title: Adaptation to microgravity, deconditioning, and countermeasures.
abstract: Humans are generally in standing or sitting positions on Earth during the day. The musculoskeletal system supports these positions and also allows motion. Gravity acting in the longitudinal direction of the body generates a hydrostatic pressure difference and induces footward fluid shift. The vestibular system senses the gravity of the body and reflexively controls the organs. During spaceflight or exposure to microgravity, the load on the musculoskeletal system and hydrostatic pressure difference is diminished. Thus, the skeletal muscle, particularly in the lower limbs, is atrophied, and bone minerals are lost via urinary excretion. In addition, the heart is atrophied, and the plasma volume is decreased, which may induce orthostatic intolerance. Vestibular-related control also declines; in particular, the otolith organs are more susceptible to exposure to microgravity than the semicircular canals. Using an advanced resistive exercise device with administration of bisphosphonate is an effective countermeasure against bone deconditioning. However, atrophy of skeletal muscle and the heart has not been completely prevented. Further ingenuity is needed in designing countermeasures for muscular, cardiovascular, and vestibular dysfunctions.
authors.affiliation: Graduate School of Health and Medicine, Gifu University of Medical Science, 795-1 Nagamine Ichihiraga, Seki, Gifu, 501-3892, Japan. ktanaka@u-gifu-ms.ac.jp.
authors.forename: Kunihiko
authors.identifier: http://orcid.org/0000-0002-9814-6379
authors.initials: K
authors.lastname: Tanaka
authors.affiliation: Department of Physiology, Faculty of Medicine, Aichi Medical School, 1-1 Yazakokarimata, Nagakute, Aichi, 480-1103, Japan.
authors.forename: Naoki
authors.initials: N
authors.lastname: Nishimura
authors.affiliation: Division of Adaptation Physiology, Faculty of Medicine, Tottori University, 86 Nishi-machi, Yonago, Tottori, 683-8503, Japan.
authors.forename: Yasuaki
authors.initials: Y
authors.lastname: Kawai
mesh_terms: D000222:Adaptation, Physiological / Q000502:physiology*; D000818:Animals; D018495:Fluid Shifts / Q000502:physiology; D006801:Humans; D018482:Muscle, Skeletal / Q000502:physiology; D013026:Space Flight; D014893:Weightlessness
keywords: Atrophy; Bisphosphonate; Bone mineral density; Gravity; Hydrostatic pressure; Orthostatic intolerance; Spaceflight