Review of Body Fluid Dynamics

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Review of Body Fluid Dynamics H. Lawrence
McCrorey
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Balance Concept zero
balance input output positive
balance () input exceeds output (gain)
negative balance (-) output exceeds input (loss)
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24-hour balance sheet (ml of
water) INTAKE OUTPUT drink
1200 urine 1500 preformed 1000
insensible 900 metabolic 300
feces 100 2500 2500
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DISTRIBUTION OF FLUIDS
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Substances used as markers for body fluid
compartments
Body fluid compartment Marker
Total body water Tritiated water
Extracellular fluid Inulin,
mannitol Plasma 125 I-albumin,
Evans blue
No marker exists for the intracellular fluid
compartment its volume is calculated as the
difference between the total body water and
extracellular fluid volume.
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Composition of Body Fluids Units
of Measure millimol (mM) milliequivalent (m
Eq) milliosmol (mOsm)
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Approximate ionic composition of the body water
compartments interstitial
Skeletal Plasma Plasma water
fluid muscle cell Ion
mEq/L mEq/L mEq/L
mEq/L Cations Na 142 152.7 145.1
12.0 K 4.3
4.6 4.4 140 Ca2 (ionized)
2.5 2.7 2.4
4.0 Mg2 (ionized) 1.1
1.2 1.1 34
Total 149.9 161.2 153
190 Anions Cl- 104 111.9
117.4 4 HCO3- 24 25.8
27.1 12 HPOv, H2POv- 2
2.2 2.3 40 Proteins 14
15 0 50 Other
5.9 6.3 6.2
84 Total 149.9 161.2 153
190
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Composition of Body Fluids
note
a. In EC, sodium conc. high, potassium low
b. plasma and IS essentially similar
except for relative absence of protein in
IS. (small difference due to Donnan effect)
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Composition of Body Fluids note
c. difference between concentrations in
plasma vs. plasma water
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• Consequence of the difference (an example)
• imagine lab findings if plasma sodium 70
  mEq/l.
• Suppose patient has hyperlipidemia and
  
• hyperproteinemia, such that these
  occupy 50 of
• plasma volume, i.e. plasma only 50
  water.
• what is the sodium concentration in plasma
  water?

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140 mEq/l, i.e. 70/500 ml 140/l
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(Na) (Cl) (HCO3) 10 normal
gap 9-16
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Review of Osmosis and Osmotic Pressure
Osmosis measurable tendency of solvent
(water) to move across semi-permeable
membrane in response to difference in
concentration of particles on two sides.
Osmotic pressure a colligative property of a
solution, determined by the number of
solute particles per unit of solution.
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Unit is the osmol

• a 1 osmolal solution (1 osm/kg water)
  decreases activity of water by 22.4
  atmospheres (17,024 mmHg)
• Thus, 1 mOsm decreases by 17 mmHg.

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Osmolality of normal plasma ? 285
mOsm/Kg water Thus, POSM 285 x 17 ??
5,000 mmHg
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although any solution has an osmotic pressure,
in order to see the effects of that pressure
there must be a hindrance to the movement of
solute.
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Osmolality vs. Tonicity
Osmolality concentration of particles per
unit of solution
Tonicity refers to whether or not there is a
net movement of water (hence, it depends
upon the particular membrane involved).
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• Two solutions having the same osmolality
  (exerting the same osmotic pressure) are said to
  be isosmotic

• when two solutions are separated by a membrane
  and there is no net transfer of water, the
  solutions are said to be isotonic

THESE ARE NOT THE SAME!
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PLASMA - INTERSTITIAL EXCHANGE
Four STARLING forces

• favoring filtration
• hydrostatic (blood) pressure
• COP of interstitial fluid

opposing filtration (favoring
reabsorption) COP of plasma
tissue pressure
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EXTRACEULLAR - INTRACELLULAR
EXCHANGE

• All exchanges of water and solutes with the
  external environment occur through ECF. Changes
  in ICF are secondary to fluid shifts between the
  ECF and ICF. Fluid shifts only occur if the
  perturbation of the ECF alters its osmolality.

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• Except for brief periods of seconds to
  minutes, ICF and ECF are in osmotic equilibrium.
  Thus, a measure of plasma osmolality will provide
  a measure of both ECF and ICF osmolality.

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• For sake of simplification, it can be assumed
  that equilibrium between ICF and ECF occurs only
  by movement of water, and not by movement of
  osmotically active solutes.

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SIX QUALITATIVE ALTERATIONS
1. Hypotonic Expansion ex. excess water
intake 2. Hypotonic Contraction ex.
sodium loss from adrenal insufficiency
3. Isotonic Expansion ex. IV drip of 0.9
NaCl 4. Isotonic Contraction ex.
hemorrhage, burns 5. Hypertonic Expansion
ex. sea water drowning 6. Hypertonic
Contraction ex. severe sweating, fever
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Glucose BUN
POSM 2(Na)

18 2.8
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Since urea not an effective osmol
glucose 18
Effective POSM 2(Na)
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At normal plasma glucose levels, glucose accounts
for only about 5 mOsm/l Thus, effective POSM ?
2 (Na)
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Regulation