Body Compartments & Fluid Movement. Fluid & Electrolytes. Wittmers. Sarah Eisenschenk.
11.30.09.
Things in italics are from the BRS Phys book!
I. Overall Fluid balance
A. Water input (intake)
1. ingestion of fluid and food = 2 Liters/day
2. metabolic water = 0.2 Liters/day
C6 H12O6  6O2  6CO2  6H2O
B. Water output (loss)
1. insensible water loss
a. respiratory = 0.4 Liters/day
b. skin = 0.4 Liters/day (doesn’t include sweat, just diffusion)
2. Sweat = 0.1 (range up to 2 Liters/hr & somewhat regulated, but no a reg water loss syst)
3. Feces = 0.1 Liters/day
4. Kidney = 0.5 to 20 Liters/ day (REGULATED!)
II. Body fluid compartments
A. Magnitude: (60-40-20 rule)
Total body water = 60% BW (ie. 42 L of 70 kg man)
Intracellular fluid = 40% BW (28.0 L) or 2/3 of TBW
Extracellular fluid = 20% BW (14.0 L) or 1/3 of TBW
Intersitital fluid = 75% ECF (11.0 L), thus is 1/4th of TWB (3/4 x 1/3)
Plasma volume = 25% ECF (3.0 L), thus is 1/12th of TBW (1/4 x 1/3)
Transcellular fluid = 1-2 Liters
(Note also the cap flow & lymph flow.)
B. Solute concentrations
ICF- major cations are K+ & Mg2+, major anions are prot & organic phosphates.
ECF- major cations are Na+, major anions are Cl- & HCO3Plasma- prots
ISF- little prot
Plasma-ISF memb is very permeable. Difference is only due to charged particles.
Cell is negatively charged inside.
C. Osmosis
1. Definition: movement of water across a membrane due to a solute concentration gradient. (Two components must be present)
If semipermeable= permeable to solvent only.
In other words, water moves down its conc gradient.
Once the P = osm P then there is not movt.
1
2
Impt to note that gives osmotic P in atm. Refer to molar conc of the particles,
in fact can say it is proportional to osm P and forget about RT.
Once piston P is = osmotic P, then Jv=0 (water movt)
Note: 0.6 osmols = 600 milliosmoles
2.
0.3 x 2 particles = 0.6 osmols
Osmosis and a leaky membrane
 theoretical   actual
 
 actual
 reflectioncoefficient
 theoretical
 is unique for each molecule and membrane system
If theoretical osm P>actual osm P than memb is NOT semi-permeable.
So, must define the reflection coef, which gives an idea of osmotic activity for a
molecule of a given memb. : Close to 1 = 0 perm, while < 1=inc perm.
Only talking about solutions here! No membs or cells! However Solution A is usually
cell.
given
Hypertonic solution contains a greater conc of
impermeable solutes than the soln on the other side of
the memb (CELL). (SHRINKS LIKE TOP)
Hypotonic solution contains a lesser conc of
impermeable solutes than the soln on the other side of
the memb (CELL). (SWELLS LIKE BOTTOM).
Tonicity= rxn of cell to the soln.
Osmolarity does not equal tonicity.
inside of a
3
See Sketch below. In this scenario, the rat rbcs are in hypotonic (SWELLS) and hyposmotic solution.
This direct relationship is not always the case. In this example, there are fxnal changes: the rbcs
start as biconcave discs. Once swellsphere. Max volv change of the rbc is at 150 mosm.
Urea is permeable to cell memb, so 300 mosm NaCl
ICF and 300 mosm ECF (isosmotic), but the urea
enters the cell  lysis as water follows. (hypotonic)
300 mosm urea + 300 mosm NaCl is hyperosmotic
compared to 300 mOsm of ICF. But urea crosses
memb, preventing shrinkage of cellisotonic soln.
Gave ethanol IV for methanol poisoning  hemolysis
of rbcs (red urine), this was like giving urea. Could
have added glucose or NaCl (imperm) making the
solution slightly hyperosmotic w/o lysis. Easier
4
solution is to give the guy glass of alcohol.
