PH value of blood

Due to their high concentration, plasma proteins—and hemoglobin in the erythrocytes in particular—provide about one-quarter of the blood s buffering capacity. The buffering effect of proteins involves contributions from all of the ionizable side chains. At the pH value of blood, the acidic amino acids (Asp, Glu) and histidine are particularly effective. 

The history of discovery of this relationship is an interesting story, one well told in a review by Cameron (1989). To students trained in the context of medical physiology, a pH value of 7.4, the pH value of blood at normal human body temperature, 37°C, is—or at least was in the past—presented as the normal pH value. In fact, a pH of 7.4 is a normal pH value for blood only near 37°C at lower temperatures, a pHB of 7.4 is acidic relative to the true physiological pH at that temperature (figure 7.16). In retrospect, this fact should have been obvious from an understanding of protein biochemistry. However, it took several decades for the tem-perature-pH relationship to be fully accepted and even longer for it to be understood mechanistically in terms of protein chemistry. 

Figure 7.16. Temperature effects on the pH values of blood, cytoplasm, the mitochondrial matrix, the pK of the histidyl imidazole group (pK-imid) and the neutral pH of water (pN). Parallel lines shown for muscle cytosol and blood encompass the range of values that have been reported for these two variables.

The body fluids can be regarded as buffer solutions, with the normal pH values of the extracellular fluids (including blood) and intracellular fluids being 7.4 and 7.2, respectively. 

Because of the importance of pH in regulating so many physiological processes, any changes in pH that occur as a result of changes in body temperature would seem likely to add another element of thermal stress to the system. Thus, one might predict a priori that the pH values of the blood (pHB) and the cytosol would need to be closely conserved in the face of changes in body (cell) temperature if homeostasis is to be... 

Ammonium NH4 is free NH3 dissolved in water. It is defined as the sum of free NH3 and ionized NH4, which is in a pH-dep>end-ent dissociation balance with NH3, i.e. alkalosis shifts the balance towards free ammonia. With a normal blood pH value of 7.4, more than 90% of the ammonium are available as NH4. ... 

The pH measurement system is calibrated against primary standard buffers admitted either manually or automatically into the sample chamber. The buffers are phosphate solutions that should meet National Institutes of Standards and Technology (NIST) specifications. Calibration buffers meeting NIST specifications are available from the manufacturer of an instrument, usually in containers of appropriate size and shape for mounting as a reservoir on the instrument. The pH values of the low and high calibrator buffers axe set by the manufacturer but always lie close to 6.8 and 7.4 at 37 °C. The tolerance of the specified values should be less than or equal to an SD of 0.003 to achieve SDs of 0.005 to 0.01 in measuring blood pH. Unopened containers should be stored at room temperature. When visual observation or an instrument display warns of low concentrations in the reservoir, recommended practice is to replace the reservoir with a newly opened container rather than to replenish fluid in the current one. Pooling several almost empty containers is not recommended. 

The residual liquid-junction potential, combined with the uncertainty in the standard buffers, limits the absolute accuracy of measurement of pH of an unknown solution to about 0.02 pH unit. It may be possible, however, to discriminate between the pH of two similar solutions with differences as small as 0.004 or even 0.002 pH units, although their accuracy is no better than 0.02 pH units. Such discrimination is possible because the hquid-junction potentials of the two solutions will be virtually identical in terms of true a. For example, if the pH values of two blood solutions are close, we can measure the difference between them accurately to 0.004 pH. If the pH difference is fairly large, however, then the residual hquid-junction potential will increase and the difference cannot be measured as accurately. For discrimination of 0.02 pH unit, large changes in the ionic strength may not be serious, but they are important for smaller changes than this. 

Previous Work. As shown by the results of many experimentors, cerebral blood flow rate (under normal blood pressure conditions) is primarily determined by the C02 tension in arterial blood (12,14, 21, 22, 45, 46). The pH value of the extracellular fluid in brain is regarded as an essential factor for regulating vascular diameter (47, 48, 49,50). Cerebral hypoxia causes the C02-dependent regulation of the cerebral blood flow to change or vanish (9, 21, 25, 40, 45, 46, 51, 52). 

