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Blood drains from the brain via two primary pathways: the internal jugular veins (IJV) and the vertebral venous plexus (VP) (Gisolf et al., 2004). On Earth, the IJV drainage pathway predominates supine (66% of blood flow), whereas the VP is the main pathway upright (Doepp et al., 2004). In microgravity, the drainage pathway through the IJV is reduced to varying

Purpose of reviewWeightlessness increases both bone loss and kidney stone formation risk. The large interior volume of the International Space Station (ISS) has allowed for a mix of exercise devices to help maintain the skeleton. But space exploration is changing. Long stays on the ISS will be replaced by journeys in smaller spacecraft both to and around the Moon. Small private space stations are under development. This will limit the ability to do exercise countermeasures, which can increase both bone loss and kidney stone risk. This review examines this risk and how it can be minimized in this new era of spaceflight. Recent findingsSimple, low-mass, low-power ways to track bone loss and kidney stone risk in space are being researched. Tracking urinary calcium concentration in the first morning void and targeting additional countermeasures (e.g. bisphosphonates) to those who run consistently high levels is one promising approach. SummaryNew exploration spacecraft would not have the room and capability to replicate the current 2 h, daily exercise countermeasure programme on the ISS. A monitoring approach, perhaps using urinary calcium as a marker, is needed to find those at greatest risk. This would allow countermeasures to be targeted individually and used efficiently.

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