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 O2 at baseline
activity and the majority of the remaining O2 exchange takes place within the
first few capillary branches. Most capillaries release little O2 at baseline
acting as an O2 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 O2 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 O2 delivery
capacity to support cerebral tissue in disease.
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