Printed in Great Britain DISTRIBUTION OF OXYGEN TENSION IN THE BLOOD AND WATER ALONG THE SECONDARY LAMELLA OF THE ICEFISH GILL

In spite of the extensive studies that have been made in recent years on the structure and function of gills, especially in fish, little is known about the O2 tensions at different points along the exchange surface. Although the terminal water and blood tensions have been measured for a number of fish (Stevens & Randall, 1967; Piiper & Baumgarten-Schumann, 1967, 1968; Holeton, 1970; Garey, 1967), investigation of this aspect of gill function is extremely difficult. Measurements of the tension at different points along the lamellar surfaces have been made only for the crab gill (Hughes, Knights & Scammell, 1969). A know-ledge of the distribution of O2 tensions along the exchange surface is important theoretically and also for practical use in relation to diffusing capacity etc. In many recent studies the term APO (Randall, Holeton & Stevens, 1967; Holeton, 1970; Jones et al. 1970) has been used as a measure of the effective O2 tension difference. It assumes a linear relationship between gas tension and content, and that the difference between water and blood O4 tensions is constant along the exchange surface. Piiper & Baumgarten-Schumann (1968) recognized this deficiency and applied a Boh