Cardiovascular Responses to Limiting Oxygen Levels

The capacity for metabolic energy (adenosine triphosphate, ATP) production via oxidative phosphorylation is much greater (~ 15 ×) than by anaerobic glycolysis, and vertebrates have, therefore, evolved efficient respiratory and circulatory systems to take oxygen up from the environment and transport it to the tissues. Consequently, reduced oxygen availability (hypoxia) or no oxygen (anoxia) in the environment poses a significant challenge to most vertebrates. For example, the vertebrate heart itself, being an aerobic organ with a high ATP demand, is inherently sensitive to oxygen limitation. The freshwater, estuarine and marine aquatic ecosystems in which fish reside are prone to hypoxia and low dissolved oxygen conditions of varying severity, periodicity and duration. Thus, hypoxia is a common environmental challenge faced by fishes. The focus of this chapter is two-fold. A significant proportion of the chapter summarizes the cardiovascular responses to oxygen limitation and their regulation in water-breathing and air-breathing fishes intolerant of surviving prolonged periods of severe hypoxia or anoxia, as well as those species that can survive prolonged periods of very little to no oxygen. Second, the survival strategies that the severe hypoxia/anoxia-tolerant species employ to endure prolonged periods of oxygen deprivation are described. Here, the differing anoxia survival strategies utilized by severe hypoxia/anoxia-tolerant species to balance cardiac ATP supply and demand, and to cope with metabolic wastes, are highlighted. In this section, there is special emphasis on the anoxic survival strategies used by one of the champions of vertebrate anoxia survival, the crucian carp ( Carassius carassius ). This species spends up to half of its life deprived of oxygen.