The nucleus pulposus is made of 88% water. Water can come in and out of the disk by means of small pores, which act as byways to and from the bone of the nearby vertebra. Loading of the spine, (which happens in sitting and standing) pushes the water out of the disk. Laying down allows the water to come back in. Unfortunately, the aging process is such that over time, more water leaves the disk than enters.
Unlike the muscles and organs of our bodies, the intervertebral disk has no blood supply. This means that for an intervertebral disk to receive necessary nutrition (and for wastes to be carried away), it has to rely on mechanical means and the flow of water.
The nucleus pulposus can be visualized as a soft ball located right in the center of the disk. According to I.A. Kapandji, it has the ability to swivel in all directions. This swiveling action alters the tilt and rotation of the vertebra above and below it, (1)allowing for and buffering the effects of movement on the spine. The swiveling of disks is in response to the direction in which the spine is moving.
1. From I.A.Kapandji, The Physiology of the Joints Vol. 3 The Trunk and the Vertebral Column 2d ed. (Churchill Livingstone), 30-31.
