Body Fluids and Fluid Compartments Notes

Body Fluid Compartment And Its Measurement

Volume of Body Fluid Compartment

Indicator dilution principle

It is based on principle of conservation of mass i.e. Total mass of substance after dispersion in fluid compartment will be the same as the total mass injected into the compartment.

If we inject a indicator substance of know volume and concentration into a compartment of unknown volume, allowing it to disperse in compartment and then analyzing the extent to which substance is diluted we can calculate the volume of compartment.

Mass A = concentration A X volume A = C1 X V1

Mass B = concentration B X volume B = C1 X V1

Mass A = Mass B

C1 X V1 = C2 X V2

Volume of compartment B (V2) = (C1 X V1)/ C2

If I ml solution of l0mg/mg is injected in a compartment and final concentration achieved is 0.01mg/ml then unknown volume of compartment can be calculated as

Volume B = 1ml X l0mg/ml / 0.01mg/ml = 1000ml

Ideal substance should have following properties

• It should be measurable.
• It must be non-toxic.
• It must not be metabolized / secreted / excreted.
• It should remain in same compartment.
• It should be unchangeable by body during mixing.
• It must not have any effect on its own distribution in fluid / mix thoroughly.

In case it is metabolized then volume can be calculated as

Amount of substance metabolized/excreted = C3 X V3

(C1 X V1) – (C3 X V3) = C2 X V2

Volume of compartment B (V2) = (C1 X V1) – (C3 X V3) / C2

Measurement of different body compartments

Measurement of total body water

It is measured by indicator dilution principle. Substance used to measured are

• Deuterium oxide 2H20
• Tritium oxide 3 H2O
• Antipyrine

Measurement of ECF

It is difficult to measure because

• Limit of space is ill define
• Few substance mix in the space while other lies exclusively in ECF
• Lymph cannot be separated from ECF and is measured with it
• Substance enter CSF slowly because of Blood Brain Barrier
• Equilibrium is slow in joint fluid, aqueous humour, avascular tissue, dense connective tissue, cartilages and bones

Direct method

It is measured by indicator dilution principle. Substance used to measured are

• Inulin
• Mannitol
• sucrose
• Radioactive sodium
• Radioactive chloride
• Radioactive iothalamate
• Thiosulfate ion

Indirect method

Osmolarity = total osmole / volume, so volume = total osmole/Osmolarity

Thus ECF volume can be determined by total osmotic content/ Osmolarity

Measurement of ICF

It cannot be measured directly but indirectly as ICF = total body water – ECF

Measurement of plasma

It can be measured by substance which can bind plasma proteins

• Evan blue dye
• Sr albumin labeled radioactive iodine

Its average value is 3500m1

Measurement of Intestitial fluid volume

It cannot be measured directly as

• It is difficult to sample interstitial fluid
• Substance that equilibrate interstitial fluid also equilibrate plasma

So it is indirectly calculated as Intestitial fluid volume= ECF – plasma

Measurement of total blood volume

Indirect method

Blood = plasma + Haematocrit,

I = p+H,

P = I-H

Then blood volume = plasma / (l-H)

Direct Method – It is measured by indicator dilution principle. Substance used to measure is 5l Cr-labeled RBC.

Measurement of Red cell volume

Indirect method – RBC volume = total blood volume – plasma volume

Direct Method – It is measured by indicator dilution principle. Substance used to measured is

• 51 Cr = radioactive chromium
• 59 Fe = radioactive iron
• 32 P = radioactive phosphorus
• Antigenic tag