170 Introduction to Anatomy & Physiology 53. Explain how a stress fracture can happen, in terms of osteoblasts and osteoclasts. Further explain why stress fractures are particularly common in the tibia and metatarsals, given the functions of those bones. 54. Compare and contrast rheumatoid arthritis and osteoarthritis. In your own words, explain anatomically and physiologically what is occurring in each condition. 50. In the acronym R.I.C.E. for self-treatment of sprains, the C stands for _____. 51. _____ is an autoimmune disorder in which the body’s own immune system attacks healthy joint tissues. Thinking Critically 52. A 10-year-old boy and his 42-year-old father were building a tree house when the branch they were standing on broke away from the tree. The father sustained two broken ribs, but the boy had only a few bruises. Explain. Building Skills and Connecting Concepts Analyzing and Evaluating Data Instructions: The astronaut in Figure 4.42 is exercising on a special treadmill system in the International Space Station. Without this type of resistance training, he could lose 1% to 2% bone mass per month. Consider this information as you answer the following questions. 55. If the astronaut spends three months in space and never exercises, what percentage of his bone mass might he lose? 56. You read in this chapter that bones account for about 15% of human body weight. If this astronaut weighs 170 pounds, how much do his bones weigh? 57. If he fails to exercise for six months in space, how much weight might this astronaut lose in bone mass? 58. If this astronaut spends 12 months in space, but only exercises for 3 months, what percentage of bone mass will he lose? Figure 4.42 NASA/JSC Copyright Goodheart-Willcox Co., Inc.