130 Introduction to Anatomy & Physiology at the sites of force application (often muscle attachments). As a result, the bones of people who are physically active are usually denser and stronger than the bones of people who are sedentary. 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 height. Dynamic activities such as running and jumping involve landing impacts, which cause motion of fluid within the bone matrix. This motion is particularly effective at triggering the actions of osteoblasts to build bone. Altogether, bones account for only about 15% of 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, however. Being overweight is almost always the result of carrying excess body fat. Atrophy of Bones 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. 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 the 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. Check Your Understanding 1. What factors can cause bone to hypertrophy, increasing mass, density, and sometimes circumference? 2. What factors can cause bone to atrophy, losing mass and mineralization? laid out in concentric circles called lamellae (la-MEHL-ee) around the Haversian canals, and it is the lacunae that house the osteocytes. Each Haversian canal, with its surrounding layers of lacunae, forms a structural unit called an osteon, or Haversian system. Within the system are tiny sideways canals called canaliculi (kan-ah- LIK-yoo-ligh). The canaliculi connect with the lacunae, forming a comprehensive transportation matrix for supply of nutrients and removal of waste products throughout the Haversian system. The multiple Haversian systems are joined by perforating (Volkmann’s) canals, also running sideways, that connect the Haversian canals. Check Your Understanding 1. What is the periosteum? Name at least three functions of the periosteum. 2. Where is the endosteum found? Is it inside or outside the periosteum? 3. What covers the ends of long bones? 4. Describe the functions of osteoblasts and osteoclasts. 5. Describe and analyze the effects of less elasticity on the body as people age. Remodeling of Bones Living adult bone is a very active tissue, always changing at a microscopic level in bone mineral content (and thereby strength) and sometimes in size or shape. This occurs through osteoblast and osteoclast activity during a process called remodeling. Forces, such as gravitational force, muscle forces, forces received when you push or pull on something, and impact forces when you bump into something, all influence bones. The remodeling process converts the size and direction of the forces acting on bone to changes in bone mineral density. (Refer to Chapter 1 for more information about the effects of forces on the body.) In some circumstances, the remodeling process also causes changes in the size or shape of bone. Hypertrophy of Bones Generally, when bone is subjected to larger (stronger) forces, it tends to hypertrophy (high- PER-troh-fee), with increases in density and growth Copyright Goodheart-Willcox Co., Inc.