Capillarization

Capillarization refers to the extent of the capillary network supplying a tissue. In skeletal muscle physiology, it describes the number and distribution of capillaries in relation to muscle fibers. These microscopic blood vessels provide an important exchange interface between circulating blood and muscle tissue, supporting the movement of oxygen, nutrients, metabolites, and other substances between blood and muscle cells .¹

Muscle capillarization can be quantified in several ways. Capillary density is the number of capillaries within a given area of muscle, while the capillary-to-fiber ratio describes the number of capillaries relative to the number of muscle fibers. Researchers may also measure how many capillaries contact individual fibers or account for fiber size and perimeter when describing capillary supply .^1,2

These measurements are related but are not interchangeable. Capillary density depends partly on muscle-fiber size. If muscle fibers become larger without a proportional increase in capillary number, capillaries per unit area can decrease even though the number of capillaries relative to fibers has not fallen. The capillary-to-fiber ratio is less directly affected by changes in fiber size and therefore provides different information about the structure of the capillary network .^1,2

Capillarization can increase through angiogenesis, the formation of new capillary vessels from existing blood vessels. Exercise training can increase skeletal muscle capillarization, although the magnitude and pattern of change depend on factors including initial training status and how capillarization is measured .^1,2