130 Anatomy & Physiology Essentials Copyright Goodheart-Willcox Co., Inc. 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. Because gravity is also a force that continuously acts on bones, heavier people tend to have greater bone mass and density than people who are light- weight for their heights. Bone accounts for only about is deposited in the bone matrix through the release of another hormone, called calcitonin, from the thyroid gland. These two hormones chiefly focus on homeo- stasis of blood calcium but also indirectly affect the density of bones. Hypertrophy Generally, when bone is subjected to stronger forces, it tends to hypertrophy, with increases in density and growth at the sites of force applications (often muscle Effect of Microgravity on Bone 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 subject to extremely low gravitational force, known as microgravity, “Houston, we have a problem!” Scientists studying the effects of space flight on humans have long recognized that bone density and total body calcium are diminished during time spent outside of Earth’s gravitational field. 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 space flights. Astronauts on the International Space Station spend 25% of each working day participating in a complex, multimodal exercise program. Remember that lifting weights in space is not exactly exercise, given that the weights are weightless! The exercise program for US astronauts includes running while tethered to a treadmill and also working out with the advanced Resistive Exercise Device shown in the photo. 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 flight. Research Notes NASA NASA astronaut Dan Burbank, Expedition 30 flight commander, exercises using the advanced Resistive Exercise Device, or aRED, in the Tranquility node of the International Space Station.