STUDENT INVESTIGATION 3.3

Background

Each RBC contains about 200 - 300 million hemoglobin molecules, and each hemoglobin molecule has four atoms of iron, which attract four oxygen molecules. (The oxygen is obtained from the lungs and it attaches to the iron so that it can be delivered to the cells and tissues of the body.) RBCs being manufactured in the marrow obtain iron from the plasma. This iron is then incorporated into the hemoglobin. In fact, most of the iron in the body is in the hemoglobin of the RBCs. When RBCs are destroyed, iron is returned to the plasma where it will be used again to form new RBCs. Using a radioactive marker (or tracer) method (using radioactive iron - Fe59), we can determine the rate at which the marrow removes iron from the plasma, and, therefore, determine the rate at which the marrow is producing the RBCs.

The method to measure the amount of radioactive iron in the blood involves using a scintillation counter. The radioactive iron is continually decaying and it gives off gamma rays. The scintillation counter will count the number of gamma rays being given off by the radioactive iron. The number of counts that is registered by the scintillation counter is proportional to the amount of radioactive iron present in a blood sample.

The protocol used in determining the production rate of RBCs is the following:
(You will not be carrying out the actual protocols, but you will carry out a graphing exercise to illustrate the concept.)

1. Three astronauts on a mission are serving as the subjects for this study.
2. Each astronaut is given an injection of a radioactive iron marker (called Fe59). (At this point the iron races to the bone marrow to be absorbed and to be used in the production of the RBCs.)
3. Every half hour, a blood sample is taken from each astronaut to continue over a period of two hours. The samples are collected and analyzed to determine how quickly the Fe59 is taken up by the marrow. (The rate at which the Fe59 disappears from the blood is an indication of how efficiently the bone marrow is working to absorb the Fe59, and, therefore, how efficient the bone marrow is in producing RBCs.)
4. The data is shown in Table 3. Plot this data on a graph that your teacher will provide to you like Figure 8 and then answer the questions that follow.

Table 3. Data reflects the percentage of Fe59 remaining in the plasma for each astronaut. The measurements were made at regular time intervals after injection so the disappearance rate of Fe59 could be determined The Fe59 disappearance rate reflects the rate of absorption of Fe59 by the bone marrow.

Figure 8. Graph of the efficiency of bone marrow uptake of iron from the blood. The area in the triangle represents the normal (Earth value) range.

1. Plot the results of your bone marrow Fe59 absorption study from all three astronauts on a copy of the graph in Figure 8. Do the results suggest a normal, decreased, or increased ability by the marrow to absorb Fe59 in space compared with Earth? What does this suggest about the ability of the bone marrow to produce RBCs?
2. How many molecules of oxygen are carried by an RBC? (Hint: Read the Background section for this Student Investigation).