Fig. 9. Involvement of Hb in circulatory oxygen transport. (A) The range between the loading and unloading oxygen partial pressure (P_O2; bold solid lines) of medium-sized Hb-poor and Hb-rich animals under different ambient oxygen tensions (P_O2amb) was mapped onto the oxygen equilibrium curve of Hb (thin solid lines). The ordinate shows the concentration of chemically bound oxygen ([O₂]), which is the product of the fractional oxygen saturation of Hb (S) and haem concentration ([Haem]; Hb-poor: 115 µmol l^–1, Hb-rich: 666 µmol l^–1). Oxygen equilibrium curves (S vs. haemolymph P_O2) were calculated according to the Hill equation assuming a cooperativity coefficient of 1.6 (Kobayashi et al., 1988) and half-saturation oxygen tensions of 1.0 kPa (Hb-poor) and 0.5 kPa (Hb-poor; Zeis et al., 2003a), respectively. (B) Differences between the loading and unloading [O₂] in relation to P_O2amb for Hb-poor (top) and Hb-poor animals (bottom). Black and white bars refer to the physically dissolved and chemically bound portions of transported oxygen, respectively. A value of 12.3 µmol l^–1 kPa^–1 was assumed as solubility coefficient for oxygen in haemolymph (Pirow and Buchen, 2004).