Abstract
Two types of titration curve (log P CO 2 vs. pH and whole blood [HCO 3 −] vs. pH) were obtained in anaesthetized dogs by altering F i CO 2 . Both plots reveal a significant progressive decrease in slope from arterial (a) to mixed venous ( v ) to coronary sinus (cs) blood. Lines drawn between corresponding points on the arterial and mixed vanous or coronary sinus curves have slopes determined by the Respiratory Exchange Ratio (R) and are called R lines. Differences in slope between arterial and venous ( v or cs) curves may be due to changes in blood flow or of delivery of CO 2 to the blood. The former is indicated by constant R values at different CO 2 tensions whilst the latter is a reflection of changing R values due to fluctuations in CO 2 stores. The myocardium is not an important store for CO 2 but CO 2 has a profound effect on coronary blood flow and it is this that causes the difference in a-cs slopes. The reverse is true for skeletal muscle where the charging and discharging of CO 2 stores brings about the difference in a- v slopes.
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