External Vertebral Venous Plexuses
The external venous network, known as the external vertebral venous plexuses (plexus venosi vertebrales externi), is situated directly on the outer surface of the vertebral column's bony structures. Depending on its topography relative to the vertebrae, this vascular network is divided into two distinct yet interconnected parts:
- Anterior external vertebral venous plexus (plexus venosus vertebralis externus anterior). This structure encompasses the anterior aspect of the spine. Its primary function is to collect venous blood from the anterior elements of the axial skeleton. Specifically, this plexus receives blood from the vertebral bodies themselves, the adjacent anterior longitudinal ligament, and neighboring muscle groups, such as the deep muscles of the neck.
- Posterior external vertebral venous plexus (plexus venosus vertebralis externus posterior). This plexus is located on the opposite, dorsal side. Its vessels densely envelop the posterior surface of the vertebral arches, as well as their transverse and spinous processes. Blood flows into this portion of the venous bed from the deep back muscles, the skin of the back, and the posterior bony elements of the vertebrae themselves.
Internal Vertebral Venous Plexuses
Unlike the superficial veins, the internal vertebral venous plexuses (plexus venosi vertebrales interni) are hidden inside the vertebral canal (canalis vertebralis). They are extensive, accompanying the central nervous system along its entire length: these plexuses begin at the level of the foramen magnum and descend all the way to the lower end of the sacral canal.
Topographically within the canal, the internal plexuses run along the inner surface of its bony walls. They are situated strictly within the epidural space, meaning they lie external to the dura mater protecting the spinal cord.
The internal system is likewise not uniform. Classification identifies two main venous channels that are abundantly interconnected by large blood vessels:
- Anterior internal vertebral venous plexus (plexus venosus vertebralis internus anterior).
- Posterior internal vertebral venous plexus (plexus venosus vertebralis internus posterior).
Basivertebral Veins
A specialized component of the spinal drainage system is represented by the basivertebral veins (vv. basivertebrales). These are specialized intraosseous blood vessels that provide venous drainage directly from the interior of the bone tissue.
Basivertebral veins form and run within specialized canals located directly inside the spongy bone (trabecular bone) of the vertebral bodies. These vessels course from the central regions of the bone toward its posterior surface (i.e., toward the vertebral canal). Upon exiting the vertebral body, the basivertebral veins drain into the anterior internal vertebral venous plexus, emptying the collected venous blood.
Anastomoses and Formation of Venous Rings
The venous system of the spine functions as a unified whole thanks to a remarkably developed network of anastomoses (connections between vessels). This ensures reliable and uninterrupted blood outflow.
Within the vertebral canal, the anterior and posterior internal plexuses regularly exchange blood. At the level of each vertebra, they connect with one another via transverse anastomoses. As a result of these connections, closed vascular structures—venous rings—are formed around the spinal cord.
Furthermore, there is a direct communication between the internal and external venous beds. This communication follows strict anatomical logic:
- The posterior internal plexus communicates via vessels with the posterior external plexus.
- The anterior internal plexus sends anastomoses to the anterior external plexus.
Thus, blood can freely redistribute between the vertebral canal cavity and the external tissues of the spine.