Figure 7. The human body.

When an astronaut goes into space, his or her body will immediately begin to experience a multitude of changes that cause the astronaut to feel different and even look slightly different! The body is an extraordinary and complicated system that automatically detects and responds to dramatic environmental changes that surround it, particularly the lack of gravity. The entire body is involved in this complex and rapid response to microgravity, and we are just beginning to form a picture of what is happening inside the body. The body is an integrated system with all of its parts in constant communication with each other and interdependent upon one another. When on Earth, the body as a whole system establishes an "Earth-normal" condition; when in space, the body (again, as a whole system) establishes a "space-normal" condition. Both conditions are appropriate for their respective environments. The differences between them are the main topic of the rest of this book. We will be focusing primarily on the body's cardiovascular system (heart and blood vessels), blood, muscles, bones, the sensory and balance system, and the kidneys and fluid control system (Figure 7).

Figure 8. Astronauts floating in microgravity.

Now, imagine that you are on your first ride on the space shuttle. After launch, huge thrusters provide enough power to carry the spacecraft quickly through the clouds and out of the Earth's atmosphere. Welcome to weightlessness! Your feet rise from the floor and you are ready to turn somersaults in the cabin, walk along the walls and "ceiling," and balance bulky objects, even another crew member, on the tip of your finger (Figure 8). You have become an instant acrobat. In weightlessness, there is no natural "up" or "down" determined by our senses. You don't even know the orientation of the various parts of your body at first, particularly your arms and legs, because they have no weight for you to feel and sense where they are. In space, your body becomes confused by the sudden change in what it has learned to expect here on Earth. It will now begin to tell you so.

Figure 9. The vestibular organ.

Upon entry to weightlessness, nearly all astronauts are troubled to some extent by a condition called space motion sickness, which is similar to car or sea sickness. On Earth, your brain has learned how to process the combined signals from your eyes (what you see), your ears (what you hear), and the nerves in your skin (what you touch) to give you information about where your body is in relation to the world around you. In the space environment, the sight, hearing, and touch signals do not match as they do on Earth (mostly because there is no "up" or "down" to relate to, you cannot feel the floor on your feet, and you cannot sit down to feel the chair beneath you), and this sudden input of confusing signals to the brain causes many astronauts to feel sick.

Your brain has also learned how to process information as you move to determine your orientation at any point of your movement. Within the inner ear, there is a balance organ called the vestibular organ (Figure 9), and as you accelerate your body linearly (in a straight line forward or backward, right or left up or down) or angularly (in a rotating fashion), your vestibular organ can "measure," or inform, the brain how fast you are moving and in what direction in relation to gravity. Then, the information from your senses of sight and touch, and information from your muscles and joints, are integrated by your brain to allow you to fully understand your movement. In space, with virtually no gravity, the signals from the vestibular organ combined with what is seen and felt by the other body sensors are all giving conflicting information to your brain about the orientation of your body, at least based on what your brain "expects" from your experience on Earth.

Astronauts suffering from space motion sickness may get headaches, lose their appetite, feel as though there is a "knot" in their stomach, and find it very difficult to work efficiently. Some astronauts actually get very sick and vomit. In space, microgravity is constant, and the brain and the body learn to adapt many of their functions and begin to work together relatively quickly. Fortunately, for most people, the symptoms of space motion sickness seem to last only through the first few days of the mission.

Figure 10. Cosmonauts from the former Soviet Union are carried from their tending site after a long space journey.

Other body functions do not adapt as quickly, and in fact the changes in certain other physiological functions may prove to be lasting and could cause serious problems, especially when astronauts return to the "normal" gravity of Earth. Sometimes it is not apparent when our space shuttle astronauts emerge upon landing, but you can believe that all of them feel shaky as they walk to the podium to receive their welcome home. Most Russian (formerly the Soviet Union) cosmonauts, after spending months in space, are actually carried away from their spacecraft in a specially designed stretcher (Figure 10). This is because their cardiovascular system (cardio = heart, vascular = veins and arteries, all relating to the flow of blood around the body) has chan ged while in the space environment. Their muscles and bones have weakened, and, of course, their balance system is not yet accustomed to gravity. They are generally "out of shape" compared with what is required to function in the constant gravity field of Earth.

Figure 11. An example of "puffy face".

Figure 12. Inflight measurement of the leg circumference to determine the extent of "bird legs" that the astronaut has developed.

Let's examine briefly how the cardiovascular system changed. While in space, the body no longer experiences the downward pull of gravity to distribute the blood and other body fluids to the lower parts of the body, especially the legs. In fact, the blood and fluids make what is called a headward shift (move toward the head), which means that these fluids are redistributed to the upper part of the body and away from the lower extremities. This phenomenon carries with it some interesting effects. While in space, the astronauts even look different. They have a puffy face (Figure 11) because they have more fluid in the upper body, filling the facial cavities that are normally dry, and they have legs that are much smaller in circumference (called "bird legs") because they have less fluid in the lower body (Figure 12).

