Intuition would suggest to most people that the condition of weightlessness, as experienced in outer space, would be rather gentle on the human body. The truth of the matter, however, is that lack of gravity (microgravity) subjects the body to considerable changes and wear-and-tear. This has serious implications for the well-being of astronauts. Many studies are under way aimed at understanding the effects of microgravity on the human system and at devising ways to counteract the debilitating effects.
In the early days of the space programme, the focus was on rockets and other hardware, not on biological research. However, it soon became apparent that the astronauts were developing disturbing symptoms. These included nausea, backaches, space anaemia (drop in red blood cell count by up to a third), and dehydration. On return to Earth, astronauts were weak and prone to fainting. Their muscles were reduced in size and the lower body bones were brittle.
The initial answer to this was to devise sophisticated exercise programmes that the astronauts would perform in space in an effort to maintain muscle tone and reduce bone loss. These exercises were not very successful. NASA now has a comprehensive research project studying the effects of microgravity on the body. This programme is carried out, not in space, but on the ground. Rather conveniently, these studies can be done using volunteer patients who are simply confined to bed for prolonged periods. This works because the adjustments the body makes in coping with lying down are almost identical to the adjustments it makes in adapting to the near-weightless conditions in space. The subjects in the bed-rest studies lie on beds that are tilted towards the head by exactly six degrees off the horizontal.
Your spine is normally curved and gravity-compressed. One of the many things that happen in microgravity is that the spine starts to straighten and stretch. Space-shuttle astronauts grow by several centimetres as the vertebrae in the spine pull apart and, in the process, many of them suffer severe lower back pain. After a few days in space the spine becomes maximally stretched and usually the pain goes away. However, the stretching can damage the spine.
The bed-rest studies have shown that the backache originates in the spinal ligaments. Interestingly, a noticeable lengthening of the spine can be detected even after a single good night's sleep. Some people notice that they must adjust the position of the rear-view mirror in the car each morning in order to compensate for their overnight growth.
Normally, gravity pulls blood towards the lower extremities of the body, away from the head. Certain receptor nerves detect the pressure difference and send out signals to redirect blood back to the brain. In space, pressure gradients disappear and blood moves from the lower extremities to the upper body. Nerve receptors in the head and neck now signal the body to lose fluid and urination increases. Meanwhile, the nerves forget how to respond to gravity. When the astronaut returns to Earth the nerve receptors no longer send out signals to ship blood to the brain to counteract gravity. This causes the brain to power-down and the astronaut faints. Bed-rest studies have shown that astronauts can jolt their circulatory systems back into appropriate action by drinking water and eating salt just before landing.
IT IS well known that astronaut muscles waste and the cardiovascular system loses condition on long space missions. Physical exercise is essential to counteract these effects. Orbiting Russian cosmonauts spend several hours a day working out. Exercise is also important in combating another side-effect of space travel - osteoporosis, or irreversible bone loss. When the load of gravity comes off the bones in space, they start to break down as the body resorbs calcium from the bones. The main load-bearing bones of the legs and hips are the most vulnerable. This phenomenon is also well known in paraplegic victims, who lose 30 per cent of the bone-mass in the lower body within nine months of losing the use of the legs.
The minerals lost from the leg and hip bones are not lost from the body, but, apparently, migrate to bones in the upper skeleton, especially the head. Bed-rest studies have demonstrated that in 30 days the heads of volunteers became 10 per cent denser (physically, not intellectually!). In space, the astronauts' legs are not used much, and most of the work is done with the arms. The body may be compensating for this by moving mineral from an area of inactivity to a more active area.
The only reliable way of building up bone mass is fairly heavy exercise, such as running or weight-lifting. The bones must be loaded in excess of one's body weight. In space it is difficult to devise exercises that will produce adequate loads on bones in the absence of gravity. No doubt exercise machines will be developed in the future that will apply appropriate loads to bones in space. In the meantime it may be necessary on the longer space flights to use drugs that block bone resorption and formation.
The bed-rest studies also have valuable lessons for us about how to live on Earth. The body is a very special type of machine. Most machines gradually wear out with use. The body, on the other hand, grows stronger with appropriate use, and becomes debilitated when under-used. The sedentary lifestyle that most of us have is unhealthy and should be supplemented with regular aerobic and strengthening exercises. Moreover, many of the symptoms that healthy people develop during bed rest are also changes associated with ageing. These include weakened muscles, dizziness on standing, bone loss, general debilitation, and resistance to insulin. These changes are not caused primarily by the process of ageing, but by inactivity. These changes are therefore preventable.
William Reville is a senior lecturer in biochemistry at UCC