Before a scientist dives into an examination of why certain changes occur in the body, he or she must obtain a quantitative understanding of the symptoms of the changes. First of all, symptoms provide evidence that a change that has taken place. The quantitative measurements of those symptoms tell you the magnitude, or how much, of a change has taken place. Therefore, you must know the magnitude of the symptoms before you can begin to understand the source of the symptoms.
For instance, if you fall while skiing down a mountain and you feel extreme pain in your knee, the magnitude of the pain is a symptom that suggests you might have a serious injury. If you feel just a slight amount of pain only when you touch it, this lesser magnitude of pain could suggest that you might have just bruised your knee. And if you do not feel pain at all in your knee, then there is no reason to think that there is anything wrong with it and you can get up and continue skiing. The magnitude of the pain will determine what should be done next to diagnose the source of that pain.
For the investigation of the erythrokinetics of space flight, the first step for Dr. Alfrey's team was to determine the magnitude of the volumetric changes (volume is the amount of space that a substance occupies) of the major blood components, plasma and RBCs, as well as changes in the total volume of blood. Once the magnitude of the changes is understood, then Dr. Alfrey can begin to diagnose the source of the volumetric changes.
It has been known for some time that fluid volumes in the body decrease in microgravity. More specifically, scientists were aware that there are volumetric changes in some of the major blood components. Every astronaut studied after returning from space has been shown to have a decreased red blood cell mass (RBCM), a decreased plasma volume (PV), and, therefore, since
PV + RBCM = TBV,
there is also a decrease in the astronaut's total blood volume (TBV). The condition that results from the decreased RBCM has been termed "space anemia." A major part of Dr. Alfrey's investigation was to measure the magnitude of changes in PV, RBCM, and total blood volume (TBV) in astronauts. Hematocrit measurements were also performed. The unique feature of Dr. Alfrey's study was that, in conjunction with the determination of how much the fluid and RBC volumes decreased in space, he also carried out the most complete set of measurements to determine why the changes took place. In this section, we will only discuss the volumetric (how much) measurements that were carried out.
PV and RBCM were measured using the dilution method (described in Student Investigation 3.2). Remember, the dilution method involves injecting the astronaut with a known quantity and concentration of marker substance (a dye or a labeled molecule) that mixes with the liquid portion of the blood (plasma) and becomes evenly distributed throughout the bloodstream. Once the marker is evenly distributed, a blood sample is taken. By comparing the concentration of the marker that was initially injected with the concentration of the marker in the blood sample, the scientist can determine how much plasma must be in the system to dilute the marker to its new concentration. Using the formula
one can determine the PV and the RBCM.
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Figure 9. Blood samples collected from the astronauts are stored in a carefully packed Blood Sample Kit. The kit has Velcro attachments to secure the blood samples so that they do not float around the cabin of the spacecraft. Blood that will be returned to Earth for analysis is stored in the refrigerator or freezer to preserve it. |
For Dr. Alfrey's determination of PV, astronauts were injected with specially prepared blood protein molecules called albumin. Before injection, the albumin molecules were labeled with a radioactive material, 125-iodine, that attaches to the albumin and emits gamma rays. This radioactive property (the emission of gamma rays) served as the "marker" to distinguish the injected albumin molecules from those that the astronauts already have circulating in their bloodstream. The dose of radioactivity that the astronauts were exposed to was very low and safe.
At this point, you may be wondering what a gamma ray is. Well, in 1905, a thoughtful young man by the name of Albert Einstein came up with a new theory that simplified our understanding of light. Remember, light is a form of electromagnetic (EM) radiation. According to Einstein, light and other forms of EM radiation consist of individual bundles of energy that radiate from a source. These bundles of energy are called photons. The EM radiation (light) that you see shining from the Sun is composed of photons. The EM radiation (light) from a light bulb in your classroom or home is composed of photons. Not all EM radiation (Figure 10) is visible but all EM radiation is composed of photons, and all photons carry various amounts of energy. Gamma rays are very high energy photons that are not visible to the naked eye.
Note: All forms of electromagnetic (EM) radiation consist of photons. You have already been exposed (no pun intended!) to the EM spectrum before in the Introduction section of the book It is shown again to remind you that gamma rays fall in the very high energy range of the spectrum.
When this radioactive albumin is injected into the astronaut, it circulates around the bloodstream until it is mixed fully and distributed evenly with the rest of the blood. It takes about 30 minutes for the albumin to spread through the bloodstream. After this time, a blood sample is taken from the astronaut, and the amount of labeled albumin in that sample is measured by counting the number of gamma rays emitted from the blood sample. The number of gamma ray emissions is directly proportional to the number of radioactive labeled albumin molecules in the sample.
