Capillary density

Capillary density is the number of capillaries serving a given area of muscle. Capillaries are where the exchange actually happens — oxygen and fuel out to the tissue, carbon dioxide and metabolites back — so density sets how well a muscle can be supplied and cleared.

It matters because it is one of the peripheral adaptations that raises endurance independently of the heart. More capillaries mean a shorter diffusion distance, better oxygen delivery at a given cardiac output, and faster clearance of the metabolites that accumulate during hard work.

In practice it responds particularly well to sustained moderate-intensity aerobic training, which is another reason a large base of easy volume is not wasted time. Type I fibres carry the highest density.