STUDENT INVESTIGATION 4.2Performing a Serial Dilution to Demonstrate the Principles Behind the Measurement of Glomerulor Filtration Rate
The measurement of glomerular filtration rate (GFR) in space is one of the very important measurements to make in order to understand how the body responds to the "upward" fluid shift that occurs in space. In fact, it is one of the major components of Dr. Leach's space flight experiment. The technique that is used to measure GFR is known as the plasma clearance method. The term "plasma clearance" is somewhat self-explanatory; it is used to express the ability of the kidneys to clean, or "clear," the plasma of various substances. This technique is essentially the same as the "dilution method" that we discussed in the previous chapter because it involves the determination of how fast the body is able to eliminate a chemical tracer, or "marker" substance from the bloodstream. The difference between the "plasma clearance method" and the "dilution method" is that the tracer substance that is chosen for the plasma clearance method will be filtered out of the bloodstream and eliminated in the urine. The tracer that is chosen to carry out the dilution method is selected for its ability to stay in the bloodstream and not be eliminated by the kidneys.
Specifically, the plasma clearance method to measure GFR is based on a determination of how quickly a chemical tracer that is injected into the bloodstream of the astronauts is removed from the blood plasma by filtration in the glomeruli of the kidneys. Thus, the method involves determining the rate of loss of the tracer.
The tracer used in this technique must have the following properties:
First of all, you will be using a concentrated acid for this experiment. Therefore, review the laboratory safety instructions with your teacher. Above all, use safety goggles and be aware of the chemical wash facilities (at least the sink) in the event that you splash the acid onto your skin. Secondly, when mixing acid and water, always remember: never add water to acid. This will cause a violent and dangerous reaction. For this experiment, you will only add acid to water.
For this activity, you will be diluting acid/water solutions five different times. As an analogy, think of the water in each test tube as your blood supply and think of the concentrated hydrochloric acid (HCL) as the tracer substance that will be injected into your bloodstream. Each time you remove some of the solution from one of the test tubes, you should imagine this as the process of glomerular filtration where some of the tracer substance is being eliminated from the bloodstream. Finally, you will add a special indicator substance to each of the completed HCL/water solutions so that you are able to analyze the relative differences in HCL concentration of the six solutions. This will give you an idea of how different tracer concentrations determined from blood samples taken from the astronauts will indicate the rate at which the tracer is being eliminated from the system.
Remember, analogies can only serve as a partial representation of what is really happening. It is important to remember that for the actual determination of glomerular filtration rate, the time between blood samples must be carefully recorded. You will be asked to think about why and to respond to a question about this at the end of the experiment. You may want to discuss this with your experiment group.
Finally, since this is a demonstration experiment, there is no need to develop an hypothesis about the results that you expect. There are, however, a few objectives for you to keep in mind that will define what you are supposed to learn from this activity. From your experiment, you should be able to: