Fig. 1. Representative example of SeRIS microelectrode measurements showing calculated ion fluxes along the surface of the anal papillae in vivo. The length of the arrows and the arrowheads indicate the magnitude and direction of flux, respectively. The papillae were arbitrarily divided into quarters such that regions 1 and 2 comprised the proximal half of the papillae and regions 3 and 4 the distal portion of the papillae. (A) H⁺ flux. H⁺ is released by the papillae into the bathing medium. Note that H⁺ flux is higher in the proximal regions of the papillae. Vertical scale bar, 6.5 pmol cm^-2 s^-1; horizontal scale bar, 90 µm. (B) Cl^- flux. Cl^- is taken up by the papillae from the bathing medium. The papillae that measurements were taken from was positioned approximately 500 µm below the other papilla that is seen in the field of view. Vertical scale bar, 42 pmol cm^-2 s^-1, horizontal scale bar, 54 µm.