Fig. 6. Exogenous H₂S is rapidly consumed by trout (A) or rat (B) blood in vitro effectively maintaining H₂S concentration close to 0. The concentration of H₂S (as total sulfide) was measured in real-time with a polarographic H₂S sensor. Serial additions of H₂S (as Na₂S), each sufficient to raise total sulfide to 10 µmol l^–1, did not increase sulfide by more than 1.5 µmol l^–1. The rate of sulfide consumption in trout blood was tenfold greater, presumably due to the presence of mitochondria in trout erythrocytes. (C) Effect of exogenous sulfide (as Na₂S) on sulfide concentration of dorsal aortic blood in an unanesthetized trout. An extracorporeal pump circulated blood from the dorsal aorta across the sensor and returned it to the caudal vein. The arrow indicates bolus injection of Na₂S into the caudal vein cannula. The amount of Na₂S injected was theoretically sufficient to raise plasma sulfide to 30 µmol l^–1 when fully mixed in the plasma. Inset shows injection of the same amount of sulfide into a recirculated volume of Hepes buffer equivalent to the trout's plasma volume and pH. Sulfide is rapidly removed from the plasma in vivo, but not from the buffer. w, wash. Adapted from Whitfield et al. (Whitfield et al., 2008), with permission.