Antireflux mechanisms in veins draining the upper territory of the vertebral column and spinal cord in man.

The terminal segments of vertebral, right supreme intercostal and accessory hemiazygos veins from 28 cadavers of various ages were histologically examined to assess the possible presence of intrinsic mechanisms facilitating the delivery of venous blood from the upper vertebral and spinal cord territories. In these veins, blood flow is assisted by gravity (veins of receptive type); however, the tunica adventitia showed a significant amount of longitudinally oriented bundles of smooth muscle fibers together with elastic fibers, i.e., an intrinsic structure similar to that of propulsive type veins. Both the muscular and elastic components increased with age. One or two complete valves were found at the opening of these veins into their major receptors. It can be assumed that the structural changes of the venous wall and the ostial valves play a role in avoiding or limiting undue reversion of the blood flow in many physiologic and pathologic conditions, and in protecting the delicate nervous structures of the vertebral canal from possible damages due to temporary or chronic venous stasis.