The O 2 and CO 2 Transport System in Teleosts and the Specialized Mechanisms That Enhance Hb–O 2 Unloading to Tissues

Abstract

Teleost fishes comprise nearly half of all vertebrate species, which perhaps represents the most extensive adaptive radiation in vertebrate evolution. This success is in part through adaptations in the modes that they use to take up, transport, and deliver oxygen (O2) to the tissues. The present chapter reviews the basic concepts of O2 transport in fishes with a focus on hemoglobin (Hb) and red blood cell (RBC) function, and associated features that enhance the capacitance of blood for O2 (βb): the Bohr and Root effects, RBC β-adrenergic sodium proton exchangers (RBC β-NHE), and the retia mirabilia (teleost vascular counter-current exchangers). The RBC microenvironment plays an important role in modulating Hb characteristics in vivo during challenges to O2 transport such as hypoxia, exercise, and increases in temperature. In addition, a novel mechanism of enhanced Hb–O2 unloading in teleosts is described and discussed, one that relies on the heterogeneous distribution of plasma-accessible carbonic anhydrase (PACA). This is an intrinsic characteristic of the cardiovascular system, and a vastly understudied area not only in fishes. Thus, the available data on PACA distribution in fishes are presented in a phylogenetic context, concluding with the putative sequence of events that may have led to this remarkable mode of Hb–O2 unloading in modern teleosts. In combination with other well-studied teleost Hb and RBC characteristics, a heterogeneous distribution of PACA in the cardiovascular system may greatly enhance βb, and thus tissue O2 delivery, and support a higher rate of tissue O2 consumption without increasing [Hb] or blood flow. This is a research area worthy of further investigation that may improve our understanding of O2 transport in this most specious group of vertebrates.