Breathing may be primarily driven by the unceasing rotation of residual gravitational ischemia in the brain—'skull-trapped' regions of reduced oxygen tension

The 'top half' of the brain sits on the 'bottom half' of the brain as a weight burden. Within the somewhat spherically shaped cranial vault, gravitational compression of the 'bottom half' of the brain against the inside of the skull, in theory, may generate ischemia, with underlying brain regions experiencing decreased blood flow and decreased oxygen tension. Some questions remain as to whether gravity is involved in the causation of breathing in humans. Is 'gravitational ischemia in the brain' a principle driver of breathing, underlying even the well-studied brainstem neuro-synchronization and automaticity mechanisms of breathing? Relevant reports from the general scientific literature were identified. A focused topical analysis of their intertwined concepts, together with their cumulative aggregate data, was done to explore the current status of this topic, as well as some possible directions for future research. One potentially testable hypothesis suggests that gravity may be a principle underlying driver for breathing, in humans, possibly working through glial cells (astrocytes), scattered throughout the brain, continuously sensing gravitationally-induced regional hypoxia in different regions of the brain, at different times, generated during different head positions, never completely resolved, and leading to a continuous repetitive triggering of the already well-established breathing automaticity mechanisms. Its position within the existing breathing neuro-circuit cascade would be at the very beginning. A second hypothesis explores the inverse of 'gravitational ischemia' in the brain, occurring in astronauts during long-duration spaceflight, and called 'gravity-opposition physiology'. This second hypothesis may potentially provide a framework for understanding Spaceflight Associated Neuro-Ocular Syndrome.