Note: Look at osmolarity prior to water movt to determine hyper vs hypo, then look at tonicity once particles/water move across memb.
Fluid and osmotic changes with pathology and treatment.
Review of normal state:
70 kg subject (ICF =40%,ECF=20%,TBW=60%)
volume
Con (mosm)
Total sol
ECF
14
300
4,200
ICF
28
300
8,400
TBW
42
300
12,600
AKA. “Isosmotic vol expansion” ECF Vol incs, but no change in osm bc NaCl is imperm, no water shift (so no change in ICF vol).
Plasma prot & hematocrit dec, RBCs do not change size.
Arterial BP incs.
Iv. infusion of 3% NaCl solution
Solution analysis of particle concentration
3% NaCl = 3g/100ml = 30g/L
NaCl = 58 g/mole
molar cconcentration = 30/58= 0.517molar
osmolar concentration = 1.035 osmolar
If you give 2L of the solution this is equivalent to 2.07
moles of particles or 2,070 milliosmoles
Hyperosmotic soln incs ECF vol & Conc. Don’t
Conc osm has to equilibrate btwn compartments, so 333 mosm.
even look at TBW yet. Here can find the
Qualitative answer of how far water went can be found here.
direction water will go (qualitative answer),
14,670/333=44. Hypertonic soln, yet hypertonic. ICF decd (SHRINK)
into the ECF.
Aka “Hyperosmotic volume expansion”. Osmolarity of ECF incs bc osmoles have been added to ECF. Water shifts from ICF
to ECFinc of ICF osm until it equals ECF osmincd ECF Vol & dec ICF Vol.
Plasma prot & hematocrit dec.
~ to administration of urea. Incd ECF, decs conc osmol,
Solutes stay the same bc didn’t add any. Water flows
into cells. (Hypoosmotic)
Conc osm has to equilibrate btwn compartments, so
286 mosm. Know that ICF increased (so did ECF
slightly), so hypotonic (SWELL)
5
AKA “Hyposmotic volume expansion”. Osmolarity of ECF decs. ECF vol incsICF osm decsincd ICF Vol.
Plasma prot conc decs. Hematocrit unchanged (water shifts to RBCs, incg their vol & offsetting the diluting effect of
the gain of ECF vol.)
Problem in dehydration
*Presenting information: Your patient has not been able to drink for the past two days. No vomiting or diarrhea was
reported; however, she has been doing some moderate exercise. Shows signs of sever dehydration.
*Data: body build average, weight 153 lbs. with a plasma osmolarity of 340 mosmoles.
*How much fluid would you have to replace to repair the deficit?
Convert body weight 153 lbs./2.54lbs.kg-1 = 60 kg
volume
Con (mosm)
Total sol
ECF
12
340
4,080
ICF
24
340
8,160
TBW
36
340
12,240
First, find the volume of TBW needed to return to Con of 300 mosom. Manipulate this equation: conc=solutes/Vol.
Next solve for the vol difference. Then solve for the other volumes (ECF & ICF), using vol=solutes/conc.
Calculations
Make the assumption that no salt is lost
What would the total body water have to be to get back to 300 mosmols/liter
Volume = 12240 mosmols/ 300mosmols/L
= 40.8 L
Vol replaced = 40.8-36 = 4.8 L
To return the plasma osmolarity to 300 mosm/liter
Questions:
1. When would a cell swell in a hypertonic or hypotonic solution?
2. In order for osmosis to occur what two components must be present?
3. T/F: If theoretical osm P>actual osm P than memb is NOT semi-permeable.
4. Given the following scenario: A rbc is in a solution of 300 mosm urea + 300 mosm NaCl, would the solution be considered hyper, iso, or hypo osmotic? Hyper, iso, or
hypo tonic?
5.
Which way will the water move in this scenario w/ infusion of 3% NaCl?
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Answers:
1. Hypotonic solution.
2.A semi-permeable membrane & solute concentration gradient.
3. T
4. Hyperosmotic. & isotonic.
5. Into the ECF.