A recently introduced three-dimensional analytical model of O2 diffusion to heavily working muscle that considers myoglobin-facilitated O2 diffusion inside the muscle fiber and a carrier-free layer separating erythrocytes and fiber is able to furnish the following new insights in O2 supply to red muscle at high performance. 1) Fiber PO2 profiles are essentially flat, and the major PO2 gradients are located in the perierythrocytic region, in good agreement with experimental findings [T. E. J. Gayeski and C. R. Honig, Am. J. Physiol. 251 (Heart Circ. Physiol. 20): H789-H799, 1986]. No specialized anatomical pericapillary barrier structure is required to explain these results. 2) A functional barrier to O2 diffusion has been identified that consists of the carrier-free layer and of the pericapillary muscle fiber portions. There are three reasons that make these structures act as a diffusion barrier: a "geometric reason," a "diffusivity-related reason," and a "myoglobin-related reason." 3) PO2 fields of adjacent red blood cells (RBCs) practically do not interact. 4) Small scale heterogeneities in capillary and RBC spacing are compensated for by high myoglobin-facilitated fiber diffusivity. Limiting factor for diffusional O2 transport is the number of RBCs present on the fiber surface.

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GLEICHMANN, U., D. H. INGVAR, D. W. LUBBERS, B. K. SIESJO and G. THEWS. Tissue PO, andpCO, of the cerebral cortex, releated to blood gas tensions. Acta physiol. scand. 1962. 55. 127-138. -Quantitative measurements of the PO, and the pC0, on the intact surface of the cerebral cortex were made in anesthetized dogs. Simultaneous measurements of the cortical metabolic rate of oxygen and carbon dioxide, as well as of the arterial and cortical venous PO, and pC0, (GLEICH-MANN et al. 1962), allowed calculation of the average tissue PO, and pC0, according to THEWS (1960 a and b). The results indicate that the PO, and the pC0, electrodes measure tensions which are close to the calculated average tissue PO, and pC0, respectively. I t has long been recognized that direct measurements of the tissue oxygen and carbon dioxide tensions could yield useful information on important events in the central nervous system. A step forward was the development of polarographic electrodes for measurement of the oxygen tension in tissue (DAVIES and BRINK 1942). Since that time a number of qualitative studies of the oxygen tension of the cerebral cortex have been performed (DAVIS, MCCULLOCH and ROSEMAN 1944, DAVIES and REMOND 1947, MOCHIZUCK~ 1951, MEYER and DENNY-BROWN 1955 SIESJO 1960, 1962, MEYER and GOTOH 196 1 ) . Although quantitative measurements have been possible with the "recessed" type of platinum cathode (DAVIES and BRINK 1942, DAVIES and BRONK 1957 ; $. CATER 1960) such measurements have not been performed systematically to any larger extent (See discussion). On leave of absence from the Institute of Physiology, Cologne. Present adress: Department of Applied Physiology, Marburg. On leave of absence from the Institute of Physiology, Kiel.