Copyright Goodheart-Willcox Co., Inc. Chapter 5 The Skeletal System 131 Somewhat surprisingly, bone atrophy has also been observed in elite swimmers who spend hours a day training in a swimming pool. Swimming involves a large amount of muscle activity, but the buoyancy of the water counteracts much of the force of gravity. So, while swimmers are in the water, their bones are subjected to greatly reduced stresses. Loss of bone mass and strength is an even more significant problem for astronauts, who spend time completely out of Earth’s gravitational field. The loss of bone in astronauts in space is so rapid that it is currently one of the major factors preventing a manned space mission to Mars. 15% of a person’s body weight. This tends to be true whether a person is underweight, of average weight, or overweight. No one is overweight because of heavy bones. Being overweight is almost always the result of carrying excess body fat. Atrophy People who are subject to reduced forces are prone to bone atrophy, or loss of bone mineral density and strength. This has been observed, for example, in individuals who are bedridden for long periods of time. SECTION 5.2 REVIEW Mini-Glossary epiphyseal plate growth area near the ends of long bones where osteoblast activity increases bone length modeling process in which new bone is created through osteoblast activity during the formation and growth of immature bones ossification process of bone formation osteoblasts specialized cells that build new bone tissue osteoclasts specialized bone cells that resorb bone tissue remodeling process through which adult bone can change in density, strength, and sometimes shape Review Questions 1. Describe the functions of osteoblasts and osteoclasts. 2. What is bone modeling? 3. Explain the process of ossification in the growing fetus. 4. Describe the processes by which bones grow in length and circumference. 5. Why do bones become more brittle in old age? SECTION 5.3 The Axial Skeleton Objectives • Explain the ways in which the skull of an infant is similar to and different from an adult skull. • Describe the regions of the spine and the shapes and sizes of the vertebrae in each region. • Explain the structures and functions of the thoracic cage. Key Terms atlas axial skeleton axis cervical region coccyx cranium facial bones fontanel intervertebral discs lumbar region mandible maxillary bones median sacral crest sacral canal sacral hiatus sacrum skull sternum sutures thoracic cage thoracic region vertebra