Hodgkin s disease a malignant condition characterized by progressive enlargement of lymph nodes, spleen, and lymphoid tissues homeostasis maintenance of equilibrium with respect to the physiological functions of the body (stability of the metabolism, body temperature, pH value of the blood, blood pressure, etc.) by means of regulatory systems... 

The nested detector (Fig. 5.10/) will find its use whenever an intrinsic property of the sample should be measured prior to its further treatment in a FIA system. This may be the determination of the original pH value of the sample, its conductivity, color, or content of solid particle or blood cell. The nested detector may also be tuned to a given value of a selected parameter, which, when reached, will trigger the valve to inject the entrapped portion of the analyte into the FIA system for further investigation. 

Healthy blood is very weakly basic, with a pH range of 7.35-7.45. If the pH drops below this range of values, the result is a condition called acidosis. Severe acidosis (pH < 7.0) leads to depression of the central nervous system, which starts malfunctioning (leading to disorientation and, if the condition is very severe, possibly a comatose state). If the pH rises above 7.45, the condition is called alkalosis. This condition causes hypertension and muscle spasms, and it can even lead to death. As a result, the pH value of our blood is highly regulated by buffers, which are molecules that consume any extra protons or hydroxide ions and keep the pH constant or variant in a narrow interval. 

Total buffer capacity of blood 46-52 mmol I a pH value of 7.4 is equivalent to 39.8 nmoll hydrogen ions. 

The pH values of commraily used infusion solutions vary between 4 and 8 depending on the type of formulation. A lower pH is acceptable if solutions are not buffered. The pH of 5 % glucose solutions amounts from 4.0 to 5.0. This is necessary to prevent degradation of glucose (see Sect 13.6.4). Infusimi fluids that are used to correct the pH of the blood have a deviant pH value (e.g. pH 8.0 for sodium hydrogen carbonate infusion). 

Figure 11 shows X/Xq as a function of the -potential. The -potential was determined in a 10 M KCl solution at the physiological pH value of 7.2. Comparable parabolic curves were obtained with a maximum of blood compatibility within a i -potential range between -4 and -8 mV. The -potential of the water-... 

It is necessary to understand that the pH scale is logarithmic and not arithmatic. Thus, when it is said that two solutions differ from each other by 1 pH unit, it means chat one solution has 10 times the hydrogen ion concentration of the other. Thus, vinegar (pH 3.0) has H" concentration approximately 10,000 times greater than that of blood (pH 7.4). Table 1.4 lists the pH values of some important and commonly used aqueous fluids. 

Henderson-Hasselbalch Equation n A formula relating the pH value of a solution to the pK value of the acid in the solution and the ratio of the acid and the conjugate base concentrations pH = pIQ + log( [A—] / [HA]), where [A—] is the concentration of the conjugate base and [HA] is the concentration of the protonated acid. For the bicarbonate buffer system in blood,... 

At 37°C this buffer has a pH value of 7.382 with a temperature coefficient of -0.026 pH. It is used with physiological measurements because of its compatibility and solubility with this type of sample. It does not appear to inhibit many enzyme systems and has a temperature coefficient near that of blood (-0.015 pH/°C). 

Xenia Beebe et al. evaluated the maximum charge injection capacity of activated iridium wire electrodes in bicarbonate buffered saline as 2.1 m C/cm and l.OmC/cm for anodic-first and cathodic-first, 0.2ms charge balanced biphasic current pulses, respectively [1]. Bicarbonate buffered saline has a pH between 7.35 and 7.45 (similar to the human blood plasma) and is comparable to PBS with a pH value of 7.4. The counter electrode was an iridium foil. The reference electrode was a saturated calomel electrode. Cyclic voltammetry at lOOmV/s revealed a water window of -0.6 V +0.8 V, the same as versus Ag AgCl. 

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