I. Renal Function Measurements: Glomerular Filtration Rate (GFR) and Renal Plasma Flow (RPF)Three of the biggest jobs that the kidneys have are: (1) to cleanse the blood, (2) to regulate and maintain an appropriate fluid and chemical balance in the body, and (3) to produce the urine. Each of these functions is closely related to the other two, not only because each involves the removal or addition of fluid and chemicals from the blood, but also because each of these functions takes place in the kidney's nephrons. The starting point in the nephron for each of these functions is the glomerulus. It is the "gateway" that the blood must pass through in order to be cleansed by the kidneys.
The glomerular filtration rate (GFR) and the renal plasma flow (RPF) are both rate measurements that help scientists and physicians characterize the status of renal function. When looked at together, they provide information about how fast the kidney can process the blood through its various structures and, therefore, can indicate how efficiently the kidneys operate. What about space flight? Does the renal system respond to the fluid shies that occur in microgravity by participating in the increased elimination of fluids? In particular, does GFR increase to eliminate the "flood" of fluids that the body detects in the "upper" part of the body? And is this flood reflected as an increase in RPF? Dr. Leach measured both of these variables in astronauts before, during, and after a space flight mission, and we will look at those results in a moment. First, let's see how GFR and RPF were measured.
GFR, remember, is the rate at which water and dissolved substances are filtered out of the blood and into the kidneys. Recall that as the blood flows through the glomerulus, the water and the various salts and other dissolved substances flow through the glomerular-capsular membrane and into Bowman's capsule. A "normal" GFR in an average adult who is at rest is about 125 ml/min. This is not how much blood passes through the glomerulus each minute, but instead, it is how much filtrate is removed from the blood each minute.
RPF is the rate at which plasma flows through the kidney. Of course, we learned in the last chapter that blood consists of about 55% plasma and about 45% cellular components (mostly RBCs). By estimating the rate of plasma flow through the kidneys, one can obtain an estimate of the rate of total blood flow through the kidneys. You might ask why the plasma flow rate is measured and not the blood flow rate. Well, the reason for this lies in the technique that is used to measure the plasma flow rate. We will discuss this in detail in a moment, but suffice it to say that the plasma flow rate is measured because it is only the plasma that carries the tracer substance. And it is the plasma that is filtered out of the glomerulus and into the nephron, not the blood cells. Therefore, the plasma delivers the tracer substance to the kidney, where it will be washed out of the system. It is the rate of elimination of the tracer from the plasma and into the urine that will provide the scientist with the information needed to calculate how fast the blood is flowing through the kidney. The normal rate of plasma flow through both kidneys of a 70 kg human on Earth is about 650 ml/min, and the normal rate of blood flow is about 1200 ml/min. Plasma makes up about 55% of the total blood volume and the plasma flow rate is about 55% of the total blood flow rate. This is no coincidence!
GFR and RPF are both measured using a technique known as the plasma clearance method. As stated earlier, this technique is based on a determination of how quickly a chemical tracer is removed from the blood plasma by the action of the kidneys. In other words, the plasma clearance is the rate of elimination of the tracer from the plasma. For the determination of both GFR and RPF, non-radioactive tracers are used. A product known as Inutest (a derivative of the polysaccharide inulin) is used as the tracer for the determination of GFR because it meets the criteria found in Table 3.
Table 3. Criteria used for the secretion of Inutest as a tracer for the determination of glomerular filtration rate.
In summary, Inutest mixes evenly in the blood and is only removed by glomerular filtration. The glomerular filtrate, then, contains the same concentration of Inutest as does plasma, and as the filtrate flows down the kidney tubules, all the filtered Inutest continues on into the urine while 99% of the filtrate is returned to the blood. As time goes on, the concentration of Inutest in the plasma decreases at a rate that reflects how fast the nephron is able to filter the blood in the glomerulus. Therefore, the plasma clearance per minute of Inutest is equal to the GFR.
As an example (Figure 12), let us assume that it is found by chemical analysis that: (1) there is 0.1 gram of Inutest in each 100 ml of plasma, and (2) that 0.125 grams of Inutest passes into the urine per minute. By dividing 0.1 into 0. 125, one finds that 1.25 100-milliliter portions of glomerular filtrate must be formed each minute in order to deliver to the urine the analyzed quantity of Inutest. In other words, since plasma clearance of Inutest occurs at a rate of 0.125 grams per minute (measured from the urine), this means that 125 ml of glomerular filtrate is formed per minute.
Table 4. Criteria used for the selection of PAH as a tracer for the determination of renal plasma volume.
