- space medicine : that branch of aviation medicine concerned solely with conditions to be encountered by man in space.
-
microgravity : the minute amount of gravitational
force existing in outer space; it results in a weightless condition and
enhances the likelihood of certain diseases, e.g. osteoporosis
.
Spinal injury patients provide a much better model than those with osteoporosis
for the weightless conditions in space because they lose bone at a similar
rate to astronauts (lose about 2% every month, even with exercise, vs.
2-3% every decade in age-related osteoporosis). In a study, 8 patients
who did not take zoledronate lost 16-18% of their femur bone mass over
a year, while 7 patients using the drug lost only 6%. - deconditioning : a change in cardiovascular function after prolonged periods of weightlessness, probably related to a shift of a quantity of blood from the lower limbs to the thorax, resulting in reflex diuresis and a reduction of blood volume.
- hibernation would help astronauts to cope with the psychological demands of decades-long return journeys to destinations such as Saturn. And because less space and food would be needed on such missions, the spacecraft would be lighter and easier to launch. An injection of D-Ala,D-Leu-enkephalin (DADLE), a substance with opium-like properties, is known to trigger hibernation in ground squirrels during the summer season, when the animals would normally be awake. It also seems to send cultures of human cells to sleep: the cells divide more slowly and their gene activity drops when the molecule is applied. One downside of hibernation is that it leads to loss of muscle strength, a problem that also afflicts patients confined to bed after an operation : such bedridden patients retain more strength if they receive dobutamine, a drug used to boost the strength of heart musclesref, so a similar treatment might work during hibernation. The Madagascan fat-tailed dwarf lemur (Cheirogaleus medius) was revealed in 2004 as the first primate known to hibernateref.
-
See also astronomy and
astrophysics.
Returning astronauts have experienced altered immune function and increased vulnerability to infection during spaceflights dating back to Apollo and Skylab. Lack of immune response in microgravity occurs at the cellular level. Differential gene expression was analyzed to find gravity-dependent genes and pathways. Inhibited induction of 91 genes was found in the simulated freefall environment of the random positioning machine. Altered induction of 10 genes regulated by key signaling pathways was verified using real-time RT-PCR. Impaired induction of early genes regulated primarily by transcription factors NF-kB
,
CREB,
ELK, AP-1,
and STAT
after crosslinking the T-cell receptor contributes to T-cell dysfunction
in altered gravity environments. PKA and PKC are key early regulators in
T-cell activation. Since the majority of the genes were regulated by NF-kB,
CREB, and AP-1, we studied the pathways that regulated these transcription
factors. The PKA pathway was down-regulated in vg. In contrast, PI3-K,
PKC, and its upstream regulator pLAT were not significantly down-regulated
by vectorless gravity. Since NF-kB, AP-1, and
CREB are all regulated by PKA and are transcription factors predicted by
microarray analysis to be involved in the altered gene expression in vectorless
gravity, the data suggest that PKA is a key player in the loss of T-cell
activation in altered gravityref.
