Cell Volume Regulation

Hello All,

Lots of questions this week which is reasonable considering the exam on Monday.  I do want to give you one piece of advice, however, before answering your questions:  BREATH!!!  (You all made it here to RUSM, therefore you ALL have the potential to be AMAZINGLY successful.  Make sure to remember that when you enter the exam.  Take a deep breath, and you will be able to tackle anything that you set your mind to!)

Also, remember that for the thought and study question banks you MUST USE the ACTUAL G-drive on campus to open the question sets otherwise they will NOT WORK.  You CANNOT access them 'online' or the quizzes will not work, so please try to access them on campus from the ACTUAL G-drive in order to access those question banks.

Okay, onto the questions.  First a few left-over questions from Membrane Transport Mechanisms:

1.  I am getting a little confuse between the major difference between facilitated diffusion and simple diffusion that uses a channel.  Both are gated, neither need ATP and both are specific for a substrate.  What is the difference between the two?  The only one I am finding is that the facilitated gets saturated and undergoes a conformational change.  This also gets me a little confused on why channels don't get saturated when they have a gate that is specific?

Great question and I see your confusion.  Indeed they DO both have gates and they DO both allow for the movement through the membrane down their concentration gradient.  The big difference is the saturation and here is why:  facilitated diffusion involves a transporter that is only open to one side of the membrane at a time and shuttles the molecules that it is moving across the membrane.  Channels, on the other hand are either open or closed in that the gate is closed and no molecules can move or it is open and there is a channel straight through the membrane that molecules can move through.  Therefore, no shuttling takes place and the flux of the movement of molecules through the channel is not limited by the kinetics of the transporter as is facilitated diffusion.  I can see how you would think that the channels themselves cannot cross all of the molecules through the membrane in-time (similar to people leaving a stadium through only one door), but under PHYSIOLOGICAL conditions, the concentration gradients and number of channels is not such that it reaches that maximal flux so only facilitated diffusion through transporters (and active transport mechanisms) reach that maximal flux.


Now onto Cell Volume Regulation.  I have grouped some questions that are asking about the same concept together and try to provide an answer to all concerns about that concept (Your questions are in blue and my answers are in red):
 
2.  Would you mind sending an explanation for question #10 from today's practice problems? It was the only one I had trouble with. I am not sure what conversions to make or what the 10% excretion referred to in regards to the calculations.  

Number 10, asking about extracellular fluid volume found in the volunteer having mannitol injected intravenously, I wasn't sure how to calculate this. I was going to use the indicator dilution method, but we're only given the concentration of mannitol in the body (conc. B) and the starting mass of mannitol being injected. Is there a simple solution that I'm just over thinking? Or am I completely missing what I should be finding to complete the calculation?

Here is the question: 

Volunteer gets 10 g mannitol through IV. After sufficient time [ ] was measured to be 65 mg/100 mL. 10% was excreted in urine. What is the ECFV?

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.

65 mg/100mL:  Could you please explain what a number like this really means and how to utilize it in a calculation effectively? I don't want to just have to guess on these types of questions. There must be a small mathmatical concept that I am not putting together to utilize this data effectively.

I think that possibly if you look at this again without 'medschool brain' you will see that this is just a basic fraction such that you can simply put it into the equation as above or move the 100mL to the top of the equation above such that 9000mg * 100 mL / 65 mg = 13486 mL.

 

3. A RBC will shrink the most when placed in which of the following solutions? The answer was 200 mM KCl. Other choices were: 250mM urea, 150 mM NaCl, 300 mM mannitol and 100 mM CaCl2

  

Number 3, asking which solution the RBC would swell most in, I think I'm probably just thinking about this wrong. I chose 200mM KCl, but the answer is 250mM Urea. Is this because the osmotic coefficient for urea is 0? I think maybe I chose the answer for the cell shrinking rather than swelling, but I just wanted to make sure.

I see how 200mM KCl would be tempting, but remember that inorganic salts separate into ions in solution, so 200mM KCl would actually be 400mOsM making it hypertonic to the cells driving the water out of the cells and trying to make them shrink as the water is 'sucked' towards the more particles.  The cells would swell the most, however, in a solution that has LESS particles/volume than the cells themselves and since all of the other answer choices are either 300mOsM or more, 250mM urea (although it will cross the plasma membrane itself and the volume changes will somewhat balance out), will cause the most swelling to the cells.

