Intraosseous circulation from physiology to disease.

Blood flow within bones is unique in two ways: (1) the blood circulates within a closed cavity in which pressure must remain constant, a feat achieved in part thanks to the considerable distensibility of the intraosseous vessels and, above all, veins; (2) the intraosseous circulation allows traffic of minerals between the blood and bone tissue and sends the blood cells produced within the bone marrow into the systemic circulation. In contrast, the arterioles and capillaries within bones have the same anatomic structure as those located elsewhere in the body and are susceptible to arteriosclerosis, arteritis, or thrombosis. The mechanisms that regulate blood flow within bone are incompletely understood, probably because they are difficult to study in vivo. The cytokines and growth factors that regulate intraosseous angiogenesis also regulate bone remodeling, and close links exist between the blood supply to bone and bone formation and resorption: most diseases characterized by increased bone resorption are associated with increased bone vascularization. The vascular bud located at the center of bone multicellular units (BMUs) may determine the timing of bone resorption and bone formation. Avascular bone necrosis and bone infarction may result from acute blood vessel occlusion (thrombosis, lipid emboli, fat cell hypertrophy with compression of intraosseous capillaries), whereas arteriosclerosis may contribute to the development of osteoporosis.