Space Physiology
The heart, lungs, and blood vessels function differently in space than they do here on Earth. You already know this by now, but we are going to examine this statement in some detail by focusing on Dr. Blomqvist’s investigation, which has already been carried out in space. We will look at some of the space flight data that has been collected in order to understand the adaptation process that the body undergoes in space. We will then look at how the body readapts to Earth once the astronauts return.Scientists have hypothesized that the cardiovascular system begins to adapt to weightlessness as soon as the blood and other body fluids move from the feet, legs, and lower trunk to the upper body, the upper trunk, and the head. This fluid shift causes the heart to enlarge, at first, to handle the increased blood flow (Figure 13). Even though the body still contains the same total fluid volume at this point, additional fluids have accumulated in the upper body in a way that doesn’t happen of Earth during normal activities. The brain and other body systems interpret this increase in blood and other fluids as a "flood" in the upper body. The body then reacts to correct this situation by getting rid of some of the "excess" body fluid. Astronauts bec ome much less thirsty than normal, and the kidneys, with the help of the endocrine system (which produces and delivers biochemical messengers, hormones, throughout the body), increase the output of urine. Both actions decrease the overall quantity of fluids and electrolytes (ions such as sodium and potassium) in the body, and lead to a reduction in total circulating blood volume. Recall that the heart enlarged somewhat when more blood moved into the upper body. Now, however, once the fluid levels are decreased and the heart does not have to word against gravity, the heart shrinks somewhat in size.
| Figure 13. The fluid shift phenomena of space flight. (a) On Earth, gravity exerts a downward force to keep fluids flowing to the lower body. (b) In space, the fluid tends to redistribute toward the chest and upper body. At this point, the body detects a "flood" in and around the heart. (c) The body rids itself of this perceived "excess" fluid. The body functions with less fluid and the heart becomes smaller. (d) Upon return to Earth, gravity again pulls the fluid downward, but there is not enough fluid to function normally on Earth. |
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Why is it important to understand the changes that occur in the functioning of the cardiovascular system in space? There are a number of answers to that question. First, we want to be certain that astronauts remain healthy in space, and that they will continue to lead normal lives after they return to Earth. That is, we want to know whether the changes that occur in the functioning of the human body in space are reversible. Second, we want the astronauts to be comfortable in space, and we want to be certain that they can perform their important duties on each space flight mission. In fact, we want to do everything possible to ensure the success of the missions that thousands of people have worked so hard to make possible. Third, this type of research has important implications for people on Earth. You yourself may have experienced dizziness or a fainting feeling when you have stood up too quickly. This momentary sensation is the result of the rapid shift of about a pint of body fluid. However, some people suffer from fainting or dizziness spells with no apparent cause. Any research directed at obtaining a basic understanding of how the cardiovascular system works can help medical doctors and their patients cope with many types of cardiovascular problems on Earth.
A comprehensive cardiovascular investigation developed by Dr. Gunnar Blomqvist and his research team was designed to examine the cardiovascular consequences of the fluid shift that occurs during space flight in order to determine how the operation of the cardiovascular system is changed in space when astronauts are at rest and when they are exercising. This investigation measured central venous pressure, arterial pressure, heart rate, and the size of the heart. The investigators also examined cardiac output during exercise and at rest, and leg volume changes at various times during a mission.
Before we begin our examination of the results from Dr. Blomqvist's study, let's review the original hypotheses that served as the foundation for the development of Dr. Blomqvist's space flight study. In addition, we will participate in some hands-on activities (called Student Investigations) that were designed to provide you with some practical experience with important concepts before you are presented with the actual space flight results.
A scientific hypothesis is a simple, direct statement about what the experiment is designed to examine. You have probably learned that a hypothesis is an "educated guess." This is true, and if the experiment is well designed, the results will either agree with the hypothesis (and therefore support it), or the results will disagree with the hypothesis (and therefore refute it). Dr. Blomqvist's investigation is designed to support or refute two simple scientific hypotheses:
Hypothesis 1
Adaptation of the heart and cardiovascular system to microgravity is
rapid and effective. The adaptation is primarily a general response to a
head ward shift of body fluids.
Hypothesis 2
Upon reentry to the Earth's atmosphere and the return to normal gravity,
the cardiovascular problems (dysfunction) experienced by the human body
are a result of the body having adapted to a space-normal condition. This
spacenormal condition is challenged by the sudden return to normal gravity.