The next small step

The microgravity experienced in space missions has serious effects on human physiology. Kevin Fong looks at the effects of prolonged weightlessness on the human body and our current understanding of the effects of microgravity on physiology

NASA

Astrodynamic considerations and existing propulsion technology limit the speed with which a crew can be delivered to and returned from the surface of Mars. A typical energy efficient mission profile might involve six months of outward bound journey, up to a year and a half of exploration on the planets surface, and a return flight lasting another six months.1 This comes to nearly a thousand days, more than twice the length of any single mission in the history of human space flight and an order of magnitude longer than routine International Space Station operations.

Several hazards await the crews of missions to Mars, including radiation exposure and the psychological stress of spending 30 months in a confined habitat, further from Earth than any human in history, with death no more than a hulls thickness away.

Physiology of microgravity

Prolonged exposure to microgravity seems to affect almost all physiological systems. Disturbances of haemopoiesis, immunosuppression, and endocrine changes have all been seen.2-4 The effects of microgravity that are of key importance to human space operations are those on the musculoskeletal, neurovestibular, and cardiovascular systems.

Effects on the musculoskeletal system

Wolffs law states that bone remodels in direct response to the forces applied. It is therefore perhaps predictable that demineralisation of bone should occur when the mechanical stress is removed and the skeleton is effectively unloaded in the weightlessness environment of space. The rate and extent of this process is considerable, with losses of 1-2% of bone mass per month in flight.5 If unabated over the duration of a mission to Mars, this bone demineralisation, with its resultant hypercalcaemia and hypercalcuria, would leave crews at substantially increased risk of pathological fractures and renal stone formation.

The osteoporosis associated with space flight has been well documented.6 The bone loss seems to be site specific, predominantly in the load bearing regions of the lower limbs and lumbar spine.5 Study data variously implicate reduced bone formation resulting from osteoblastic dysfunction and excessive osteoclastic resorption.78 Both processes are probably involved, but their relative importance and how they are orchestrated remain unclear.

In the absence of gravitational load see box), skeletal muscle alsoatrophies. Reductions in musclevolume and in peak force and velocity of contraction have been observed. The quality and quantity of muscle also change, with phenotypic shifts in muscle fibre type evident from biopsy samples.9 These changes seem to occur in muscle groups associated with load bearing functions. In these groups the intrinsic mechanical and metabolic properties of slow twitch muscle fibres, associated with high oxidative capacity and low fatigueability, seem to alter to resemble those of fast twitch fibres responsible for developing explosive force in activities such as running andjumping.9

The current regimen of countermeasures, which relies on resistive exercise and dietary supplementation, provides some protection but is not uniformly effective in preventing musculoskeletal atrophy.5 However, oral bisphosphanates have recently been found effective in reducing bone losses in healthy subjects deconditioned by 17 weeks of bed rest and will soon be evaluated in spaceflight crews (personal communication, W H Paloski, NASA Human Adaptation and Countermeasures Office).

Effects on the cardiovascular system

Prolonged exposure to microgravity seems to be associated with a prolonged QTc interval on electrocardiograms,10 and limited data from studies with Holter monitors--devices used to monitor an electrocardiogram continuously--suggest an enhanced potential for the generation of abnormal rhythms.11 On returning to Earth, many astronauts have orthostatic intolerance: even after short flights, of nine to 14 days, up to 60-70% of returning crew members are unable to complete a 10 minute stand test without experiencing syncope or presyncope.12 Longer flights are associated witha higher incidence of orthostaticintolerance.

The mechanisms underlying this phenomenon have been well investigated. The headward fluid shifts that result from loss of gravitational loading seem to be misinterpreted by the body as evidence of hypervolaemia and thus lead to endocrine changes that encourage intercompartmental fluid shifts and deplete the intravascular space. Although the resultant hypovolaemia has a key role, other cardiovascular elements also seem to contribute to the postflight orthostatic hypotension. This is shown by the inability of either fluid loading or mineralocorticoid administration to fully ameliorate this postflight phenomenon.1314

Investigations have revealed alterations in total peripheral resistance, vascular reactivity, and sympathetic drive.1516 Volume repletion and use of extrinsic vasopressor agents have reduced some but not all of the symptoms associated with postflight orthostatic intolerance.

Effects on the neurovestibular system

Space flight is associated with disorientation, space motion sickness, and impaired ability to acquire and track visual targets.17-19 The early phases of low earth orbit missions are associated with space motion sickness, and a study of 24 shuttle missions found that this was experienced by nearly 70% of astronauts flying for the first time.20 The symptoms tend to subside after acclimatisation of 24-72 hours, after which the dominant neurovestibular effects are disorientation and impaired visuomotor tracking. On return to Earth, these symptoms resolve but only after a period of readaptation during which performance is markedly impaired.

NASA

The absence of gravitational stimulation of the otolith organ seems to be heavily implicated in the observed neurovestibular effects. This is thought to contribute to sensory conflict and may interfere with central processing tasks associated with visuomotor skills. Over time, the central nervous system is apparently able to adapt by re-weighting sensory inputs--relying more heavily on visual cues than proprioceptive and otolithic inputs--but this adaptation is not complete, as shown by the deficits observed.2122

Postflight decrements in sensorimotor control have been well characterised from both basic science and occupational health perspectives. Early in a flight all crew members experience disrupted postural stability, locomotor coordination, and gaze control. The underlying cause seems to be adaptation of the vestibularsystem to microgravity. As missions get longer, adaptation of the somatosensory and motor control systems starts to be important. The mechanisms of this slower phase of inflight adaptation are not yet well understood, but such understanding may be critical for the success of extended duration missions beyond low Earth orbit. In longer missions the incidence of postflight autonomic dysfunction increases. For example, orthostatic hypotension, which can exacerbate the balance control deficits, may result in part from vestibular autonomic system alterations.

Discussion

Microgravity clearly exerts a profound and widespread effect on human physiology. Some of these changes represent appropriate physiological adaptations and can be thought of as an attempt to achieve new space normal homoeostatic set points. However, this space normal state is clearly not appropriate for Earths gravity and is likely not appropriate for the reduced gravity on Mars, roughly a third that of Earths.

It is said that the two most difficult feats in all of rocket science are starting and stopping. Having survived the violence of take off and a marathon six month flight, the crews of the first expeditions to Mars will be faced with a dangerous landing several hundred million kilometres from Earth. A sensible precaution would be to try to deliver the crew to Mars in an optimal state for the landing and for the ensuing programme of planetary exploration. How this might best be achieved remains a matter of some debate.

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