35 - Chapter 35 GAS TRANSPORT It was noted in Chapter 32...

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Chapter 35 GAS TRANSPORT It was noted in Chapter 32 that hemoglobin participates in buffering H + ions and that its ability to do so varies with its oxygenation state. This results from a conformational change in the protein when it takes up O 2 , with the release of one H + ion. Similarly, non-oxygenated hemoglobin has a high affinity for H + . A "solution" of oxyhemoglobin is thus more acid than is non-oxygenated hemoglobin. This phenomenon is termed the Haldane effect : Hb • H + + O 2 ± HbO 2 + H + As noted earlier, this buffering role of hemoglobin minimizes pH differences between arterial and venous blood. There are two important consequences of these properties of hemoglobin. Increased acidity in peripheral tissue resulting from general metabolic processes, by mass action, assists in displacing O 2 from hemoglobin and making it available for diffusion into the tissues. This phenomenon is called the Bohr effect : available to tissue M Hb • H + + O 2 ± HbO 2 + H + M t i s s u e a c i d i t y f r o m m e t a b o l i s m Increased partial pressure of oxygen (P O2 ) in the pulmonary vascular bed aids in H + release and, because this H + is buffered by HCO 3 - , the mechanism facilitates CO 2 release from blood and makes it available for diffusion into the alveolus of the lung: Hb • H + + O 2 ± HbO 2 + H + N P from alveolar air P P CO 2 + H 2 O ± H 2 CO 3 ± HCO 3 - + H + P yielded up to alveolar air The amount of oxygen combined with hemoglobin is mainly controlled by P 02 . The concentrations of gases are described in terms of P 02 and P CO2 in Box 35-1. The unit of measurement of partial pressures or concentrations is torr . In addition to P 02 , pH (via the Bohr effect) and temperature both influence what is called the oxygen dissociation curve . This is the relationship between the degree of association of O 2 with hemoglobin, expressed as the percentage saturation, for different P O2 values. Because O 2 combines with Hb to form HbO 2 , increased P O2 should increase formation of HbO 2 merely by mass action. The curve shown in Figure 35-1 shows that most combining of Hb and O 2 occurs at a P O2 of about 60 torr. The plateau above 65 to 70 torr is important because it ensures that nearly complete saturation of hemoglobin still occurs even though P O2
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This note was uploaded on 03/23/2009 for the course ANSCI 1110 taught by Professor Brucecurrie during the Fall '08 term at Cornell University (Engineering School).

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35 - Chapter 35 GAS TRANSPORT It was noted in Chapter 32...

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