Fig. 9. The in vivo saturation curve describes the relationship between haemoglobin (Hb) oxygen-saturation, measured in the circulatory system of an animal, and the oxygen partial pressure of the ambient medium (PO2amb). The shape and position of the in vivo saturation curve are highly variable because they depend not only on the intrinsic oxygen-binding properties of Hb but also on the respective position in the circulatory system, on the haemolymph concentration of Hb and on the systemic and metabolic characteristics of the animal. A simple model of the circulatory system (A) demonstrates the positional effect and the influence of Hb concentration. Oxygenated haemolymph leaves the respiratory organ and enters the tissue compartment where, at the proximate position, the in vivo saturation curve is measured (solid lines in B and C). Because oxygen is released from Hb and transferred to the metabolizing tissue, the in vivo saturation curve measured at the distant position appears to be shifted to the right (dotted lines). This rightward shift is more pronounced at lower (B) than at higher (C) Hb concentrations.
