Copyright Goodheart-Willcox Co., Inc. Chapter 5 The Skeletal System 139 person is lying in bed during overnight sleep, the discs absorb water and expand slightly. During periods of upright standing and sitting, when the discs are bearing weight, they lose a small amount of fluid and compression occurs. For this reason, people may be as much as three-fourths of an inch taller when they first arise in the morning. Injury and progressive aging reduce the water retention capability of the discs, accounting for diminished standing height in elderly individuals. Because the discs receive no blood supply, they must rely on changes in posture and body position to produce a pumping action that brings in nutri- ents and flushes out metabolic waste products with an influx and outflow of fluid. Because maintaining a fixed body position curtails this pumping action, sitting in one position for a long period of time can negatively affect disc health. Abnormal spinal curvatures can develop due to genetic or congenital abnormalities, or as a result of the spine being habitually subjected to asymmetrical forces (Figure 5.18). Exaggeration of the lumbar curve is called lordosis, accentuation of the thoracic curve is called kyphosis, and any lateral deviation of the spine is known as scoliosis. Intervertebral Discs Intervertebral discs composed of fibrocartilage provide cushioning between all articulating vertebral bodies that are not fused. These discs serve as shock absorbers and allow the spine to bend. The differences in the anterior and posterior thicknesses of these discs produce the normal cervical, thoracic, and lumbar curves. In a normal adult, the discs account for approxi- mately one-quarter of the height of the spine. When a Vertebral arch Articular processes Superior articular process Superior articular process Inferior articular facet Inferior articular facet Transverse process Transverse process The major components of a typical vertebra (superior view) Lateral and slightly inferior view Inferior view Vertebral body Vertebral body Spinous process Spinous process Inferior articular process Pedicle Pedicle Arrow passing through vertebral foramen Vertebral body Vertebral foramen © Body Scientific International Figure 5.16 Three views of a typical vertebra.