CO 2 back-diffusion in the rete aids O 2 secretion in the swimbladder of the eel

Abstract

In order to estimate the relative importance of back-diffusion of O 2 and CO 2 for their partial pressure enhancement in the swimbladder, we have determined O 2 and CO 2 partial pressure content and pH in microsamples collected non-obstructively frm the afferent and efferent rete vessels in the European eel. 1. 1. The P O2 increased significantly along the arterial vessels of the rete (from 33 to 303 Torr), with no change in O 2 content, suggesting O 2 not to be exchanged in the rete counter-current system. 2. 2. A coresponding increase of P CO2 (from 4 to 35 Torr) was accompanied by a significant rise in CO 2 content (from 8 to 15 mmol · LP −1), suggesting significant CO 2 back-diffusion in the rete. 3. 3. Change in P O2 during passage of blood through the swimbladder epithelium were variable and small, and the P O2 in rete venous blood was similar to that in rete arterial blood, explaining the lack of O 2 back-diffusion. 4. 4. Using blood CO 2 dissociation data, about 70% of the rise in arterial CO 2 content was estimated to derive from diffusion of CO 2, the remaining 30% form diffusion of HCO 3 −4, from venous to arterial rete capillaries, or from H + transport in the opposite direction. The data indicate that CO 2 back-diffusion in the rete does not only raise the rete arterial P CO2; it also reduces the O 2 capacity (Root effect) and thus enhances the arterial P O2.