|Space Travel 101: Fundamentals of Space Travel|
As a result of microgravity, osteoporosis occurs at a rapid rate. Osteoporosis is a decrease in bone mass, and its occurrence may be due to changes in force and stress that causes bone-forming cells to become less active. Bone eroding cells cause the resulting decrease in mass, and bones become brittle. Full recovery upon returning to Earth may not occur.
Osteoporosis is a disease characterized by an excessive decrease in bone mass that leads to an increased susceptibility to fracture. Although the exact cause of osteoporosis as a result of microgravity is unsure, a closer look at bone anatomy explains possible causes.
Osteoblasts (bone-forming cells) and osteoclasts (bone-eroding cell) are necessary for bone development and bone tissue homeostasis. Since most astronauts are over the age of twenty-five, their bones have already ossified, or hardened. However, the actions of osteoclasts and osteoblasts continually remodel them. Throughout life, osteoclasts resorb (dissolve and assimilate) bone, releasing calcium and phosphate ions into the blood. Osteoblasts then invade the region and deposit new bone tissue. Microgravity causes the rate at which osteoblasts deposit new bone layers to be reduced, while osteoclasts breakdown rate remains the same. One theory is that because the bones do not need to support body weight in microgravity, changes in force and stress cause bone-forming cells to become less active. Bones soon become brittle with the decrease in mass at a rate of approximately 1 percent per month in microgravity. This loss of bone mass causes them to be more likely to break upon returning to Earth, and full recovery is not guaranteed. In addition, the blood's calcium concentration increases slightly as osteoclasts provide calcium while the total body fluid has decreased (see “Puffy-Head Bird-Legs” Syndrome). As kidneys rid the body of excess calcium, they are more susceptible to forming painful kidney stones.