Unloading of weight bearing bones as induced by microgravity or immobilization has significant impacts on the calcium and bone metabolism and is the most likely cause for space osteoporosis. During a 4.5 to 6 month stay in space most of the astronauts develop a reduction in bone mineral density in spine, femoral neck, trochanter, and pelvis of 1%-1.6% measured by Dual Energy X-ray Absorption (DEXA). Dependent on the mission length and the individual turnover rates of the astronauts it can even reach individual losses of up to 14% in the femoral neck. Osteoporosis itself is defined as the deterioration of bone tissue leading to enhanced bone fragility and to a consequent increase in fracture risk. Thinking of long-term missions to Mars or interplanetary missions for years, space osteoporosis is one of the major concerns for manned spaceflight. However, decrease in bone density can be initiated differently. It either can be caused by increases in bone formation and bone resorption resulting in a net bone loss, as obtained in fast looser postmenopausal osteoporosis. On the other hand decrease in bone formation and increase in bone resorption also leads to bone losses as obtained in slow looser postmenopausal osteoporosis or in anorexia nervosa patients. Biomarkers of bone turnover measured during several missions indicated that the pattern of space osteoporosis is very similar to the pattern of Anorexia Nervosa patients or slow looser postmenopausal osteoporosis. However, beside unloading, other risk factors for space osteoporosis exist such as stress, nutrition, fluid shifts, dehydration and bone perfusion. Especially nutritional factors may contribute considerably to the development of osteoporosis. From earthbound studies it is known that calcium supplementation in women and men can prevent bone loss of 1% bone per year. Based on these results the calcium intake was studied during several European missions and performed an experiment during the German MIR 97 mission where researchers investigated the effects of high calcium intake (>1000 mg/d) and vitamin D supplementation (650 IU/d) on the calcium and bone metabolism during 21 days in microgravity. In the MIR 97 mission high calcium intake and vitamin D supplementation led to high ionized calcium levels and a marked decrease in calcitriol levels together with decreased bone formation and increased bone resorption markers. Tghe conclusion from the MIR 97 mission is that an adequate calcium intake and vitamin D supplementation during space missions is mandatory but, in contrast to terrestrial conditions, does not efficiently counteract the development of space osteoporosisref.
Astronauts may soon have another weapon in the fight against the muscle-wasting effects of living in space. And it's a surprisingly low-tech one: a cycle-powered centrifuge that creates its own 'gravity'. The contraption, called the Space Cycle, spins to create a force that mimics the pull of gravity. The device consists of a central spindle with a pair of attached harnesses, one of which has pedals that drive the machine's rotation. As the cyclist pedals furiously, the centrifuge's spin throws the cages out and produces a force on the 2 occupants. The person sitting across from the cyclist can then perform exercises such as squats while 'weighted down' by the force of the rotation. The invention could be a simple solution to the problem of maintaining an astronaut's muscle bulk during long periods in space, such as stints on the International Space Station or a mission to Mars. Astronauts currently undergo a rigorous fitness regime when in space, but it is still difficult to mimic the effects of gravity, and long-term space passengers face losing up to 25% of their body muscle. Astronauts risk losing muscle mass and function because their muscles are not bearing enough weight. It is important to find ways to increase load-bearing activity so astronauts can maintain strength. The researchers have also been using the device to investigate the effects of different gravitational forces on muscle development. Participants undertake exercise at various levels of centrifugal force, after which their rates of muscle growth are evaluated. The study is part of a wider programme on space health run by the US National Space Biomedical Research Institute, in Houston, Texas. The European Space Agency's life-science unit, based in Noordwijk, the Netherlands, is developing a similar centrifuge to test in 2006. More ground testing is needed to prove that centrifugation really does help bones and muscle, although its benefits for the circulation system are more clear. Other teams are looking into devices that lower the pressure around an astronaut's lower extremities, helping to suck fluid into the lower limbs and giving the heart much needed exercise in pumping blood around the body. Both kinds of device could potentially benefit the body even if the astronaut doesn't exercise inside. What's important is to trigger the body's systems with short pulses of 'hypergravity'. But for astronauts who are feeling energetic, the Space Cycle can be fitted with a range of exercise gizmos, including a treadmill or even another cycle, so that two spacefarers can pedal their way to fitness together. Actually installing such devices on the International Space Station will require much further study and consideration. The presence of a spinning wheel inside a spacecraft can establish a small torque, which could potentially shunt it off course or put strain on the joints holding it together. Transporting and installing the Space Cycle is also such a complicated job that it may only be considered for future craftref.
Web resources : ESA's Bone Loss study
Web resources : The Astrobiology Web
- National Space Biomedical Research Institute (NSBRI)
- Space Medicine
- Space Physiology
- Musculoskeletal Physiology
- Gravitational Biology
- Advanced Concepts Team at ESA
Copyright © 2001-2005 Daniele Focosi.
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