4.  In general, unless the question tells us urea is permeable are we not supposed to assume it will just pass through the plasma membrane? Or do we first consider how a higher concentration of urea in 1 compartment compares to the urea concentration in the second compartment and THEN consider its complete permeability through the membrane thus leading to an isosmotic solution? 

No.  Urea HAS an osmotic coefficient of 0, that IS a true statement and it is ALWAYS permeable to the plasma membrane unless you are told it is not.  Please do not forget everything that you learned in the first lecture and in your previous lives, small non-polar lipid-soluble substances (including urea) can diffuse directly through the plasma membrane as stated on slide 4 of the membrane transport mechanism lecture.  Therefore, for study question 1, the solutions initially start out isosmotic, but the movement of urea off-sets this leading to the movement of water as well.

 

5.  Is there anyway you can explain the estimation of body fluid volumes using the three methods? I am having trouble with the percentages that are shown based on total body mass, total body water, and ECF Volume. Maybe I am bad at the math, but not sure how to estimate as indicated in the slides. Please let me know if I should come into office hours if that is easier to explain in person or I shouldn't get to caught up with the details of these three slides. Also in reference to that, I do not actually see how we saw that the ECF of Elizabeth was low.   

I understand that the ICFV volume will increase due to the movement of water from the extracellular space to the Intracellular space, but I don't understand why the ECFV volume increases as well. Obviously, when we administer  the saline, it first goes to the Extracellular first, but is that the reason why we are saying that the ECFV increases?

And more simply, if we are saying that there is an increase in ECFV Volume, will there always be an increase in the extracellular volume no matter what (ie: isotonic, hypertonic, hypotonic)?

I do understand how the volumes change in a hypertonic and hypotonic environment, however, I am still trying to understand why in a hypertonic environment, the Osmolarity of ECFV increases. Why doesn't the ECFV decrease since it wants to "donate" water out to the hypertonic environment. I'm not sure if I am understanding the diagram correctly.   

 

At the basic level the concepts important to solving ANY problem regarding to the increase in ECFV or ICFV volume and osmolarity are this:

1.  If there is a difference in osmolarities (one system is hypertonic/hypotonic or hyposmotic/hyperosmotic to the other one) then water will move.

2.  Water will ALWAYS move towards where there are more particles.

3.  Fluids are always ADDED or SUBTRACTED from the extracellular space of the human body.

Therefore, if you add or subtract fluid from the body, the volume of the ECFV will always initially change.  If that change causes a change in the osmolarity of the ECFV, then there will be another change in water between the ICFV and the ECFV to try to bring the osmolarity of the ICFV to be equal to that of the ECFV and the osmolarity will always move in whatever direction it needs to to make these equal again.  It will do that by moving water from one compartment to another.  The diagrams demonstrate the changes in the volume and osmolarity of those two systems and the NEW equilibrium.  Now, ECFV volume may initially increase A LOT and then loose some of that volume to the ICFV so that the end result is still an increase, but not quite as great of an increase as it was to begin with, but it will still be an increase (ex. addition of hypotonic solution).

If you remember those three basic concepts above, you can reason through any question that involves a difference in osmolarities and determine what would happen to the ICFV/ECFV, a cell in a solution, OR two solutions that are separated by a membrane.

6.  In today's lecture, problem #2, how did we know that the ECFV was 9 L. I am confused  as to how we got that. Could you explain this please? 

In regards to the problem with Elizabeth, how did you determine that Elizabeth's ecfv was low? I understand how we figured out that her ecfv was 9L but I missed the connection where that ended up being her problem. What number are you comparing this to?  

Remember that in the FIRST Turningpoint question we calculated that the ECFV for Elizabeth was 9L, therefore we simply used that value again for the SECOND turningpoint question.

Elizabeth was also a 70kg patient, so if you calculate the approximation of what her ECFV should be, it should be somewhere between 12-15L depending on your approximation.  Now, since we measured it using the indicator dilution method and it was 9L, that would seem a bit low.  However, the ICFV was at a normal value based on the same approximation method at 28L, so we needed to add volume to Elizabeth's ECFV.  Again, remember I said this was JUST A HYPOTEHTICAL EXAMPLE because this would not actually happen to a patient, but in this example if we simply added isotonic saline solution (based on the 3 general concepts above), that would add volume to her ECFV, but not effect the osmolarity of either system and therefore not effect the volume or osmolarity of her ICFV.

