Dr. Alfrey's entire experiment represents the first time that the basic mechanisms responsible for the observed reduction in RBCs has been so thoroughly studied preflight, inflight, and postflight. Up to this point, we have learned a great deal about his experiment. We first examined the quantitative changes in the major blood components resulting from space flight and found that, compared to Earth values,

• PV decreases,
• RBCM decreases, and
• TBV decreases.

• PVH remains about the same,
• TBH initially increases but then stabilizes, and
• the TBH/PVH ratio initially increases but also returns to an Earth-normal value.

• erythropoietin levels in the blood decrease, and
• reticulocyte counts decrease.

Wow! You have learned a great deal about Dr. Alfrey's experiment and his results up to this point. We now have one more level of information to absorb in order to obtain a clearer picture of those mechanisms that may be responsible for the decreased RBCM. This section deals with the issue of RBC production and survival.

Here on Earth,

production rate of RBCs = destruction rate of RBCs

so that a steady state always exists in the number of RBCs in our body. That is, there is always a constant number of RBCs in our bloodstream on Earth. As RBCs become old, they are destroyed by the body to make room for new RBCs that are born to replace the old ones. If destruction of RBCs occurs before they are old, however, then not only is the destruction rate increased but the survival rate of healthy RBCs is reduced. Let's state that idea one more time because you must understand the concept fully in order to appreciate this part of Dr. Alfrey's experiment.

If RBC destruction increases, then RBC survival decreases.

As an analogy, let's think about the consequences of war. It may seem strange to bring the subject of war into a scientific discussion, but it can serve as an example to clarify what we are talking about. During a war, there is an increase in destruction of human life. This means that there is also a decrease in the normal survival time (life expectancy) of those lives that were destroyed. Therefore, as the destruction of human life increases, human survival decreases. Now let's get back to the subject of RBCs.

We already know that, in space, the number of RBCs decreases early in flight. We have also learned that a reduced production level of RBCs is responsible for at least some of that early decrease in RBCs. The main point of this part of Dr. Alfrey's experiment is to document how much the production rate decreases in space and to determine whether the survival rate of RBCs might also be affected by space flight. Survival rate is determined by measuring destruction rate (remember the war analogy?).

In order to study both RBC production and survival, the astronauts were injected with radioactive 51 Cr labeled RBCs 21 days prior to launch and blood samples were taken from the astronauts at various points after the injection. With the blood samples, two types of information could be obtained. First, the rate of RBC destruction was determined in order to understand if RBC survival is affected by space flight. Second, the rate of RBC production was determined. Now, how can survival and production rates be determined using radioactive labeled RBCs? Let's examine this question.

When the radioactive labeled RBCs were injected into the astronaut, then the astronaut essentially had two groups of RBCs circulating in the bloodstream. The two groups (radioactive and normal) of RBCs are identical in certain respects and completely different in other respects (Table 6). First of all, the two groups of RBCs function identically. That is, the radioactive RBCs (which remain radioactive their entire lifetime) carry oxygen to the cells of the body just as the normal RBCs do. Also, the radioactive RBCs circulate normally in the bloodstream and they also live as long as the normal RBCs.

There are two main differences, however, between the two groups of RBCs. New radioactive RBCs are not produced by the body when the old ones die, whereas new normal RBCs are produced. Secondly, the radioactive 51 Cr that is attached to the labeled RBCs emits gamma rays, causing the labeled RBCs to be distinguishable from normal RBCs in a blood sample through the use of a scintillation detector (as discussed in a previous section). The number of gamma rays that are emitted is proportional to the amount of radioactive labeled RBCs in the sample. These differences are the key to why RBC survival and production can be measured.

As both groups of RBCs perform their function together in the body, a small percentage of them die every day. Radioactive RBCs and normal RBCs die at the same rate. Therefore, Dr. Alfrey's team can take blood samples at various intervals after the radioactive RBCs have been injected and watch how fast they disappear. This disappearance rate of the labeled RBCs will tell Dr. Alfrey how rapidly both the labeled and the nonlabeled RBCs are being destroyed. If they are being destroyed in space at the same rate that they are being destroyed on Earth, then normal RBC survival is not in question. If, however, they are being destroyed in space more rapidly than they are on Earth, then the RBC survival rate is decreased in space.

Figure 15. An example of RBC destruction rate.

Once Dr. Alfrey has determined if the RBC destruction rate is affected by space flight, he can use that information to determine how much RBC production rate is affected by space flight. Let's illustrate this idea to make it a little bit easier to understand. Look at Figure 15. The green balls are radioactive RBCs and the white balls are normal RBCs. To begin with, there are equal numbers of green and white balls. As the balls are destroyed at the same rate, only the white balls continue to be replenished. Therefore, the ratio of green balls to white balls continually declines. That is, the green balls are becoming diluted by the continual addition of white balls. By determining how fast the green balls (radioactive RBCs) are being diluted by the white balls (normal RBCs), you can determine how fast the white balls (normal RBCs) are being produced.

Up to this point, we have discussed the basic principles behind the measurements of RBC survival and RBC production. Let's look at each measurement in a bit more detail, along with Dr. Alfrey's results.

