Questions from the Cell Volume Regulation Lecture

Hello All,

Just a heads-up that I will be available most of the weekend via email or on this blog for additional questions that you may have regarding these concepts.  I know that you are all under the pressure of the Mini on Monday, but remember that you did SOMETHING right to get here to RUSM...continue to do those right things and you will succeed as well as possible on this first exam.  If things do not go as well as you hope, re-think your strategies and re-visit your study patterns and try to adjust as needed.  You all have the ability to succeed, so remember that as you sit down to write your first exam!  GOOD LUCK!

I have not received many questions regarding the Autonomic Nervous System, but would be happy to help on those as needed as well!

 
1.  How was the percent body weight calculated for the extracellular fluid and intracellular fluid? Is it 60% body mass X (2/5) for extacellular fluid and 60% body mass X (3/5) of intraceullar fluid? I'm assuming the numbers in the slide were rounded off right?

Although I am unclear of what exactly you are asking in this question I am going to answer what I believe you are asking.  IF body weight is given, 60% of that is TBW, 40% of that is ICF and 20% of that is ECF.  Therefore, for a 70kg person, 42L would be the TBW,  28L would be the ICF and 14L would be the ECF.  Alternatively, if you approximate it from the 42L, then 60% or 25.2L would be the ICF and 40% or 16.8L would be the ICF.  These are exact representations why this value MUST BE an approximation and is a reference point for the values that you ACTUALLY get for a person using the Indicator Dilution Method and can help you clinically determine if a water imbalance may be contributing to why the person is presenting to you as their physician.

2.  I'm trying to tie together the concepts of permeability as well as osmotic pressure.  In your presentation, slide 27, you describe the colloid pressure as being the force that impermeable ions such as albumin have on permeable molecules such as Cl- anions.  But then on slide 34 you cite that saline is an example of an impermeable molecule.  Doesn't saline dissociate into Cl- ions which you described as being permeable?  Isn't what makes the Cl- ions permeable in the first place ion channels because ions can't freely pass through a plasma membrane? Can you clear up the discrepancy between saline being impermeable and Cl- anions being permeable in the case of colloid pressure so that I can better understand tonicity and how it affects only impermeable molecules?

I apologize that this is confusing as I understand it is.  For the sake of water, assume that all ions are impermeable.  For the sake of physiological situations you know that in fact ions are somewhat permeable (due to their movement through ion channels, exchangers, and pumps).  Now, what this should make clear right now is that the entire system is VERY COMPLICATED.  Therefore, we simplify the idea of tonicity to be the molecules that CANNOT cross directly through the plasma membrane (cannot undergo simple diffusion), because these would be molecules that need assistance (that is controlled physiologically, ex. change in voltage) to cross the plasma membrane.  So, for the case of water, assume that they will not cross, but as we build up the entire body we will begin to bring into play the additional measures that includes their semi-permeability under different circumstances.   

Additionally, it has come to my attention that I miss-spoke regarding what colloid osmotic pressure is.  Colloid osmotic pressure is another form of osmotic pressure that can be exerted by certain proteins or impermeable substances (such as albumin). Albumin repels anions permeable to the plasmas' membrane such as Cl- ions, preventing them from entering the plasma itself. Therefore, the plasma [Cl-] is lower than the interstitial [Cl-].  I apologize for apparently saying that albumin attracts the negatively charged Cl-, but it repels it.  This should make sense to you as Cl- and the protein albumin are both negatively charged and therefore would repel each other rather than be attracted to each other.

3. I was working on question 10 of the practice problems in lecture.  and as I calculated the answer, I had gotten around 14L instead of 15.  I was wondering if i was doing it right or just thinking i'm doing it right and rounding.  What i did was I took the conc and converted to .65g/L and then since we started with 10g and ended up with 10% excreted, it will leave us with 9g. so i took 9g and divide by concentration .65g/L to get the L.  

Indeed, you are correct.  It is GREAT that you discovered what many struggle with that 10g (and with the excreted 10% 9g) is a MASS (remember mass = volume X concentration and according to the indicator dilution method, volume1 X concentration1 = volume2 X concentration2) and that you can divide the mass/concentration to determine the volume of the compartment.  15L is indeed an approximation and demonstrates to you how you can utilize approximate numbers for a question of this sort and still get the correct answer. Also, remember that one of the caveats of MCQs is that you must pick one of the answers, so that is the answer closest to the specific number that you calculated.

4. I'm having a little trouble working out the second turning point questions (asking for ICF volume). I realize that you have to use the 9L of ECF from the first question here somewhere, but I'm not too sure how... 

Indeed, you DO need to use the 9L we calculated from the Indicator Dilution Method before.  Given the numbers in the question, you calculate out 37L as the value that the volume that 3H2O measures.  Since we know that 3H2O measures the total body volume, and the question is asking for the volume of the intracellular fluid volume compartment, you need to use a bit of arithmetic to determine what that value is.  You know that the TBW = ECFV + ICFV and you calculated the TBW in this question and the ECFV in the 
first turningpoint question, so now you can subtract the ECFV from the TBW to get the value of the ICFV. 

5. This may sound like a question I should not be asking but how do we go about retreiving the correct answers to the open ended thought questions?

Actually I don't think this is a silly question at all.  However, I am not going to actually give you the answers to these questions because ultimately they are a collection of questions that can be answered directly from your notes or the lecture.  They are geared towards getting you to begin thinking about what may be wondered and asked from the given information.  However, the practice questions are what will actually test if you understand that information that you have now thought about.  If you are having trouble figuring out where to start looking for a given question, please feel free to ask and I can give you guidance.

6. Could you say that water will move out of a compartment until both compartments have equal tonicity? as well as equal osmolarity?

A difference in osmolarity is what drives water movement.  If there is a difference the water will move.  A difference in tonicity, however, is what need to be fully rectified in order for water to quite moving.  If there is simply a difference in osmolarity, but equal tonicity, then the water AND other particles will move simultaneously allowing for equilibrium to be reached without the net movement of any water, while a different in tonicity causes a movement of water in order for equilibrium to be reached.

7. Then for number 11, I just want to clarify that the percents we should use for calculating the different volumes are as follows: TBW is 60% or body mass, ICF is 60% of total body water, and ECF is 40% of total body water?  I used these percents for the question, however I calculated an ICF of 18L and an ECF of 27L.  

PLEASE Remember that the percentages are APPROXIMATIONS.  So, with that information, which answer choice would you choose?  Hopefully you would STILL choose the same answer choice.  Remember that one of the caveats of MCQs is that you have to pick the BEST answer choice available from what is offered to you even if it is not the perfect answer choice that you would have written.

8. I am having a hard time understanding the last portion of the normal state diagram from today's lecture, I do not specifically understand why the osmolarity tends to increase or decrease in ICFV when with a hypertonic/hypotonic saline sol'n. 

The best way to go about these types of questions is to start with changes to the volume and then osmolarity of the ECFV.  If there is a change to the osmolarity of the ECFV, then there would be water movement in order to rectify this difference between the ECFV and the ICFV and therefore to have their osmolarities equal.  Therefore, which way would water move (into or out of ICFV) to bring the osmolarity of ICFV to the same osmolarity as that of the ECFV.  From this approach you should be able to answer what happens to the volume and osmolarity of both compartments.