118 Anatomy & Physiology Essentials Copyright Goodheart-Willcox Co., Inc. Hypertrophy Generally, when bone is subjected to larger (stronger) forces it tends to hypertrophy (high-PER-troh-fee), with increases in density and growth at the sites of force applications (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. 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 osteoblasts to build 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 is deposited in the bone matrix through the release of another hormone, called calcitonin, from the thyroid gland. These two hormones chiefly focus on homeostasis of blood calcium but indirectly affect the density of bones. Bones in Space Maintaining good bone health and strength requires forces that act on the skeletal system to create the right balance between resorption of older, existing bone and formation of new bone. The single most important force acting to help maintain normal bone metabolism is the powerful force of gravity. When astronauts spend time in space, where they are deprived of gravitational force, “Houston, we have a problem!” Scientists studying the effects of space fl ight on humans have long recognized that bone density and total body calcium are diminished during time spent outside of Earth’s gravitational fi eld. Generally, the longer the mission, the greater the amount of bone and calcium lost. This seems to occur due to increased bone resorption combined with largely unchanged bone formation. Bone microarchitecture is also weakened, with disruptions created in the trabecular structure. A number of strategies have been employed to counteract these changes. Research has shown that heavy resistance exercise, coupled with appropriate nutritional and vitamin D intake, can effectively reduce the loss of bone mineral density on long-duration International Space Station missions. While in orbit, every US astronaut is scheduled for an hour of exercise, either treadmill or cycle ergometer, and an hour of resistive exercise (cable ergometer). Remember that lifting weights in space is not exactly exercise, given that the weights are weightless! The exercise and nutritional intake that may be optimal for preventing bone loss are unknown and still under study. It is also unclear whether the bone loss that occurs while in space is fully reversible after time back on Earth. NASA’s Orion program currently projects a manned mission to Mars to occur around the year 2035. Using current technology, robotic missions to Mars take about eight months from launch on Earth to landing on Mars. Today more than 500 people have spent up to a year in space and have maintained remarkably good health. However, for a manned mission to and from Mars to be successful, we will likely need a better understanding of ways to address bone loss, as well as other health-related consequences of long-term space fl ight. Research Notes NASA Astronaut Sunni Williams exercising on a treadmill in the International Space Station.