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#1
05-09-2012
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 763 Threads: 77 Thanked 1,415 Times in 354 Posts Reputation: 1425
Fluid and Electrolytes Calculation

if a normal 100 kg weight person drinks 4L pure water what will be the new plasma osmolarity,TBW,ECF and ICF?
assume that normal TBW= 60 ICF= 40 ECF= 20.
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#2
05-09-2012
 USMLE Forums Guru Steps History: 1+CK+CS+3 Posts: 457 Threads: 62 Thanked 281 Times in 188 Posts Reputation: 291

I think plasma osm will decrease because water is hypotonic, ECF will increase n ICF will increase too
#3
05-09-2012
 USMLE Forums Addict Steps History: Step 1 Only Posts: 129 Threads: 9 Thanked 54 Times in 36 Posts Reputation: 64

ECF 42.6 lt
ICF 21.4 lt
TBW 64 lt
???
i gotta gut feeling i'm wrong...

#4
05-09-2012
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,355 Threads: 94 Thanked 900 Times in 518 Posts Reputation: 913

water always go from low to high concentration gradient.

If you drink (or inject water+dextrose IV) you have hypotonic solute added to ECF compartment. ECF is now diluted and concentration (osmolality) is lower, so water moves into ICF also.

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Black - normal, red -after drinking pure water
(also look for this one in a weirdo with psychogenic polydipsia)

So I guess the answer to the question will be:

+2L to ICF and +2L to ECF and of course +4L to TBW
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#5
05-09-2012
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 763 Threads: 77 Thanked 1,415 Times in 354 Posts Reputation: 1425
So.

So a person took 4l pure water. this only means that the TBW is increase 60 + 4= 64 L. the new osmolarity is calculated this way. assuming that the water contains no minerals so it has 0 osmolarity, thus the solute mass in compartment, in this case in TBW will remain the same. the normal value for sloute mass is 18 000 mOsm. 12 000 for ICF and 6000 for ECF ( totall 18 000 )

as we all know measuring body fluids is done this way V=Q/C (quantity of indicator / concentration of indicator) i.e.
TBW = 18 000 / Osmolarity -> osmo= 18 000 / 64 = 281,25

osmolarity is same in ECF and ICF. so ECF= 6000/281,25= 42,67. ICF= TBW - ECF= 16 000/281,25= 21,3.

thing here to understand in the Solute mass in compartment. Check this:

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Step 1 - 244 [✔] Step 2 CK - 246 [✔] Step 2 CS [✔] Step 3 - 228 [✔] Match [EMORY SOM] YOG: 2013, 4 Months of USCE University Hospital
 The above post was thanked by: ..sharma (05-09-2012), anvel (10-02-2015)
#6
05-09-2012
 USMLE Forums Master Steps History: 1+CK+CS Posts: 1,039 Threads: 189 Thanked 603 Times in 338 Posts Reputation: 613

Thanks for a nice question beka-CTS

although the answer is already given out I just would like to share how I approached the question.

1) TBW will incease by 4 liters, so 64

2) as for plasma osmolarity I used the same calculations as beta-CTS

3) ICF is 2/3 of TBW so 2/3 of those 4 liters will contribute to ICF increase:
2/3 x 4 = 2,67
so new ICF = 20 + 2,67 = 22,67

4)ECF is 1/3 of TBW so 1/3 of those 4 liters will contribute to ECF increase:
1/3 x 4 = 1,33
so new ECF = 20 + 1,33 = 21,33
 The above post was thanked by: anvel (10-02-2015)
#7
05-09-2012
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 763 Threads: 77 Thanked 1,415 Times in 354 Posts Reputation: 1425

You're correct casandra.
question regarding fluids and you know infusion .... are sometimes obscure. i'll post one more tomorrow and one more day after tommorow.
these questions will clarify the basic and essential concepts
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Step 1 - 244 [✔] Step 2 CK - 246 [✔] Step 2 CS [✔] Step 3 - 228 [✔] Match [EMORY SOM] YOG: 2013, 4 Months of USCE University Hospital
#8
05-09-2012
 USMLE Forums Master Steps History: 1+CK+CS Posts: 1,039 Threads: 189 Thanked 603 Times in 338 Posts Reputation: 613
:)

Quote:
 Originally Posted by beka-CTS You're correct casandra. question regarding fluids and you know infusion .... are sometimes obscure. i'll post one more tomorrow and one more day after tommorow. these questions will clarify the basic and essential concepts
Great! Thank you! We'll nail all the concepts on fluids
#9
05-09-2012
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,355 Threads: 94 Thanked 900 Times in 518 Posts Reputation: 913

Oh crap, how did I forgot about 2/3-1/3 deal...
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#10
05-09-2012
 USMLE Forums Guru Steps History: 1+CK+CS+3 Posts: 457 Threads: 62 Thanked 281 Times in 188 Posts Reputation: 291

Hi I am totally confused n need your help. Why did you add 20 in ICF when the original ICF was 40?? I know my question must sound very stupid but I am kinda jumbled up in here.
Quote:
 Originally Posted by Casandra Thanks for a nice question beka-CTS although the answer is already given out I just would like to share how I approached the question. 1) TBW will incease by 4 liters, so 64 2) as for plasma osmolarity I used the same calculations as beta-CTS 3) ICF is 2/3 of TBW so 2/3 of those 4 liters will contribute to ICF increase: 2/3 x 4 = 2,67 so new ICF = 20 + 2,67 = 22,67 4)ECF is 1/3 of TBW so 1/3 of those 4 liters will contribute to ECF increase: 1/3 x 4 = 1,33 so new ECF = 20 + 1,33 = 21,33
 The above post was thanked by: Casandra (05-09-2012)
#11
05-09-2012
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 763 Threads: 77 Thanked 1,415 Times in 354 Posts Reputation: 1425

Yes ICF is 40 and not 20. you are correct.

The main thing to get here is this formula

Volume of the compartment = Amount of the Substance / Concentration of substance in this volume.

if you really get this formula it is founded on the concept how the system is diluting the substance. in our case initial volume increased from 60 to 64, but subsantial mass is constant so 64= 18 000/ new osmolarity. New osmolarity= 18 000 / 64 = 281,25.

from this value you can easely calculate ICF and ECF.

ECF Volume = subsance mass 6000/ new osm 281,25= 21,33
ICF volume = 64 - 21,33 or 12000/2821,25 = 42,67

In my first explation i mess up with ECF and ICF
__________________
Step 1 - 244 [✔] Step 2 CK - 246 [✔] Step 2 CS [✔] Step 3 - 228 [✔] Match [EMORY SOM] YOG: 2013, 4 Months of USCE University Hospital
 The above post was thanked by: anvel (10-02-2015), kdm0101 (05-09-2012)
#12
05-09-2012
 USMLE Forums Master Steps History: 1+CK+CS Posts: 1,039 Threads: 189 Thanked 603 Times in 338 Posts Reputation: 613
.

Quote:
 Originally Posted by kdm0101 Hi I am totally confused n need your help. Why did you add 20 in ICF when the original ICF was 40?? I know my question must sound very stupid but I am kinda jumbled up in here.
sorry, you're totally right. I mixed it up. so in my explanation it should be:

3) ICF is 2/3 of TBW so 2/3 of those 4 liters will contribute to ICF increase:
2/3 x 4 = 2,67so new ICF = 40 + 2,67 = 42,67

The rest is ok.

Thanks for pointing this out
 The above post was thanked by: anvel (10-02-2015), kdm0101 (05-10-2012)

 Tags Electrolytes-, Physiology-, Step-1-Questions

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