Variation in how quickly blood moves through individual brain capillaries—termed capillary transit time heterogeneity—appears to significantly influence oxygen extraction efficiency and neuronal function. This microvascular phenomenon reveals that even when overall blood flow remains normal, irregular flow patterns at the capillary level can impair tissue oxygenation and metabolic delivery. The finding challenges traditional assumptions that total cerebral blood flow alone determines brain health outcomes. This research direction holds particular relevance for understanding age-related cognitive decline and neurodegenerative diseases, where subtle microvascular dysfunction often precedes obvious symptoms. Previous studies focused primarily on large vessel pathology or total flow measurements, potentially missing this granular level of circulatory compromise. The heterogeneity concept may explain why some individuals with apparently adequate blood flow still experience cognitive issues, suggesting that optimizing capillary-level circulation could represent a new therapeutic target. However, translating these insights into clinical interventions remains challenging, as current imaging technologies struggle to assess individual capillary function in living humans. The work represents an important shift toward understanding brain health at the microscopic level.