Limited onboard amenities.(Dispatch from the medical front)

Published at www.cmaj.ca on May 27, when Expedition 20/21 launched.

Since the dawn of human space flight nearly a half-century ago, about 500 people have lived and worked in space.

Each of us have had the opportunity to fly in space at least once. We can attest that space flight is a challenging, exhilarating and fulfilling human endeavour.

But adaptation to space flight is not easy. Since we are earthlings, it takes time for our bodies to adapt to this new environment. Some physiologic systems, such as the cardiovascular and neurovestibular systems, adapt quickly. For instance, we experience a headward shift of blood and extravascular fluid soon after arriving in orbit, which results in "puffy faces," distended jugular veins, headaches and persistent nasal congestion.

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For some of us, the first few days in space can be uncomfortable as we struggle with motion sickness. Once this is resolved, the notions of up and down become irrelevant. We are just as comfortable with our feet on the ceiling as on the floor, or even floating in the middle of the cabin.

In a weightless environment, our intervertebral discs expand and our spinal curvatures are reduced, resulting in a height increase of 4-8 centimetres. That's pretty cool for vertically challenged astronauts! The downside is that many astronauts are bothered by low back pain throughout flight, presumably due to stretching of vertebral ligaments or spinal nerve roots.