ABSTRACT Introduction In physoclist fish, that is, in fish with a closed swimbladder, oxygen has been shown to be the main gas in the swimbladder. Gas gland cells, crucial for the filling of the swimbladder, produce and secrete lactic acid. The resulting acidification of the blood releases oxygen from the hemoglobin via the Root effect, generating high oxygen partial pressures required for the diffusion of oxygen into the swimbladder. CO 2 in swimbladder gas was believed mainly to result from a wash out of HCO 3 − from the plasma due to acidification, but this reaction would diminish blood acidification. This study therefore attempted to revisit our current information about CO 2 production and also about CO 2 and HCO 3 − movements in swimbladder tissue to elucidate the role of CO 2 for swimbladder function. Methods Targeted literature search has been conducted to search for relevant publications. Results The data reveal that European eel Anguilla anguilla gas gland cells produce most of the CO 2 in the pentose phosphate shunt. CO 2 diffuses into the swimbladder lumen and into the blood, facilitated by aquaporin 1. Membrane‐bound carbonic anhydrase together with HCO 3 − transporters allow for CO 2 and HCO 3 − cycling across the basolateral gas gland cell membrane, supporting proton secretion and blood acidification. Countercurrent concentration of CO 2 , HCO 3 − and protons in the rete mirabile results in a significant acidification of arterial blood in the rete. Conclusions CO 2 production together with CO 2 and HCO 3 − movements in swimbladder tissue significantly support blood acidification and the generation of high oxygen as well as CO 2 partial pressures.
Bernd Pelster (Fri,) studied this question.