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By the time the subject returns to normal gravity, blood vessels have become "deconditioned" for Earth living, losing the ability to push blood to the brain. Without adequate blood supply, the brain shuts down, and the individual faints. Upon returning to Earth after missions of more than a few days, most astronauts become dizzy when standing upright. Sixty percent cannot pass a 10-minute stand test without losing consciousness, a condition known formally as orthostatic intolerance. With stays on the ISS lasting for months, and potential interplanetary travel expected to last two years or longer, ways must be found to compensate. First, however, circulatory mechanisms must be precisely understood in order to develop effective countermeasures. "Gravity pulls blood down to the feet normally. Arteries resist that pull," Delp explains. "In microgravity there's no weight bearing. The body responds to the lack of force by remodeling itself. Look at what happens if a weightlifter stops working out. If a muscle is no longer stressed, it loses mass." In an effort to understand the mechanisms of these cardiovascular adaptations at the cellular and vascular levels, Delp will intensively analyze the postflight tissue of rats flown on the July shuttle flight. His hypothesis is that, in microgravity, blood vessels in rat hind limbs become thinner and weaker and constrict less in response to pressure changes and to chemical signals essential to vascular health. The physiological alterations should be apparent. Because rats react more quickly than humans to space-induced physical change, Columbia's 16-day mission is the human equivalent of several months in microgravity. The rats will be housed in special enclosures that have been used successfully on a number of prior shuttle flights. The crew will make daily health checks and will replenish the water supply as needed. Following landing, the small blood vessels in hind limb skeletal muscles that provide blood-pressure resistance will be analyzed for their responses to chemical signals and pressure changes, and for changes in vessel structure and gene expression. Delp expects the experiment to yield crucial information on the basic physiological responses of individual blood vessels involved in blood flow and pressure regulation. Data derived from the examinations should eventually result in the development of treatments or countermeasures to improve crew health and performance following their return to Earth. The study is also expected to aid the elderly, who can be injured as a result of vascular deterioration. "There are similarities to what happens in microgravity and what happens in old age," Delp points out. "When the elderly go to the emergency room, the reason is likely due to orthostatic intolerance, either directly or indirectly. They can't stay upright, and when they do go down, they injure themselves." The eventual goal, he believes, is to develop devices or procedures for space travel that will pull blood down to the feet so that vessels will experience a rough equivalent of gravity levels at Earth's surface. In the short run, though, he hopes the experiment will provide information that can be used to counter microgravity's effects for astronauts living on the space station, who face longer periods of vascular recuperation after returning from extended station assignments. And for Delp, there is the personal satisfaction of understanding that which was unexplored and unknown.
Above: Delp is testing the hypothesis that blood vessels in rats will become thinner and weaker in actual microgravity, just as they do in simulated microgravity, shown in images B and D. (Images A and C show control blood vessels in normal gravity.) Author: James Schultz
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