Approach to, and interpretation of, the results from the Renal Physiology Lab

The renal lab is informative, but we always run into the problem that some individuals simply don’t serve as textbook cases of renal function.  There are a number of potential reasons for this, including what they have drunk and eaten for breakfast and lunch on the day of the laboratory, caffeine consumption, etc., but the basic fact is that you may find the measurements and calculations for some members of your group don’t appear to represent what would reasonably be predicted.  Additionally, some people were not capable of urinating every 30 min, or even more than once during the lab.  Not to worry, you’ve been provided with the results of all 5 groups (a total of  students), so you have data from subjects to examine and to help you find reasonable responses.  The only way we could tighten up the data considerably would be to supervise diets and activities for a day or two before the lab.
 
By now or by the time you will start writing the report you are/will be familiar with the function kidney plays in the homeostatic adjustment of: (1) the osmotic pressures of the body fluids; (2) the volumes of the body fluids; (3) the pH of the body fluids (normal body fluid pH is about 7.4), and; (4) the electrolyte concentrations of the body fluids.  Thus, if any of these parameters is disrupted, the kidneys will function to adjust them back to normal values.
 
Kidney Function – Summary
 

Nephrons are the functional units of your kidneys. There are millions of them.  When blood flows into the kidneys, some of the fluid portion of the blood (e., the blood plasma) is filtered into small tubular structures called nephrons. Larger molecules, such as proteins, are left in the blood, but smaller molecules pass into the nephron with the plasma.

 

The filtered plasma moves through the nephron, and as it does so the nephron uses membrane transport mechanisms to reabsorb molecules that the body needs. That is, molecules that need to be retained in the body are moved back to the blood and the other body fluids.

 

Molecules that are present in excess in the body fluids will not be reabsorbed, or will be reabsorbed only in limited amounts. Molecules that don’t get reabsorbed have no alternative but to end up in the urine, and so will be eliminated from the body.

 

So, needed molecules get reabsorbed at the kidneys, and molecules in excess get dumped in the urine – the end product being homeostasis.

 
Given these basic facts, consider the following.  If you drank 800 ml of water, and you weren’t thirsty, would you expect that in response to this unneeded water for: urine volume and rate of formation to go down, or for the specific gravity of the urine to increase? Your kidneys would be expected to get rid of the excess water, and so bring the volume and electrolyte concentrations of your body fluids back to normal.  Thus you would expect there to be a greater volume of water in the urine, a higher rate of urine formation, and a drop in the specific gravity due to the extra water diluting any electrolytes in the urine. [Since your body fluids were diluted by the excess water, the kidneys would also tend to reabsorb more electrolytes from the filtered plasma in the nephrons, instead of voiding them in the urine, and that would also help to adjust electrolyte concentration back to normal.  However, you really don’t have a way to pick this up readily from the lab data].
 
The adjustments the kidneys make happen pretty quickly.  For most subjects, the response that is seen through urine analysis will peak in about an hour or so, and then decline as the correction is completed.  This varies from person to person, as you will see by comparing different individuals who drank the same solutions, but is the typical situation.
 
How is the data presented:
There are several ways to assemble data. I have assembled the semi-analyzed data in the tabular form under two broad sections- Pre-treatment (T=0) and Post-treatment (average values for T=30-120). The post treatment values are averages of values obtained from the entire 2 hour session after drinking the test solutions. Under each section the values for the four parameters tested (Rate of urine formation, pH, Specific Gravity and amount of NaCl in urine) are provided for each subject in columns (every group has atleast 7 subjects who participated and provided the data) and for each group (A/B/C/D/E) in rows. The last column in each of the two sections provides a mean value from all the subjects in that group for each of the four parameter tested. Feel free to re-assemble the data in any other way, if that helps you look at and interpret it better.
 
Note: You will be expected to draw graphs for each of the four parameters (rate of urine formation, pH, specific gravity and NaCl) and compare the pre-treatment and post treatment values. Based on those graphs write a two-page report about your findings and conclusions.
 
 
How to Use the Data
Again, there are various ways to interpret a large volume of data such as this. You could compare within a group (e.g. changes in mean pH values in group D before and after treatment), or between groups (e.g. changes in pH values between group A and group D before and after treatment), in a subject (e.g. change in values pre-and post-treatment in subject 1 of group A, between individual subjects in the group, so on and so forth. In general, it’s a good idea to compare the mean values for a group since it would reflect a general trend instead of individual variations that might skew interpretation. When you compare data, comment on whether any change you observe is along predictable lines and if not; provide any insights into why the data surprised you. To help you with the data interpretation here are a few basic things to consider:
 

The person who is the control should not be expected to deviate in any significant way from normal values for any of the normal urine parameters, since s/he didn’t drink any of the test solutions. This assumption, of course, is completely out-the-window if the control persons drank two liters of their favorite drink at lunch!  Thus, a good question to ask is whether the control group members appear to be functioning as a representative control.
As with the controls, others in your or other groups may have produced data that don’t seem to represent homeostatic renal function. That’s ok- we are dealing with real physiology here.
In approaching the data ask the following questions:

(a) How would ingestion of solution X have perturbed any of the parameters that are regulated by the kidney, and;
(b) How could the kidney make adjustments that would be reflected in the composition of the urine, and would correct these perturbations?  That is to say, what would be reabsorbed, and what would be dumped in the urine.
 
For example, what if a person who drank the 1000 ml of water at time zero showed consistently low urine volumes and rates of urine formation, and maintained normal to high specific gravities?  Why this sort of response might be observed instead of what is expected?  How about, maybe that person came into the laboratory dehydrated, and so the 1000 ml of water simply took care of that problem, diluting the body fluids to normal values instead of diluting them beyond normal values?
 

What about NaHCO3 (sodium bicarbonate)? It is a basic/alkaline molecule.  It is also part of the buffer system that regulates the pH of the body fluids.  If it’s present in excess in the body fluids, what do you think will happen in the kidneys, and how will that response affect the pH of the urine?  The bicarbonate, to make its taste less obnoxious, is drunk in 300 ml of water.  Could that water volume affect any of the parameters you measured?  Do the data show such a response?

 

NaCl intake should produce an obvious imbalance, and so produce a response in terms of NaCl in the urine, and in terms of specific gravity of the urine. Check out rate of urine formation also, since intake of Na+ can reduce urine volume.

 

You may want to compare responses to a given solution across different subjects in a group. Is there an obvious trend for each solution, accepting the fact that some people probably won’t fit the norm?

 

Keep in mind that the time = 0 min values can be influenced by all sorts of things that happened before students came to lab. Ask yourself whether they are reasonable, and consider any trends in the post treatment values (time = 30-120 min) if the initial values are a bit unusual.

 
The lab reports are due as attachment to email (sent to me- kurian@cua.edu) by Friday April 28.
 
 

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