Figure11. A sodium iodide crystal is used to detect gamma rays. The
energy from the gamma ray is transformed by the crystal into a visible
form of energy- light flashes. |
RBCM was also determined using the dilution method. However, the "markers" used in this case were radioactive labeled RBCs. Before the mission, a blood sample was taken from each astronaut and combined with a compound called sodium chromate (Na2CrO4). This compound specifically enters the RBCs in the blood sample where a radioactive isotope of chromium (51 Cr) sticks strongly to the hemoglobin. The blood sample is then spun in a centrifuge so that the plasma separates from the RBCs. The plasma is removed and all that is left are the radioactive 51 Cr labeled RBCs. The labeled RBCs are then injected back into the astronauts where they will circulate and distribute evenly in the bloodstream.
The same blood sample that is taken to determine PV can also be used to determine RBCM. Both the 1 25-iodine and the 51 Cr are gamma ray emitters. However, the two radioactive materials emit gamma rays that are different in energy. Therefore, when the blood sample is placed into the sodium iodide crystal, the gamma rays that are emitted from the 51 Cr are higher energy photons (and therefore brighter) than those that are emitted from the 125-iodine. Therefore, the very bright flashes are counted as 51 Cr gamma ray emissions and the dull flashes are counted as 125-iodine gamma ray emissions.
PV was measured preflight, at 22 hours and about 160 hours inflight, and then again postflight. RBCM was only measured preflight and within two hours postflight, but it is assumed that it decreased in a linear fashion throughout the mission (therefore, a straight line was drawn on the graph in Figure 12 between the preflight and postflight values). Once the PV measurement values and the RBCM values were obtained, Dr. Alfrey's team could easily determine total blood volume (TBV). Remember, the TBV in your system consists primarily of two main parts, the plasma and the RBCs. Therefore, TBV = PV + RBCM. The TBV was calculated by adding PV and RBCM. The data for PV, RBCM, and the calculated TBV are shown in Figure 12.
Now, what does the data indicate? Well, from the results shown in Figure 12, it is clear that early in the mission there was a drop in the PV followed by a decline in RBCM. As the flight continued, some of the early loss in PV was regained, thereby replac ing the TBV lost due to the decrease in RBCM. These changes suggest that the TBV is regulated precisely by changing the volumes of its two components, RBCM and PV.
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| Figure 12. Plasma volume (PV), red blood cell mass (RBCM), and total blood volume (TBV) during space flight are all below preflight levels. |
The final measurement in this set involves the determination of hematocrit. You have already learned that hematocrit is a measure of the volume of RBCs in the body expressed as a percentage. What you may not have known is that there are two different kinds of hematocrit values. Most of the hematocrit determinations that are done routinely are called peripheral venous hematocrit (PVH). This means that blood is taken from the peripheral (or outside surface) veins that are easily accessible. As you know, the peripheral venous hematocrit values are usually about 42 for women and 47 for men. However, the percentage of RBCs in the blood from the surface veins can differ somewhat from the percentage of RBCs in the blood of the veins and arteries that are found deeper in the body. The deeper blood vessels usually have hematocrit values [called total body hematocrit (TBH)] that are lower than the hematocrit values found in the peripheral vessels (PVH). This is partly because there are a large number of capillaries in the deeper portions of the body that are smaller and have a lower percentage of RBCs and a higher percentage of plasma.
It is difficult and dangerous to obtain blood samples from deep within the body. This would require long needles to be penetrated deep into the core of the body. Therefore, an equation is used to calculate TBH:
| TBH = | RBCM x 100 |
| PV + RBCM |
The value in obtaining both TBH and PVH lies in their combination as a ratio (TBH/PVH). The value of that ratio can be compared with what would be considered a normal value. It is well known that, on Earth, the ratio of the TBH to the PVH (TBH/PVH) in a normal individual is .90. Dr. Alfrey's team determined the astronaut's TBH/PVH ratio for astronauts preflight, inflight, and postflight. The data for PVH, TBH, and for the ratio TBH/PVH is shown in Table 4.
From Table 4, you can see only small changes in the PVH. However, the TBH did increase early in the flight. These results illustrate the hazard in relying only upon the PVH as a true indicator of overall RBC volume in the body. The key determination for this set of hematocrit values is the ratio TBH/PVH. As mentioned previously, in healthy human beings here on Earth, a ratio value of .90 is considered normal. As you can see from Table 4, the ratio increases early inflight and then returns essentially to the preflight value.
In this section, we have merely been describing the magnitude of the symptoms that suggest that the erythrokinetics of space flight are different from that on Earth. Whether on Earth or in space, the body obviously works very hard to maintain a certain percentage of RBCs, and when that percentage changes, certain mechanisms are "turned on" or "turned off" to help maintain tight control of the number of RBCs in the body. In the next section, we will examine how the hormone erythropoietin acts to "turn on" or "turn off" the RBC faucet and how that mechanism may act differently in space than it does on Earth. Also, we will examine how the number of reticulocytes (immature RBCs) in the blood indicates the rate of production of RBCs.
| Table 4. Peripheral venous and total body hematocrit data. |
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