Gill function in an elasmobranch

Summary

Highly efficient oxygen uptake in elasmobranchs, as indicated by frequent excess of $P_{a_{{\text{o}}_{\text{2}} } }$ over $P_{E\mathop {\text{O}}\nolimits_{\text{2}} }$ has previously been ascribed to the operation of multicapillary rather than counter-current gas exchange by the gills. Analysis of models shows that, at maximum efficiency, a multicapillary system cannot account for values of $P_{a_{{\text{o}}_{\text{2}} } }$ greater than $(P_{I_{_{{\text{O}}_{\text{2}} } } } {\text{ + }}P_{E_{_{{\text{O}}_{\text{2}} } } } )/{\text{2}}$. In Port Jackson sharks Heterodontus portusjacksoni) $P_{a_{{\text{o}}_{\text{2}} } }$ commonly exceeds $(P_{I_{_{{\text{O}}_{\text{2}} } } } {\text{ + }}P_{E_{_{{\text{O}}_{\text{2}} } } } )/{\text{2}}$, which indicates the operation of a functional counter-current at the respiratory surface. The anatomical basis of this counter-current is provided by the demonstration that a continuous flow of water passes between the secondary lamellae into septal canals and thence via the parabranchial cavities to the exterior.