Capillary density is a physiological measure that takes a cross section of muscle and counts the number of capillaries, or blood vessels, contained within. This measurement can be an indicator of overall health and is also related to the ability to engage in strenuous physical activity.
Capillaries are the smallest blood vessels in the body, smaller even than veins and arteries, and they serve an essential function in transporting blood further than larger blood vessels can go. When capillary density is measured, a scientist or doctor will look at a cross-sectional area of muscle and count the number of capillaries present. Because capillaries are important in transporting oxygen throughout the body, a low capillary density can lead to an inability to exercise, though studies published by the National Institutes of Health indicate that capillary density is not necessarily an accurate reflection of oxygen transport capability.