The mass transfer coefficient for oxygen transport from blood to tissue in cerebral cortex.

The functioning of cerebral cortex depends on adequate tissue oxygenation. MRI-based techniques allow estimation of blood oxygen levels, tissue perfusion, and oxygen consumption rate (CMRO), but do not directly measure partial pressure of oxygen (PO) in tissue. To address the estimation of tissue PO, the oxygen mass transfer coefficient (KTO) is here defined as the CMRO divided by the difference in spatially averaged PO between blood and tissue, and is estimated by analyzing Krogh-cylinder type models. Resistance to radial diffusion of oxygen from microvessels to tissue is distributed within vessels and in the extravascular tissue. The value of KTO is shown to depend strongly on vascular length density and also on microvessel tube hematocrits and diameters, but to be insensitive to blood flow rate and to transient changes in flow or oxygen consumption. Estimated values of KTO are higher than implied by previous studies, implying smaller declines in PO from blood to tissue. Average tissue PO can be estimated from MRI-based measurements as average blood PO minus the product of KTO and CMRO. For oxygen consumption rates and vascular densities typical of mouse cortex, the predicted difference between average blood and tissue PO is about 10 mmHg.