Figure 1.

Schematic representation of the exchange of hyperpolarized and nonhyperpolarized ¹³C-urea across the plasma membrane of an RBC. The velocities (reaction rates) are denoted by v_e and v_i for the extra-to-intracellular direction and vice versa, respectively; and the longitudinal relaxation rate constants for ¹³C-urea in the extra- and intracellular compartments are denoted by 1/T_1e, and 1/T_1,i, respectively.