Glossary of Cell Water Balance

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Major fluid compartments
1) intracellular
2) extracellular
3) interstitial
4) blood plasma
Major intracellular ion
Major extracellular ion
osmolarity of body fluid compartments
290 mosmoles/L
intracellular fluid compartment
25 L

2/3 of total body water
extracellular fluid compartment
16 L

1/3 of total body water
interstitial fluid compartment
13 L

3/4 of ECF (1/4 of TBW)
blood plasma fluid compartment
1) blood volume = plasma volume + RBC volume (5L) (1/4 of ECF)
2) hematocrit: % of blood volume occupied by RBC
- RBC volume/ blood volume * 100
plama volume/blood volume = 1 - Hct
Barriers separating compartments
1) Cell membrane
2) Capillary Endothelial layer
Cell membrane
- Na-K pump maintains high [K] and low [Na} inside cell
- Na impermeabl to CM without channels, so [Na] determines osmolarity
Capillary Endothelial Layer
- permeable to all molecules but large ones
- albumin = osmotic effect on water
- water forced out by blood pressure
Total Body Water
measured by:
1) 3H20
2) antipyrine
Extracellular Fluid
1) 22Na
2) inulin
3) mannitol
Plasma volume
1) 131I-Albumin
2) Evans blue
Intracellular Volume
Interstitial Volume
ECF - Plasma Volume
water lost in exhalation
350 ml/day
water lost from kidneys
500 ml/day
evaporative water loss
550 ml/day
normal ingestion of water
60-40-20 rule
60% of body weight = water
40% = IC 2/3 of TBW
20% = EC
Volume Contraction
1) Diarrhea
2) dehydration
3) adrenal insufficiency
Volume Expansion
1) Infusion of isotonic NaCl
2) eating salt
isosmotic volume contraction
ex. diarrhea
Increase:Hcrit, Plasma protein
Decrease:ECF volume
No change:osmolarity, ICF volume
Hyperosmotic Volume Contraction
ex. sweating, fever, diabetes I
Increase:osmolarity, plasma protein
Decrease: ECF volume, ICF volume,
No change: Hcrit
Hyposmotic volume contraction
ex. adrenal isuffciency
Increase: ICF volume, Hcrit, plasma protein
Decrease:ECF volume, osmolarity
No change:
Isosmotic volume expansion
ex.infusion of isotonic NaCl
Increase:ECF volume,
Decrease:Hcrit, plasma protein
No change: ICF volume, osmolarity
Hyperosmotic volume expansion
ex. Salt intake
Increase:ECF volume,
Decrease: ICF volume, Hcrit, plasma protein
No change:
Hyposmotic volume expansion
ex. Syndrome of inapproriate ADH
Increase:ECF volume, ICF volume,
Decrease: osmolarity, plasma protein
No change: Hcrit

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