Chapter 6 Blood‐Gas Transport and Hemoglobin Function: Adaptations for Functional and Environmental Hypoxia

Abstract

The physiological diversity of blood oxygen transport traits in fishes appears designed to maintain tissue oxygenation under challenges from both metabolic demand and environmental oxygen supply. Brief episodes of functional hypoxia may occur during strenuous exercise when aerobic metabolism cannot be maintained and, when ambient oxygen tensions are low, more persistent environmental hypoxia may result. Hypoxic responses may be acclimatory (phenotypic plasticity) or adaptational (evolutionary plasticity). Fish adapted for an athletic lifestyle do not generally thrive under environmental hypoxia. Highly active species tend to have high O2‐carrying capacities, relatively low blood O2‐affinities, sigmoidal binding curves, marked Bohr and Root effects, and O2‐affinity is modulated by adenosine triphosphate (ATP). Fish living in habitats that are periodically low in oxygen may also have high oxygen‐carrying capacity, but generally have high blood O2‐affinities, low Hill coefficients, and hemoglobin (Hb) function is modulated by both guanosine triphosphate (GTP) and ATP. Hb function is further regulated by erythrocyte surface adrenoceptors when present. Multiple Hb components are functionally differentiated in some species, but not in others, and are not generally altered by acclimation. Low heterogeneity in Antarctic fish does not appear to be an adaptation for environmental stability. The O2‐binding properties of purified Hbs are difficult to interpret ecologically and consideration of the erythrocyte environment is critical to sensible interpretation of physiological traits. The Bohr effect depends on an ateriovenous pH gradient sustained by respiratory acidosis (CO2), whereas the Root effect is activated by fixed acid (lactate) and, unless localized in specific retial tissues, may seriously compromise effective oxygen transport in hypoxic situations. Reduced temperature sensitivity of Hb–O2 binding occurs in endothermic fishes that encounter thermal shifts at the gill exchange surface. Recent progress has been made in understanding the environmental thresholds for expression of factors compensating for hypoxia. These include globin synthesis, a role for Hb in regulation of the paracrine vasodilator NO, and changes in gene expression of HIF targets. Responses to hypoxia may be species specific, and comparisons become more difficult to interpret with increasing phylogenetic distance. The challenge for the future is to place research findings in the context of physiological ecology and behavior.