Chapter 5 The Skeletal System 119 Copyright Goodheart-Willcox Co., Inc. 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. Because gravity is also a force that continuously acts on bones, people who are heavier tend to have greater bone mass and density than people who are lightweight for their heights. Bone accounts for only about 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 5.1 REVIEW Mini-Glossary appositional growth growth accomplished by the addition of new layers to those previously formed articular cartilage dense, white connective tissue that covers the articulating surfaces of bones at joints bone marrow material with a rich blood supply found within the medullary cavity of long bones yellow marrow stores fat, and red marrow is active in producing blood cells cortical bone dense, solid bone that covers the outer surface of all bones and is the main form of bone tissue in the long bones diaphysis the shaft of a long bone epiphyseal plate growth plate near the ends of long bones where osteoblast activity increases bone length epiphysis the bulbous end of a long bone Haversian canals major passageways running in the direction of the length of long bones, providing paths for blood vessels Haversian system structural unit that includes a single Haversian canal along with its multiple canaliculi, which branch out to join with lacunae, forming a comprehensive transportation matrix for supplying nutrients and removing waste products also known as an osteon hematopoiesis process of blood cell formation medullary cavity central hollow area found in long bones ossifi cation process of bone formation osteoblasts specialized bone cells that build new bone tissue osteoclasts specialized bone cells that resorb bone tissue osteocytes mature bone cells osteon a Haversian system perforating (Volkmann’s) canals large canals that connect the Haversian canals oriented across bones and perpendicular to Haversian canals periosteum fi brous connective tissue membrane that surrounds and protects the shaft (diaphysis) of long bones remodeling process through which adult bone can change in density, strength, and sometimes shape Sharpey’s fi bers tiny connective tissue fi bers that join together to fi rmly bind the periosteum to the underlying cortical bone trabecular bone interior, spongy bone with a porous, honeycomb structure Review Questions 1. List each of the five functions of the skeletal system and describe how they benefit the body. 2. Explain the differences between cortical bone and trabecular bone. 3. Describe where the diaphysis of a long bone is in relation to the epiphysis, and where the periosteum is in relation to the endosteum. 4. Compare and contrast osteoblasts and osteoclasts and explain how they work together to reshape and remodel bones. 5. Explain why a physician would be worried about a child who broke a long bone close to the epiphyseal plate.