The flux of oxygen within tissues.

Diffusive flux of oxygen through tissues which are essentially connective and have few cells, display reduced diffusion coefficients when compared to that through an equivalent lamina of water. In general even significant reductions can be explained in terms of the exclusions imposed on small molecular weight diffusates by the large hydrodynamic domains of the connective tissue components. An alternative way of explaining this large exclusion is to point to the very large microscopic viscosities which large interacting polymers impose upon the solvent (water). By contrast, the diffusive flux of oxygen through tissues composed of contiguously packed and actively respiring cells, shows an increased diffusive flux for oxygen when compared to that through an equivalent water lamina. This increase can be explained in terms of the substantial solubility of oxygen within the membrane phase of the cells. This high oxygen partition coefficient into cell lipids has several consequences. Firstly oxygen diffusion will be directed and two dimensional rather than random and three dimensional. Secondly this diffusion will be directed towards the oxygen-consuming sites which are located at lipid surfaces. Thirdly the aqueous oxygen partial pressure will be kept low (since re-supply is constrained while consumption is continuous). This low aqueous environment permits all of the cell soluble redox systems to be maintained efficiently at low metabolic cost, as well as minimising the risk of unscheduled oxidations. Viewed from this perspective, the high value found for oxygen partition coefficient into the erythrocyte membrane suggests that evolution of membrane structure and components may have been driven in part by the selective advantages of high oxygen solubility.