Both Inutest and PAH are filtered out of the glomerulus and into Bowman's capsule with perfect ease. However, PAH is different from Inutest in that most of the PAH remaining in the plasma after passing through the glomerulus is secreted out of the bloodstream and into the nephron's proximal tubule. Therefore, only about one-tenth (actually about 9%) of the original PAH remains in the plasma by the time the blood leaves the kidneys. As an example, let us assume that it is found by chemical analysis that: (1) 1.0 mg of PAH is present in each 100 ml of plasma and that, (2) after five minutes, 29.25 mg of PAH has passed into the urine (Figure 13). This means that 5.85 mg of PAH has passed into the urine per minute. Consequently, 585 ml of plasma is cleared of PAH each minute. Obviously, if this much plasma is cleared of PAH, at least this much plasma must have passed through the kidneys in this same period of time.
Keep in mind, however, that the 585 ml of plasma just calculated would be only 91 % of the total amount of plasma flowing through the kidneys. In order to be still more accurate, you can correct this figure through a simple calculation to account for the other 9% of the PAH that is still in the blood when it leaves the kidney. Dividing 585 by 0.91 gives a total plasma flow per minute of approximately 650 ml.
Finally, you can calculate total blood flow through the kidney by taking into consideration the hematocrit (remember from the last chapter that a hematocrit is a measurement of the percentage of RBCs in the blood). If a person's hematocrit is 45%, this means that the plasma volume is 55%. We've already calculated plasma flow as 650 ml/min. Therefore, the total blood flow through both kidneys is determined by dividing the plasma flow by the proportion of plasma in the blood, or 650/. 55 = 1182 ml/min. What would be the total blood flow if a person had a hematocrit level of 42%?
For Dr. Leach's experiment, a variety of space flight equipment was utilized, including the Blood Collection and Injection Kit, the Blood Sample Storage Kit, the flight Centrifuge for the processing of blood, and the Urine Monitoring System (UMS). Recall that the Blood Collection and Injection Kit and the Blood Sample Storage Kit contain all of the necessary materials and storage capability for the blood tests that must be done inflight.
The Urine Monitoring System (UMS) shown in Figure 14 provides for the collection, volume measurement, and sampling of urine in microgravity. Measurement of the total volume of urine "voided" by each astronaut is automatically made as the urine enters the system. Each urine sample is then collected and moved to a freezer where it is preserved at-20°C. An automatic water flush capability in the system is provided to reduce cross-contamination from sample to sample.
Using some of the equipment just mentioned, the determination of GFR and RPF was carried out before, during, and after the space flight. First, to establish baseline measurement values, a blood sample and several urine samples were taken from each astronaut before the injection of tracers. Following this, known quantities of Inutest and PAH were injected into a vein in the astronaut's forearm. The exact start and finish times for each injection were recorded. After sufficient time (about 90 minutes) had elapsed for the tracer to mix uniformly in the blood, blood samples were collected at about 15- 20 minute intervals and urine samples were collected whenever the astronaut's felt the need to urinate.
The amount of tracer injected and the actual times of blood sample and urine sample collection were carefully recorded since this was a determination of the elimination rate of the tracer. Following the collection of each sample and after the time was recorded, the blood samples were centrifuged and hematocrit levels were determined for each sample. Then, the blood cells were discarded and the plasma was frozen for later analysis. The volume of the urine samples was measured as they were collected and then the samples were frozen for later analysis.
The results of the GFR measurements are shown in Figure 15. This graph indicates the percent change from preflight baseline measurements. As you can see, GFR was elevated at all times inflight. Once the astronauts returned (R+0 is landing day), it looked like the GFR was trying to return quickly back to preflight levels. The subsequent increase after six days back from space is hard to interpret because of the huge variability in the measurements among the astronauts (indicated by the error bars). Remember, many other hormonal and chemical activities must be considered before any interpretation of the data will make sense.
The results of the renal plasma flow (RPF) are shown in Figure 16. Again, the graph indicates the percent change from preflight baseline measurements. As you can see, RPF was also increased inflight. However, the pattern of increase for RPF was different from that of GFR. In particular, look at the data point in both Figure 15 and Figure 16 for flight day 8 (FD8). While GFR has already begun to decrease, RPF jumps to a much higher level. For which data point does this opposite effect happen again? One can probably conclude from this that, while certain hormones or other chemicals may affect both GFR and RPF, there are probably some different factors that influence each of them. This is very interesting and indicates the complexity of the renal/endocrine system.
Finally, the GFR and RPF values can be used to calculate filtration fraction - that is, the fraction of the plasma that is filtered in the kidney each minute - from the following equation:
If you assume that a normal value for GFR is about 125 ml/min and a normal value for RPF is about 650 ml/min, you can estimate, from the graph, an approximate value for both variables. Remember, the values that you calculate are not real experimental data. An exercise such as this is merely an example of what one can do through graphical interpretation. It is up to your teacher to instruct you how to calculate your approximations of GFR, RPF, and ultimately, filtration fraction. Let's move on to a discussion of the space flight changes in hormone and electrolyte levels.