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This chapter describes blood oxygen (O2) and carbon dioxide (CO2) equilibrium curves (also called dissociation curves), and the physiologic factors that determine the shape and position of these curves. Equilibrium curves quantify O2 and CO2 carriage in blood as graphs of concentration versus partial pressure. It is necessary to consider both partial pressure and concentration because partial pressure gradients drive diffusive gas transport in lungs and tissues, but concentration differences determine convective gas transport rates in lungs and the circulation (see Chapters 18 and 21).
This subject would be much simpler if O2 and CO2 were physiologically inert and occurred in blood only as physically dissolved gases. The concentration of a dissolved gas in a liquid is directly, and linearly, proportional to its partial pressure according to Henry's law (C = aP, where a = solubility; see Chapter 18). However, O2 and CO2 also enter into chemical reactions with blood. These reactions (1) increase O2 and CO2 concentrations in blood, (2) allow physiologic modulation of O2 and CO2 transport in blood, and (3) make respiratory CO2 exchange an important mechanism of acid-base balance in the body. The physiologic and pathologic consequences of the these three factors for O2 and CO2 exchange are considered in Chapter 21.
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