Cell Volume Regulation

Hello All,

Here are your questions from this lecture.  As always, if you have other or additional questions please do not hesitate to let me know so that we can work through the material together!  Remember, the questions are in bold and the answers in blue.  Some of you asked about practice questions, so I have provided those questions as well in red.

1.  My friend and I asked about how do we know Elizabeth is dehydrated after class today and just have a few follow up questions and need a little bit of clarification.

Elizabeth has ~9L Extracellular Fluid and ~28L with a total body volume of ~37L

Thus, Elizabeth has ~25% Extracellular Fluid and ~75% Intracellular Fluid.  

However, we want Elizabeth to have 1/3 Extracellular Fluid and 2/3 Intracellular Fluid.

Therefore, technically we can go about this either by

1.       Increasing her extracellular fluid and decreasing her intracellular fluid

2.      Or simply just increase her extracellular fluid

I presumed isotonic saline solution is used because Elizabeth was dehydrated, therefore, the total volume would be increased with this method.  

Furthermore, from what I gathered, if the extracellular fluid is below 1/3 of the total body fluid, the person is presumed to be dehydrated?

However, what if Elizabeth wasn’t dehydrated?

Would we still choose an isotonic saline solution over the hypertonic saline solution? 

If so, what is the reasoning behind choosing an isotonic saline solution over the hypertonic saline solution? 

In addition, what condition would Elizabeth have to be in to use the hypertonic saline solution?

Remember, as I mentioned today, this is NOT a typical clinical case/event that would happen.  The numbers were chosen specifically to create easy-math.  Now, however, you were just a bit confused with what I was indicating.  In the question it stated that Elizabeth was 70kg.  Therefore, based on the approximations, her TBW should be approximately 42L with approximately 28L ICF and 14L ECF.  Therefore, since her ECF was measured at 9L that would indicate that it was low.  Therefore, adding in an isotonic saline solution would increase the volume of her ECF without changing the volume (or osmolarity) of the ICF.  

Again, however, this is NOT typical of a clinical situation and in fact if you had low volume in one system, you would almost inevitably have discrepancies in osmolarity and/or volume of both systems.  Therefore, it is not as simple as calculating volumes and picking the type of tonic solution you would give to a patient.  However, giving a patient solutions in general when they are dehydrated is important, whether it be isotonic or hypotonic.

2.  I have a quick question about the last part of the lecture. I understand why if we add a hypertonic solution, our ECFV increases, but shouldn't it decrease if we add a hypotonic solution instead of increase as well? The way I'm thinking about it is: Add hypotonic solution --> (more H2O, less solutes in soln added) so less osmolar ECF, less osmolar ICF. The water will move toward the ICF thereby increasing ICFV and decreasing ECFV (since it's moving from it to ICF)? Am I thinking about this the wrong way?

I believe what you are asking is regarding the VOLUME of the ECF, but it is unclear.  To answer that question, if you add hypotonic solution to the ECF you would indeed decrease the osmolarity of the ECF while initially increasing the volume of the ECF.  That would cause a difference in osmolarity between the ECF and the ICF causing water to move from the ECF towards the ICF, thereby decreasing the osmolarity of the ICF until their osmolarities are equal, at a new lower osmolarity in both.  This would indeed cause SOME volume to move from ECF to the ICF initially, but the end result would be an increase in volume in each and a decrease in osmolarity of each.

3.  I was going over your practice questions on the regulation of cell volume lecture and I had a quick question about number 2.  The question involves adding a small amount of Urea to a system and observing the water movement.  I was wondering as far as the sequence of events is concerned, does water always move before a soluble particle can equilibrate?  Will the water always move first to account for the additional solute and THEN then particles will equilibrate and cause the water to move back? 

2.  If a small amount of urea were added to an isoosmotic saline solution containing cells, what would be the result?

A.  The cells would shrink and remain that way

B.  The cells would first shrink, but then be restored to normal volume after a brief period of time

C.  The cells would swell and remain that way

D.  The cells would first swell, but then be restored to normal volume after a brief period of time

E.  The urea would have no effect, even transiently

That's a great question and it truly depends on where precisely the water and particles are located.  If there are solute particles (such as urea) closest to the membrane they will move immediately, but if not then indeed water would move initially.  This question is really just trying to get you to think about the fact that Urea can move across the plasma membrane and would indeed mess with the water balance across the plasma membrane.

4.  I’m a little confused about the Hematocrit and Blood Volume vs. Plasma volume. Do we need to remember the approximate volumes of each in the body or will we be given that information and just be expected to be able to calculate it? It’s one of the learning objectives so I’m basically trying to understand what aspects of those topics I need to focus on. 

I apologize for not correcting that learning objective as you will now learn more about hematocrit and blood volume within the Heme/Lymph module.  The equation IS included within the notes section for you, but I will not be expecting you to calculate hematocrit or blood volume for the exam.  Remember, the notes section should support what is discussed in lecture, but if it is not discussed in lecture I will not ask you any exam questions regarding that information. 

5.  I hope you're doing well. I am currently reviewing the practice questions that you posted, and I am struggling with question 10. I am unclear as to how to determine the volume of B. I believe the concentration of B is .65 g/L, however I don't understand how to determine the volume and concentration of A to figure out the volume of B? Your guidance would be greatly appreciated.

For the practice question 10, do we need to subtract the 10% lost in urine? The answer is very close to 15L either way.

10.  As part of an experimental study, a volunteer agrees to have 10g of mannitol injected intravenously.  After sufficient time for equilibration, blood is drawn, and the concentration of mannitol in the plasma is found to be 65 mg/100mL.  Urinalysis reveals that 10% of the mannitol had been excreted into the urine during this time period.  What is the approximate extracellular fluid volume for this volunteer?

A.  10 L
B.  15 L
C.  22 L
D.  30 L
E.  42 L

Remember the equation for the indicator dilution method is based upon the idea that mass(outside) = mass(inside), and that mass = volume X concentration.  Therefore, in this question you were given the mass outside (10g) and you were given the concentration inside and asked to calculate the volume inside.  Additionally, 10% of the indicator injected was lost in the urine, so the mass outside is actually 9g, so if you calculate 9000mg/ (65mg/100mL) = ~13486 mL or approximately 14L, so the answer choice that is closest to that is B. 15L.

You are correct, you will get the correct answer (closest to 15L) either by subtracting that 10% or not, so for this question in fact either works.

6.  Are these diagrams the same for the other fluid compartments? (ie if solution was injected into the ICF, IF, PV, or TF) Thank you.  

Remember, when you inject solution into the body (typically intravenously), you are injecting it into the ECF.  That is where solution goes into first, rather than into any other fluid compartment.  Your cells are very small, so specifically injecting into the ICF is not actually feasible, as is the same with the other compartments.  The venous system is part of the plasma volume which is part of the ECF, so that is where solutions start and then water can move between compartments from there (if there is a difference in osmolarities that initiates that move).