Copyright Goodheart-Willcox Co., Inc. Chapter 5 The Skeletal System 129 mineralized to nonmineralized bone do occur. As people age, there is a progressive loss of collagen (which provides elasticity) and an increase in bone brittleness. This means that children are often able to successfully sustain falls and other accidents with- out harm, while older adults tend to be increasingly vulnerable to bone fractures. Bone mineral normally peaks in women at about 25 to 28 years of age and in men at about 30 to 35 years of age. Thereafter, bone mass is progressively lost. However, regular participation in weight-bearing exercise can dramatically slow the loss of bone mass and strength. Because women tend to have smaller bones than men, the loss of bone mass and bone mineral density is generally more problematic for women. Osteoporosis, a condition of dangerously low bone mineral density, is discussed later in the chapter. SELF CHECK 1. Explain the fetal ossification process. 2. Osteoblasts continue building layers of bone throughout life how does the body avoid increasing bone weight as the new layers are constructed? Bone Remodeling Although bones grow and change dramatically during childhood and adolescence, living adult bone is also a very active tissue. It is always changing at a micro- scopic 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 sustained when people push or pull on some- thing, and impact forces created when people bump into something all influence the bones. The remod- eling process converts the size and direction of the forces acting on bone into changes in bone mineral density. In some circumstances, the remodeling process also causes changes in the size or shape of bone. Blood calcium level can also influence bone density. If blood calcium is too low, the parathyroid glands (discussed in Chapter 9) release parathyroid hormone (PTH) into the bloodstream. PTH, in turn, activates osteoclasts to resorb bone matrix and release calcium into the bloodstream. If blood calcium is elevated above a balanced, homeostatic level, calcium Adult Development Although most bone growth occurs during childhood, bones actually grow in diameter, or width, through- out most of life (Figure 5.9). Osteoblasts in the internal layer of the periosteum build concentric layers of new bone on top of existing ones. To understand the process, it helps to visualize the way in which the rings on a cross-cut tree stump reveal the tree’s growth. While the osteoblasts are building new layers, the osteoclasts resorb layers of bone inside the medullary cavity, causing the diameter of the cavity to be progressively enlarged. This beautifully engineered process occurs in such a way that a healthy bone remains optimally functional, lightweight, and strong enough to resist daily stresses. While osteoblast and osteoclast activities tend to maintain bones at functional sizes and shapes through- out life, age-related changes in the proportion of Epiphyseal plate Jose Luis Calvo/Shutterstock.com Figure 5.8 Bones grow in length at the epiphyseal plates (the dark pink area). Medullary cavity Osteoblast activity builds new bone Osteoclast activity resorbs bone – – – – – – – – Periosteum Cortical bone Endosteum + + + + + + + + + + + + + + + + + + © Body Scientific International Figure 5.9 Cross section of a long bone showing normal bone throughout life.