Large arteriolar component of oxygen delivery implies safe margin of oxygen supply to cerebral tissue

What is the organization of cerebral microvascular oxygenation and morphology that allows adequate tissue oxygenation at different activity levels? We address this question in the mouse cerebral cortex using microscopic imaging of intravascular O2 partial pressure and blood flow combined with numerical modeling. Here we show that parenchymal arterioles are responsible for 50% of the extracted O(2) at baseline activity and the majority of the remaining O(2) exchange takes place within the first few capillary branches. Most capillaries release little O(2) at baseline acting as an O(2) reserve that is recruited during increased neuronal activity or decreased blood flow. Our results challenge the common perception that capillaries are the major site of O(2) delivery to cerebral tissue. The understanding of oxygenation distribution along arterio-capillary paths may have profound implications for the interpretation of BOLD fMRI signal and for evaluating microvascular O(2) delivery capacity to support cerebral tissue in disease.