"Capillary density" refers to the number of tiny blood vessels called capillaries in a specific area of the body. It indicates how well oxygen and nutrients can reach and nourish tissues or muscles in that area. Full definition
This very specific high rep training physiologically PREPARES your body for future muscle growth by increasing capillary density (capillaries are the tiny blood vessels where nutrient exchange happens) and the number of mitochondria (the energy factories that reside within the cells of your muscles). (fitness-ebooks.com)
They found that at 6 months of age, these mice had reduced capillary density and could run only half as far as normal 6 - month - old mice. (sciencedaily.com)
The cross-section increases comes from stored fuel like glycogen, capillary density etc... (strengthsensei.com)