainder as compounds with proteins such as hemoglobin. The bicarbonate ion is produced by the following reversible reactions  

Reaction (B) is very rapid. Reaction (A) is slow, but becomes very rapid in the presence of the enzyme carbonic anhydrase, which exists in the erythrocytes. Carbon dioxide produced by the gas exchange in tissues moves into erythrocytes, while bicarbonate ions produced by reactions (A) and (B) in the erythrocytes move out into the plasma. 

Carbonic acid, H2C03, is a weak acid that dissociates by the above reaction (B). In general, a solution of a weak acid HA which dissociates into H + and A- will serve as a buffer solution. Thus, respiration in lungs contributes to physiological buffering actions. 

The normal pH value of the extracellular fluids at 37 °C is about 7.4, while that of the intracellular fluids is about 7.2. This can be explained by the buffering action of carbonic acid. In general, when a weak acid HA dissociates into H + and A, the following relationship holds  

Most of the C02 in blood exists as HC03 produced by the dissociation of H2C03. For this dissociation reaction, the value of log [1/K] = pi at 37 °C is 6.10. The ratio [HC03 / H2CO at 37 °C is maintained at approximately 20 by the respiration in the lungs. Then, Equation 14.4 gives pH = 7.4. 

Most of the CO2 that is physically absorbed by blood becomes H2CO3 by the above-mentioned reaction (A), in the presence of carbonic anhydrase. Thus, [H2CO3] is practically equal to [CO2], which should be proportional to the partial pressure of CO, that is,/)CO2. 

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Carbon dioxide devices were originally developed by Severinghaus and Bradley (59) to measure the partial pressure of carbon dioxide in blood. This electrode, still in use today (in various automated systems for blood gas analysis), consists of an ordinary glass pH electrode covered by a carbon dioxide membrane, usually silicone, with an electrolyte (sodium bicarbonate-sodium chloride) solution entrapped between them (Figure 6-17). When carbon dioxide from the outer sample diffuses through the semipermeable membrane, it lowers the pH of the inner solution ... 

Different enzymes exhibit different specific activities and turnover numbers. The specific activity is a measure of enzyme purity and is defined as the number of enzyme units per milligram of protein. During the purification of an enzyme, the specific activity increases, and it reaches its maximum when the enzyme is in the pure state. The turnover number of an enzyme is the maximal number of moles of substrate hydrolyzed per mole of enzyme per unit time [63], For example, carbonic anhydrase, found in red blood cells, is a very active enzyme with a turnover number of 36 X 106/min per enzyme molecule. It catalyzes a very important reaction of reversible hydration of dissolved carbon dioxide in blood to form carbonic acid [57, p. 220],... 

As with oxygen, the amount of carbon dioxide physically dissolved in the plasma is proportional to its partial pressure. However, carbon dioxide is 20 times more soluble in plasma than is oxygen. Therefore, approximately 10% of carbon dioxide in blood is transported in the dissolved form. 

Carbon Dioxide. A method has been adapted for the determination of carbon dioxide in blood. It can be seen from Fig. 23 that after... 

The conversion of dissolved carbon dioxide in blood to HCO3 and H30 is catalyzed by the enzyme carbonic anhy-drase. The Michaelis-Menten constants for this enzyme and substrate are = 8 X 10 mol and k2 = 6 X 10 s. ... 

The classical manifold architecture in Fig. 8.22, upper was exploited in the pioneering work incorporating GD in flow injection analysis for the spectrophotometric determination of total carbon dioxide in blood plasma [265]. Details of the separation unit are shown in Fig. 8.23, left. The donor stream with the sample zone was acidified and the released CO2 diffused through the membrane towards the acceptor stream, which was an alkaline cresol-red indicator solution. Analyte collection resulted in a transient lowering of the pfi of this stream and hence a transient modification to the monitored absorbance. The recorded peak height was proportional to the CO2 content in the injectate. 

This thermodynamic coupling between oxygen and carbon dioxide exhibited in water will also occur between any two gases in any solvent. A physiological example of this is the solubility of oxygen and carbon dioxide in blood, where it is found that increasing the partial pressure of Oo at fixed CO2 partial pressure results in an increased oxygen concentration in blood and decreased carbon dioxide concentration. Also, the situation is reversed if the CO2 partial pressure is increased at fixed O2 partial pressure. This phenomenon was first experimentally observed in 1914 and is referred to as the Bohr effect. 

Spectrophotometric Determination of Carbon Dioxide in Blood with Gas Diffusion Separation 231... 

Fig.9J FI manifold for the determination of carbon dioxide in blood vAth gas-diffusion separation. CR, water carrier S, sample K 0.2 M sulphuric acid SP. membrane gas-diffusion separator A. buffered acceptor with acid-base indicator D, detector and W, waste outlets for donor and acceptor streams [12].

IFCC reference measurement procedure for the substance concentration determination of total carbon dioxide in blood, plasma or serum. Clinical Chemistry and Laboratory Medicine 39 283-289. 

A Sensitive Procedure for Determining Carbon Dioxide in Blood or Tissue Utilizing Gas-Solid Chromatography Toxicol, and Appl. Pharmacol. 1 135-143 (1959) CA 53 14208g... 

Gas Chromatographic Determination of Oxygen and Carbon Dioxide in Blood Lab. Delo 1972(6) 327-330 CA 77 85155s... 

XR. Su, B.S. Yu, H.W. Tan, X.R. Yang, L.H. Nie, S.Z. Yao, Flow-injection determination of total ammonia and total carbon dioxide in blood based on gas-diffusion separation and with a bulk acoustic wave impedance sensor, J. Pharm. Biomed. Anal. 16 (1998) 759—769. 

The Henry s law constant, Kh, is commonly reported in moles per cubic metre per kilopascal (mol m kPa )- This form of the law and these units make it very easy to calculate the molar concentration of the dissolved gas, simply by multiplying the partial pressure of the gas (in kilopascals) by the appropriate constant. Equation 3.14 is used, for instance, to estimate the concentration of Oj in natural waters or the concentration of carbon dioxide in blood plasma. 

This chapter describes diffusion for these and other multicomponent systems. The formalism of multicomponent diffusion, however, is of limited value. The more elaborate flux equations and the slick methods used to solve them are often unnecessary for an accurate description. There are two reasons for this. First, multicomponent effects are minor in dilute solutions, and most solutions are dilute. For example, the diffusion of sugars in blood is accurately described with the binary form of Fick s law. Second, some multicomponent effects are often more lucid if described without the cumbersome equations splattered through this chapter. For example, the diffusion of oxygen and carbon dioxide in blood is better described by considering explicitly the chemical reactions with hemoglobin. 

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