Measurement of total body water

Measurement of total body water

• Using the dilution method by injecting a substance which diffuses freely in all fluid compartment Substance used :

1. Deuterium oxide (D2O)
2. Tritiated water 3(H2O)
3. antipyrine and amino-antipyrine

• A sample of plasma obtained to measure concentration=Q/C

Measurement of ECF :

• Two types of substance use:

1. Saccharides: eg inulin, sucrose, mannitol
2. diffusable ions: eg chloride, bromide, Na+

Measurement of ICF :

• No substances can diffuse only in ICF
• Total body water – ECF

        Measurement of plasma :

• Substance used:

1. Evans blue; which bind to plasma protein
2. Albumins labeled with radioactive iodine

Measurement of blood :

• Dilution method using red blood cells labeled with radioactive chromium 51(Cr) By use of plasma volume 

Interstitial fluid :

• Interstitial fluid = ECF – plasma volume

Factors affecting body fluids :

1. Osmosis: Movement of solvent molecules into a region in which there is a higher concentration of solute to which the membrane is semi-permeable.   
2. Diffusion: It is an expansion of a gas or substance through a cell membrane, due to continuous random movement of its molecules. Water will follow this molecule into or out of the cell 
3. Gibbs Donnan effect: the presence of nondiffusible anions like proteins and organic phosphate within the cell will affect the distribution of diffusible Anions –ve charged and Cations +ve charged.  [K⁺ _X] [Cl^-_X] = [K⁺_Y ] [Cl^-_Y ]