One way to imagine this headward shift of fluids is to imagine what happens to a vessel, such as a balloon, filled with fluid when it is placed in space (Figure 13). On Earth, the fluid in the balloon will be pulled downward by gravity and will therefore register a weight on the scale. In space, the fluid will redistribute to fill the entire balloon in a more uniform way and will not register a weight. Like the fluid in a balloon in space, the fluid in your body will no longer be pulled down by gravity and it will redistribute to fill the cavities in your head and upper body.

Figure 14. The effects of gravity.

Upon entering the microgravity of space, the body senses an overabundance of fluids (a flood) in the chest and head area. Pulmonary (or lung) and heart sensors that the body depends on for fluid regulation detect this flood and send messages to the kidneys to eliminate the excess fluid that has accumulated in the upper body. In addition, astronauts do not feel thirsty and decrease their drinking far below normal. The result of both decreased drinking and increased fluid elimination (mostly through urination) reduces the body's fluid level below the normal level on Earth (Figure 14). The heart no longer has to work as hard, partly because there is less fluid to pump through the body, but also because it takes less energy to float around a spacecraft than to walk or run around on Earth. Because it no longer has to work so hard, the heart shrinks. In space, we have found that the astronaut can function quite well in this new condition. All evidence indicates that this new condition, or "state," of the body is an appropriate one for space; it is a "space-normal" condition.

Another interesting consequence of leaving the gravity field of Earth is that the astronauts no longer need the full strength of the skeletal and muscular systems for support of their "upright" posture. This is because they do not stand up in space. When the muscles and bones are not used, they deteriorate or "decondition" somewhat. The bones lose calcium and become weaker and muscles atrophy (become weaker and partially waste away). For short space flights of a week or so, these changes are small and pose no real problem, but for longer space flights, they are potential causes of concern. Therefore, it is necessary to exercise vigorously in space to maintain a certain amount of muscle tone and strength so that the astronauts will be strong and healthy when they return to Earth.

Figure 15. In order to exercise on this ergometer (exercise bike), the astronaut must be held down with shoulder pads.

It was a special challenge to develop an exercise system for use in space. If the astronauts were to push off of the "floor" with their feet, they would fly across the spacecraft cabin, stopped only by the opposite wall. Therefore, astronauts use bungee cords, or large elastic "rubber bands," to hold themselves down as they use a treadmill. They also use shoulder pads to hold them down (Figure 15) as they exercise on an exercise bicycle (ergometer). By employing a regular exercise regimen, they can maintain a certain amount of strength in the bones and muscles; however, the level of strength is certainly not equivalent to that which they had on Earth.

What happens when the astronauts do return to Earth? As the shuttle reenters the Earth's atmosphere, the pull of gravity is immediately felt by the astronauts. It quickly becomes clear that the lack of gravity in space has taken its physical toll. The heart is smaller and weaker and has undergone cardiovascular deconditioning relative to the physiological needs on Earth. Body fluids have been diminished. The muscles have atrophied and the bones have weakened. And don't forget that the vestibular, or balance, system has become used to a new set of signals that the brain has learned to understand. It is now time to change from a "space-normal" condition back to an "earth-normal" condition. Can all the changes be reversed once the astronauts are back on Earth? We are just now beginning to answer this question. There are still many critical issues we must face in order to build a space program for tomorrow, when humans will go into space for long periods of time and then return home to lead a normal life.

Various measurements are taken on each shuttle mission in order to gain more information on how the human body changes in space. In addition, a series of shuttle missions is completely dedicated to carrying out life sciences research in space. Successful human exploration of space depends on the health and well-being of the people who travel and work there, and the various experiments that we will discuss in the remainder of this book have been done to investigate and better understand some of the physiological issues we have just discussed. These experiments were designed by scientists and carried out in space by astronauts. An added benefit from much of this research is that it will help us to better understand many ailments suffered by people on Earth, such as hypertension (high blood pressure) and other heart and cardiovascular problems. In addition, bone and muscle research in space can give us added important insights into muscle degeneration and osteoporosis (loss of calcium in the bones), all of which are health problems facing society today.

Each chapter in this book is called a "Focus" because we will be focusing on different parts of the body and on how space flight influences the way the body functions. We will start with Focus 1, where we will discuss how the various space missions have been designed to study the human body. Focuses 2-7 discuss specific body systems individually, describing how each system functions on Earth, how each system changes in the space environment, and how experiments have been carried out to study these changes in detail. We will also look at some of the engineering challenges we face when designing research equipment for use in space. Finally, the results of the experiments will be discussed, which will lead us to wonder what the next set of questions should be to study in the future. A theme that we will always keep in mind as we journey through the various body systems is that the body, as a whole, is a fully integrated system and each subsystem that we examine is only part of the story.

The NASA Life and Biomedical Sciences and Applications Division, the National Institutes of Health, and the scientific community welcome you to the world of life sciences biomedical research done in the most interesting and unique laboratory ever known: outer space!

Figure 16. One day, hopefully in our lifetime, humans will live and work on the space station.