THE SPACE FLIGHT INVESTIGATION

Recall that the space flight investigation developed by Dr. Alfrey was designed to examine the erythrokinetics of space flight. The fluid shift that we've already discussed in the previous chapter has a major impact on the activities of the RBC. This, of course, makes sense, because RBCs are part of most of the fluid (blood) that shifts toward the upper part of the body. Therefore, not only is there a fluid shift but there is also an RBC shift. In response to these shifts, not only is there an elimination of fluids from the body soon after an astronaut arrives in space, but there is also an elimination of RBCs. We know this because the hematocrit stays about the same in space. That is, the ratio of plasma to RBCs in the blood on Earth is the same as the ratio of plasma to RBCs in space. The question is: Are RBCs eliminated because of a decrease in their production or because of an increase in their destruction or because both occur simultaneously?

Why is this question important? Well, a reduction in production is definitely different than an increase in destruction. Here on Earth, we understand well how the body's oxygen demand can influence the production of RBCs. However, in a healthy person, the destruction of mature RBCs should only occur when RBCs have gotten old or have somehow been damaged. On Earth in a healthy average sized adult, this only happens to about 0.8% of the total RBCs per day. Of course, in almost all cases, these are also replaced everyday. An increased destruction rate would mean that healthy, young RBCs are being destroyed and this is something we have never seen in healthy human beings. Therefore, the question is important because answering it could provide important information about how the RBC system works.

In order to fully characterize RBC activity in space, Dr. Alfrey's team designed a full set of measurements that look at different aspects of RBC production and survival. The preflight, inflight, and postflight portions of this experiment were done on two space missions and the results are very interesting. We will review each major measurement set to see the changes in preflight, inflight, and postflight conditions of the astronauts. For each measurement set, we will include information about why the measurements are important, the equipment and techniques used to make the measurements, the expected results, and the actual results. These measurement sets were designed to study changes in:

(1) plasma volume, RBC mass, total blood volume, and hematocrit;
(2) erythropoietin levels and reticulocyte counts; and
(3) RBC production and survival.

Hypothesis 1

Red blood cell mass is reduced during space flight as a result of a decrease in red blood cell production, which in turn is due to a decrease or inhibition of erythropoietin production.

Hypothesis 2

Red blood cell mass is reduced during space flight as a result of increased destruction of red blood cells.

Just as in the previous chapter, all of the results that you will see involve real experimental data. They were obtained during the course of preparing for, during, and after one or more space flight missions. After you review the scientific result s that are presented in each of the next three sections, you will be asked to break into small groups to work on the development of a presentation that members of your group will deliver to your classmates. Each small group will take one of the three sect ions and develop a plan for how to presenting the information in a clear and concise way. Your teacher will review with you the four main points in the previous chapter (page 97 in the section entitled "Speaking of Space") that you must consider in the de velopment of your presentation.

But first, let's move on to examine Dr. Alfrey's investigation.