Vascular capacitance describes the pressure–volume relationship of the circulatory system. The venous vasculature, which is the main
capacitive region in the circulation, is actively controlled by various neurohumoral systems. In terrestrial animals, vascular capacitance control is
crucial to prevent orthostatic blood pooling in dependent limbs, while in aquatic animals like fish, the effects of gravity are cancelled out by
hydrostatic forces making orthostatic blood pooling an unlikely concern for these animals. Nevertheless, changes in venous capacitance have
important implications on cardiovascular homeostasis in fish since it affects venous return and cardiac filling pressure (i.e. central venous blood
pressure), which in turn may affect cardiac output. The mean circulatory filling pressure is used to estimate vascular capacitance. In
unanaesthetized animals, it is measured as the central venous plateau pressure during a transient stoppage of cardiac output. So far, most studies of
venous function in fish have addressed the situation in teleosts (notably the rainbow trout, Oncorhynchus mykiss), while any information on
elasmobranchs, cyclostomes and air-breathing fishes is more limited. This review describes venous haemodynamic concepts and neurohumoral
control systems in fish. Particular emphasis is placed on venous responses to natural cardiovascular challenges such as exercise, environmental
hypoxia and temperature changes.
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