Circulatory Anatomy in Bimodally Breathing Fish

SYNOPSIS. The development of air-breathing organs in bimodally breathing fish has necessitated a degree of vascular remodelling in order to enhance gas exchange and support other homeostatic activities. Macrocirculatory changes include several plumbing schemes that allow perfusion of the gills, air-breathing organ, and systemic circulations in a variety of in-parallel and in-series arrangements. The incorporation of structural adaptations designed to minimize admixture of oxygenated and deoxygenated blood in transit through the heart as well as vascular shunts further increases the efficiency of the gas exchange process. A number of anatomical modifications in capillary architecture and endothelial cell structure are found in air-breathing fish and appear to be unique to these vertebrates. The physiological significance of the microcirculatory adaptations remains, to a large extent, speculative.