Figure 16. Percent changes in the disappearance rate of Cr/gHb (round data points, indicating RBC product ion) and TCrRBC (open square data points, indicating RBC survivals following injection of 5 1Cr labeled RBCs 27 days prior to launch until fund]ing day. The divergence of the two lines indicates a reduced production of RBCs during flight.

A. RBC Survival

To measure the rate of survival of RBCs, the disappearance rate of total RBC 51Cr radioactivity was measured. This is shown as TCrRBC in Figure 16, which is a short way of writing "total RBC chromium." That is,

the disappearance rate of the total RBC chromium (TCrRBC) is proportional to the RBC survival rate.

A major point to understand about this test is that the disappearance of total RBC 51 Cr radioactivity is caused by either early death or removal. Early death would include hemolysis (premature bursting of a healthy RBC) and phagocytosis (the attacking and killing of healthy RBCs by cell-eating phagocytes). The removal of RBCs refers to phlebotomy (taking too many blood samples for tests like these).

This test depends on the measurement of radioactivity in the total amount of labeled RBCs. If RBCs die earlier than they normally do (from hemolysis or phagocytosis), or if too much blood is removed in the process of taking blood samples (phlebotomy), the total number of RBCs will decrease more quickly than normal. Dr. Alfrey can make corrections in his data to account for any loss of blood due to phlebotomy; therefore, the disappearance rate of TCrRBC will indicate if unexpected hemolysis or phagocytosis is taking place. If so, RBC survival will be decreased and if not, RBC survival will be normal.

To determine the survival rate of RBCs, Dr. Alfrey's team took blood samples at various intervals after the initial injection Of 51 Cr labeled RBCs. The same labeled RBCs that were injected into the astronauts to determine RBCM (which we talked about in a previous section) are used to carry out measurements of both RBC survival and production. For each blood sample that is taken, the amount of radioactivity in the sample is determined by using the scintillation detector that we discussed earlier. From doing the original RBCM measurement from a blood sample taken soon after the injection of radioactive RBCs, Dr. Alfrey knows how much radioactivity was in the astronaut's system to begin with. This initial measurement is considered the 100% reference point to compare each subsequent measurement. Therefore, the level of radioactivity in the next blood sample that is taken will have some percentage less than the first measurement. Then the next blood sample that is taken will have some greater percentage less than the first measurement, and so on. If these percentages are plotted on a graph relative to the time that the samples were taken, the slope of the curve connecting the points will indicate the disappearance rate of the labeled RBCs from the bloodstream. This disappearance rate indicates how fast the RBCs are being destroyed and whether RBC survival is normal or reduced.

The open square data points in Figure 16 indicate the rate of survival (TCrRBC) of RBCs for the astronauts in Dr. Alfrey's experiment. As you can see, measurements were taken five times preflight, and a final measurement was taken after the mission. No inflight measurements were taken for this determination; however, it is assumed that the rate of survival continued in a linear fashion from the preflight value to the postflight value. Therefore, a straight line was drawn between those data points.

B. RBC Production

To measure the rate of production of RBCs, the disappearance rate of 51 Cr per gram of hemoglobin in the blood sample is determined. This is shown as Cr/gHb in Figure 16,.which is a short way of writing "chromium per gram of hemoglobin." That is,

the disappearance rate of chromium per gram of hemoglobin (Cr/gHb) is proportional to the RBC production rate.

The main point to understand about this test is that as new unlabeled RBCs are produced in the bone marrow, they are released into the bloodstream. The release of new unlabeled cells dilutes the circulating labeled RBCs. Over time, the ratio of labeled RBCs to unlabeled RBCs decreases. The rate at which the labeled RBCs become diluted with unlabeled RBCs is proportional to the rate of new RBC production.

This measurement is not affected by the destruction of RBCs or by the removal of RBCs when blood samples are taken. That is because this measurement does not depend on the actual amount of RBCs in the system (as the RBC survival measurement does); it only depends on the relative ratio of unlabeled RBCs to labeled RBCs. RBC production can be determined, then, by looking at relative changes in the ratio of radioactive RBCs to normal RBCs. The change in this ratio can be determined by looking at how fast the radioactive RBCs become diluted by the production of normal RBCs.

The round data points in Figure 16 indicate the rate of production (Cr/gHb) of RBCs for the astronauts in Dr. Alfrey's experiment. As you can see, measurements were taken five times preflight, six times inflight, and a final measurement was taken after the mission.

Now, we have covered RBC survival and production separately up to this point and two main questions have emerged:

1. Does the survival rate of RBCs in space differ from the survival rate of RBCs on Earth?
2. Does the production rate of RBCs in space differ from the production rate of RBCs on Earth?

We can now answer these questions. The preflight portion of Figure 16 indicates both the production and survival rates on Earth. As you can see, both rates are identical until the astronauts enter the space environment. At that point, the rate of production of RBCs slows down (compared to the rate of production on Earth) while the survival rate continues normally. The divergence of the two lines indicates a reduced production of RBCs during the flight. From this data, Dr. Alfrey concludes that the decrease in production together with normal age-related death accounts for the observed decrease in RBCM.