Skeletal Responses to Spaceflight

Abstract

The skeletal system of vertebrate animals, including humans, has been evolving for millions of years under the constant influence of gravity. Gravitational loading and function have determined the shape, size, composition, and strength of bones. If gravitational force is decreased, then changes in the skeleton will occur; however, the extent and duration of these changes are not known. Data from spaceflights suggest that the changes in bone and calcium metabolism begin early in flight and continue for at least 3 months. Major concerns for long-duration spaceflight include (1) decreased skeletal strength, which might complicate return to the earth and human exploration of planets and (2) deficits in the skeletal mineral pool, particularly calcium and phosphorus, which play a critical role in maintaining the function of most cell and organ systems. This chapter discusses gravity and skeletal development, bone composition, and bone regulation and focuses on spaceflight data and ground studies in both man and growing rats. Skeletal adaptation to weightlessness appears to alter primarily the biomineralization process. The consequence of this alteration is a decrease in mass and strength of the weight-bearing bones. Primarily from ground-based studies, the gravity-inducted relocation of bone mineral from one area of the skeleton to another has come to light. The response of the skeletal system to spaceflight is highly complex, with losses and redistribution of bone mineral operating simultaneously to adapt bone architecture and composition to a new environment.