Total Body Water = ECF (Interstitial Fluid + Plasma) + ICF ECF = 20% Body weight (14L in a 70kg man) ICF = 40% Body weight
Measurement of Body Fluid Volumes
Volume of Distribution of drug/compound = amount injected – amount excreted / concentration of substance in sample measurement
Plasma volume =
If one knows the plasma volume and the haematocrit (percentage of blood made up of cells) then the total blood volume can be calculated by :
Plasma volume x 100/100-haematocrit
Red cell volume = volume occupied by all circulating red cells
Radioactive Chromium, Iron and Phosphorous
Extracellular fluid Volume (ECF) = measure using Inulin/Mannitol/Sucrose
(Interstitial fluid volume cannot be measured directly)
Intracellular fluid volume cannot be measured directly: BUT TBW – ECF = ICF
Total Body water is measured using Deuterium Oxide (D20) “heavy water” TBW smaller in women (as fat higher and fat contains no water) TBW decreases with age
A mole is the molecular weight of the substance in grams 1 mole = 6 x 10 power 23 molecules 1 mole NaCL = 23 + 35.5g = 58.5g
pH = negative logarithm of (H+) A buffer = a solution that binds or releases H+ ions e.g. carbonic acid They operate to maintain the ECF at pH 7.40
Diffusion = Fick’s law of Diffusion = net rate of diffusion
Osmosis = diffusion of solvent molecules into a region in which there is a higher concentration of solute to which the membrane is impermeable.
Osmotic pressure (P) = nRT/V
Plasma Osmolality = 2xNa + Urea + Glucose (usually 290)