Fig. 4. Analysis of O₂ supply to isolated papillary muscles. (A) Partial pressure of O₂ (P_O) profile through the cross-section of a cylindrical muscle. 1 atm=1.013x10⁵ Pa. The thickest line shows the P_O profile for a preparation with a radius equal to the mean value (0.41 mm) of those used in the study. The results for the smallest and largest preparations used (upper and lower lines, respectively) are also shown. Analysis was performed using the following parameters: P_O at muscle surface, 0.95 atm (horizontal broken line); diffusivity of O₂, 2.39x10^-5 cm² atm^-1 min^-1 at 27°C, adjusted to 30°C using a Q₁₀ of 1.04 (Mahler et al., 1985); steady-state rate of active metabolism, 9.7 mW g^-1 (present study; equivalent rate of O₂ consumption, 29 µl min^-1 g^-1); basal metabolic rate, 4.4 mW g^-1 at 27°C (Baxi et al., 2000); Q₁₀=1.31 (Loiselle, 1985b). (B) The relationship between net efficiency and the muscle radius for all preparations. The slope of a straight line fitted through the data did not differ significantly from zero.