Microgravity- induced organ and system-level deconditioning: a network physiology perspective.

Microgravity removes the gravitational loading that anchors fluid distribution, mechanotransduction, vestibular calibration, and vascular homeostasis, exposing the human body to one of its most profound physiological challenges. Rather than producing isolated organ-specific changes, it induces a deeply interconnected, multi-system deconditioning response emerging from disrupted inter-organ communication. This narrative mini review synthesizes evidence from human spaceflight missions, ground-based analog models, and cellular studies to characterize organ- and system-level adaptations from a network physiology perspective. Skeletal muscle atrophy begins within days, with strength loss preceding structural loss due to early neural contributions. Bone mineral density falls 1%-2% per month at weight-bearing sites, with incomplete recovery after return to Earth. Cardiac remodeling, plasma volume contraction, and impaired baroreflex gain converge to produce post-flight orthostatic intolerance. Cephalad fluid redistribution elevates intracranial pressure and contributes to Spaceflight-Associated Neuro-Ocular Syndrome. Thymic involution and macrophage dysfunction impair immunity, while gut microbiome dysbiosis generates systemic inflammatory and metabolic consequences via the gut-brain axis. Endocrine shifts-including insulin resistance, cortisol dysrhythmia, and hypercalciuria-constitute an interrelated metabolic syndrome of spaceflight. These changes do not occur in isolation. Myokine-osteokine crosstalk, inflammatory amplification across the muscle-bone-gut axis, and gut-derived metabolites disrupting liver-brain metabolism represent proposed mechanisms of network-level deconditioning propagation. Critically, dynamic inter-organ coupling during spaceflight has not yet been directly measured-a fundamental research gap. Terrestrial Network Physiology studies of intermuscular and cardiorespiratory coupling networks serve as methodological precedents for future in-flight investigations. Any proposed network reconfiguration framework is explicitly hypothetical. Future research must prioritize integrated, sex-specific, network-aware countermeasure design and multi-omics biomarker development for long-duration exploration missions.