Fluid Balance

Body Organization

Cells exist in “internal sea” of ECF
Composition – primordial oceans
ECF = Interstitial Fluid + Blood plasma
TBW = ECF + ICF
18% protein,7% mineral,15% fat
60% - WATER

Body Water
60% Total body weight
40% intracellular
20% extra cellular
Extracellular – 25% intravascular
- 75% interstitial

Body Water
70 kg normal adult
TBW = 42 l
ICF = 28 l
ECF = 14 l

Measurements – fluid vol.
Inject subs that stay in one comp.
Vd = amount injected/conc. of subs
Mix evenly through the comp.
No effect on the dist of H20
Must be unchanged
Easy to measure

Plasma Volume
Dyes bound to plasma proteins
Evans blue (t-1824)
Sr. Alb labeled with radioactive iodine
Average value = 3500ml(70kg adult)


Blood vol.
Hematocrit (Hct)- % of blood vol made up of cells
Total blood vol. = plasma volume x 100/100- Hct
Red cell volume = total blood vol.-plasma volume


ECF Volume
Difficult to measure
Limits of the space are ill defined
Transcellular fluids
Inulin
Mannitol and sucrose
20% of body weight

Interstitial fluid vol.
Cannot be measured directly
ECF VOL – Plasma vol.
ECF: ICF - infants & children>adults


ICF VOLUME
Cannot be measured directly
ICF = TBW – ECF
TBW – Deuterium oxide D2O
Tritium oxide and aminopyrine
Water content of lean body tissue – 71 to72 ml/100gm of tissue

Conc. Of Solutes - Units
Mole
Millimoles, micromoles
Molecular weight
Equivalents

Moles
Gram molecular weight of a substance
One mole = 6 x 1023 molecules
SI unit for amount of substance
Mol. Wt – ratio of mass of one molecule of a substance to the mass of 1/12 the mass of an atom of carbon-12
Dalton – unit of mass equal to 1/12 the mass of an atom of carbon-12

Equivalents
1 eq is 1 mol of an ionized substance divided by its valence.
Water
Small, highly diffusible, polar molecule
Solvent – hydrate proteins, sugars and other substances-
Dissolving properties
Thermal properties
Ionizes very slightly (pH 7.0)
Bulk Flow & Diffusion
Bulk flow –movement of the solution mass in the direction of hydrostatic pressure gradient
Diffusion – molecular movement down conc. Gradient
Disperse local accumulation of molecules if it is free to move
Osmotic Pressure
Fluid compartment – principal solute
Mainly restricted to that compartment
K+ ICF
Na+ ECF
Plasma proteins: intra vascular space
Osmotic pressure – total number of particles


Osmosis
Solution – stable mixture of solute & solvent
Solute– dispersed individual molecules
Solvent – liquid bulk
Solute mol. Occupy space
Water mol. Net movement by diffusion.

Osmosis
Water – diffuse from region of high solvent conc. to low conc.
The extent of tendency for solvent to move – measured by use of semi permeable membrane. -hydrostatic pressure to produce equal and opposite bulk flow
Sum of conc. of all solute particles
Oncotic Pressure
Plasma proteins – albumin, globulin
Large mol wt & size –unable to leave intravascular space
Pressure diff between plasma and interstitial fluids
Colloid osmotic pressure
Amounts to only 0.7 mosmol/kg water

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