0305 - Fluid Compartments - RM

Question 1

Outline the fluid compartments in the body. What are their approximate volumes?

Answer 1

The fluid compartments are the various real and virtual compartments that make up Total Body Water (TBW).There are two major fluid compartments, and several sub-compartments:Intracellular Fluid (ICF 25-28L) - ICF is slow-flowing, via changes in tonicity (i.e. via osmosis). ICF contains high K=, MG2+, and low Na+, Cl-, Ca2+ Extracellular Fluid (ECF 12-14L) - ECF is more specialised and structured than ECF. There are two main sub-compartments of ECF, which comprise Functional ECF (fECF):Plasma (PLF - 3L) - liquid portion of blood, very fast moving and links to excretion/secretion of fluid through intestines, skin, lungs, kidneys. Plasma has a high protein content and is separated from ISF by the CAPILLARY MEMBRANE.Interstitial Fluid (ISF - 9-11L) - a non-moveable (i.e. slow-moving) virtual compartment that surrounds and bathes cells. It has a low protein content (except for lymph). It links to three sub-compartments (5L total), all of which have very slow kinetics.Transcellular Fluid (TCF - 1L) - formed by transport activities of cells (i.e. cells made to secrete fluid). Covers any bodily fluid, and is generally contained within other compartments, such as joint capsules (synovial fluid), spinal column (CSF), stomach, bladder etc.Bone Water (BW - 2L)Dense Connective Tissue Water (DCT - 2L)

Question 2

What are the approximate ratios of ICF:ECF and ICF:fECF?

Answer 2

ICF:ECF =3:2ICF:fECF =2:1

Question 3

Which body fluid compartments can be easily measured? How would you calculate their volume?

Answer 3

Only TBW, fECF, and PLF can be easily measured. Inject a known volume and concentration of tracer dye, allow it to dilute, and then calculate the volume based on the new concentration of tracer dye (C1V1=C2V2). You would need to correct for excretion and metabolism of the tracer. TBW is generally measured by water isotopes (such as heavy water), which can penetrate all compartments.True fECF can’t be measured, so a ‘20 hour bromide space’ is used by convention.PLF can be measured by using anything that binds to albumin (such as Evan’s Blue). However, albumin can leak into ISF, resulting in an over-estimation that requires correction.

Question 4

Which body fluid compartments cannot be easily measured? How would you measure them?

Answer 4

ICF = Measure TBW, them measure and subtract fECFISF = Measure fECF, then measure and subtract PLFTCF cannot be measured in full - you would measure a sub-compartment (such as bladder or joint compartment)Total blood volume is typically measured by haematocrit - calculating % of blood that is PLF, and extrapolating a PLF calculation (though this is innaccurate).

Question 5

What are the major ionic components in ECF and ICF

Answer 5

ECF - Major cation = Na+ (>120mM), major anion Cl- (100mM). Major buffer is HCO3. (High Na+, high Cl-).ICF - Major cation = K+ (>120mM), some Mg++. Major Anion HPO4– (Hydrogen Phosphate 100mM), which is also major buffer. There is also some protein (neg. charge).ECF = High Na+, High Cl-, Low K+ICF = Low Na+, low Cl-, High K+

Question 6

Outline the concept of free and bound ions. Give some examples of each

Answer 6

Free ions are those that are are ‘floating freely’ dissolved in bodily fluids. Bound ions are fixed (bound) to specialised proteins.All monovalent (Na+, K+ etc) ions are ionised (free).Most divalent/trivalent ions are bound.Ca++ and Mg++ have a special role, and are 50% free, 50% bound.