  

7.  After class today, I asked you if water still moves through the membrane if the solutions are isotonic, but not isoosmotic.  You said yes, the water moves with the solute until they are equal.   I wanted to clarify one thing - do they move in opposite directions?  It seems like that would be the most productive way to equalize concentrations.  For example, two equal amounts of water separated by a membrane, you put urea on the left side which can move through the membrane.  Urea would move to the right, and Water would move from the right to the left.  Is that correct?

Essentially YES! You have caught a great nuance of the complexity of water within a system such as the body...water will always move if there is a difference in osmolarity, but it is never just as simple within the human body as a difference in tonicity of one substance, but rather there are some molecules moving across the membrane (both through the membrane and actually through channels and transporters) and the water constantly moving as well to continue to balance out the osmolarity.

8.  500 ml of 7 g/L Evans blue dye and 500 ml of 90 g/L antipyrine are given to a patient intravenously. Two hours later a blood sample is taken and the concentration of Evans blue dye and antipyrine in the plasma are both 1 g/L. Given that there is no loss of the indicators in their urine, what is the patient's plasma volume?

To measure plasma volume you would use an indicator for that compartment because you can.  Evans blue dye happens to be a specific indicator for that compartment.  Therefore we can use the data presented above for Evans blue dye: 500 mL of 7 g/L was injected and the concentration after equilibration was 1 g/L.  Therefore, (.5L * 7g/L)/1g/L = 3.5L

I know that there are versions of the above question floating around that ask a different volume and some of which have an incorrect indicated answer, but for the above question that would be the calculation.  If you have an alternate version, see if you can reason through how YOU would calculate the answer for that version.

9.  A few of us were talking through the thought questions from the Cell Volume Regulation lecture and came across a question. For #6, we were discussing the possibility of having both a hypoosmotic and isotonic system. However, we suggested the possibility of a reverse process of a hyperosmotic and isotonic system (such as the example with Gibbs-Donnan, the cell, and the Na+/K+ pump), where instead, the extracellular fluid is hypoosmotic to the intracellular, and there may be an ion pump that allows for isotonic balance.

Our question is: is there an actual biological example of system both hypoosmotic and isotonic?

First let me say I am SO HAPPY that you are working together to work through the thought questions.  A. your fellow students really are AMAZING resources for you so I am happy you are utilizing them and B. what a great way to study the material and determine if you have an understanding of the material!

Remember that tonicity measures only the molecules that cannot cross the plasma membrane but osmolarity measures ALL of the molecules (both those that can and those that cannot cross the plasma membrane).  Therefore it would be impossible to have a system where the number of total molecules were less than the number of molecules that could not cross the plasma membrane.  Therefore in fact it is IMPOSSIBLE to have a situation that is both hypoosmotic and isotonic, you can only have a system where the osmolarity is equal to or greater than the tonicity.

10.  Consider a solute found in the Extracellular Space and membrane permeability to this solute is very low. What would be the effect on  intracellular osmolarity if there were loss of Extracellular fluid containing this solute and the fluid lost was hypo-osmotic? 

  Alright I understand that the solute can not get into the intracellular fluid therefore water will move to the higher solute concentration.  Since water is moving to the outside , intracellular osmolarity would decrease, correct?  I am not sure what would occur since it is hypo-osmotic? This means that it is already a lower [] of things in the ICF correct ? 

 

So essentially the above question is saying a loss of hypotonic (because it is hyposmotic and the solute permeability is very low giving it an osmotic coefficient of almost 1) extracellular fluid would cause what to happen?  Remember, a loss of hypotonic solution would leave behind a hypertonic extracellular solution (because you took away more volume and less solutes so left would be more solutes and less volume).  Therefore, the ICFV would sense that and respond by water being pulled toward the more solutes, losing volume from the ICFV (but leaving the solutes behind) to increase the osmolarity of the ICFV while somewhat decreasing the osmolarity of the ECFV until they are both higher than they originally were.