Spinal or lumbar disks are very forgiving yet can also cause problems. When, for example, the fibrous ring (annulus fibrosus) surrounding the intervertebral disk moves axially or laterally, this is called a disk protrusion. In cases where the gelatinous, spongy center presses down on the fibrous ring so hard that it tears, we are talking about a herniated disk (prolapse). When gelatinous substance mass already escapes the fibrous ring and enters the spinal canal, we are looking at a sequestered disk prolapse.
The disks between individual vertebrae serve as a kind of buffer. They do not fulfill this task on their own, though, as the double "S" formed by the spine acts as a shock absorber, too. The individual vertebral disks consist of a rather solid but nevertheless elastic outer fibrous ring and a soft, viscous gelatinous nucleus which itself is not supplied with blood. Depending on the age and condition of the disk, this soft center contains up to 90 percent water and its fibers can store up to 1,000 times their mass of moisture, soaking it up like a sponge. The pressure exerted by this nucleus keeps the individual vertebrae at some distance from each other.
When we move, the soft center, or gelatinous nucleus, shifts a little. When we bend forward, it moves backward, and when we bend backward, it moves forward a bit. Normally, such a disk is very flexible. Since it does not have any blood vessels, it is nourished by movement only. During movement, the spine is put under strain and released again and thus absorbs nourishing liquid from the vertebral body. This way, it becomes nice and plump. Due to strain in the course of the day, part of this liquid is squeezed out of the disks again, the nucleus shrinks a bit and the vertebrae move a little closer to each other. This, by the way, is also the reason why we may be a bit shorter in the evening than in morning. At night, lying down, the disks again soak up fluid from the surrounding tissue, regenerate and in the morning, they again present the spine's tightly filled shock absorbers. But: The older we get and the less we move, the more the disk's ability to regenerate and retain water diminishes. And then it can happen that the fibrous ring becomes brittle, protrudes or even gets torn.