1. THE AUSTRALIAN NATIONAL UNIVERSITY
Overview of Body Fluid Compartments and Important Intra- and Extracellular Ions Christian Stricker Associate Professor for Systems Physiology ANUMS/JCSMR - ANU

3. At the end of this lecture students should be able to
Aims
At the end of this lecture students should be able to
define the different fluid compartments;
state approximate compartment sizes;
describe how volumes of different fluid compartments are measured;
recognize major ionic components in different compartments (cations and anions); and
express the concept of free and bound ions.

4. Contents Fluid compartments Determination of compartment size
Tracers for various compartments
Ionic contents in different compartments (cations and anions)
Concept of “free” and bound concentration

5. Importance Fluid replacement
Which compartments are affected?
What needs replacing?
How much should be given?
What is the route of administration?
Also requires osmotic conside-rations (see resp. lecture).

6. Total Body Water (TBW) Average H2O content (70 kg body weight):
Standard man: 42 L H2O (60%)
Standard woman: 38.5 L H2O (55%)
Variations between individuals/sexes
Largely due to adipose tissue (breasts)
73.2 ± 3% of “fat free” body weight (BW) is H2O (most mammalia).
Variations in water content between tissues
Plasma: 93% H2O (& 7% ‘plasma solids’)
Kidney: ~83% H2O
Bone: 20% H2O
Fat: 10-15% H2O

7. Variation Due to Sex and Age
after Schmidt/Thews 1977
Neonate: % of total body water (TBW)
12 month: 60% TBW
>60 years: 50% TBW

8. Top Fluid Compartments
2 top-level compartments:
Intracellular fluid (ICF): 25 L ≈ 60%
Extracellular (ECF): 17 L ≈ 40% fECF 12 L
Where is the rest ? (see below)
Top compartments are separated by a lipid (cellular) membrane:
Transporters, channels, carriers and diffusion required.
Water cannot “diffuse” through the membrane: special channels.
Fluids produced from fECF.
after Schmidt/Thews 1977

9. Intracellular Fluid Compartment
Is contained in ~1014 cells; separated by a cell membrane.
Fluid follows “passively” with changes in tonicity within ECF (see “osmosis” lecture).
Neglects intracellular organelles (mitochondria, Golgi apparatus, ER, etc.).

10. Extracellular Fluid Consists of plasma and interstitial fluid.
Plasma is inter-linking compartment that is movable.
Most excretions/secretions occur from plasma.
Interstitial fluid is non-movable (i.e. only slowly; “inter stare” = what is in-between (cells); water also binds to “dry” substance - i.e. bone minerals, connective tissue, etc.).

11. Extracellular Sub-Compartments
2 sub-compartments of fECF with different exchange rates in particular locations:
Plasma (PLF, 3 L)
Interstitial fluid (ISF, 9 L)
The two are separated by an endothelial “membrane”.
after Schmidt/Thews 1977

12. Special ISF Compartments
At rest, very slow exchange rates from ISF to
transcellular H2O (1 L; CSF, joint fluids, intestinal fluids, prostatic fluid, sperm fluid, etc.);
varies with activity…
bone H2O (2 L); and
dense connective tissue H2O (2 L).
Variable numbers (±10%) in different books/monographs.
after Schmidt/Thews 1977

13. Exchange Rates of ECF
Plasma fluid (high protein content; high bulk flow - fast exchange).
Interstitial fluid (ISF, ~20% TBW): link between plasma and ICF (low protein content; slower exchange).
Transcellular fluid (variable protein content; slow exchange depending on activity).
Dense connective tissue and bone water (DCT/BW; very slow exchange).
ECF - DCT/BW - TCF = PLF + ISF = fECF = 12 L.
ICF : ECF = 60 : 40; ICF : fECF = 60 : 28 ≈ 2 : 1.
after Schmidt/Thews 1977

14. Tracers for Fluid Compartments
Ideal tracer should
be non-toxic,
rapidly and evenly distribute throughout the target compartment,
not enter other compartments,
not be metabolised,
not be excreted during equilibration period (or excretion is able to be corrected for),
be easy to measure, and
not interfere with body fluid distribution.
Despopoulos & Silbernagl 2003
“Non-ideal” tracers can be used, if adjustments are made for
excretion (correction for excreted amount) and/or
metabolism (correction for 1st order kinetics, for example).

15. Tracers Used for TBW
Water isotopes (stable; not radioactive) i.e. deuteriated water (2H2O), H218O. Measured using NMR.
Water isotopes slowly penetrate all compartments and, therefore, also label water within.
Measured by doctors in nuclear medicine.
Historical: antipyrine (phenazone - analgesic; can cause bone marrow suppression: agranulocytosis).

16. Extracellular Tracers
Crystalloids (“sugar” derivatives): mannitol, inulin, etc. These tracers do not enter cells, but equilibrate in ECF (kinetics) unevenly: under-estimate.
Ionics (isotopes, i.e. 82Br-, 34SO42-, etc.): These substances equilibrate well, but also enter cells in small amounts: over-estimate.
“True” ECF cannot be measured; it is conventional to refer to measured space by tracer and equilibration time (i.e. 20 h 82Br-space).
Two kinetic parts to ECF:
Fast: functional ECF (fECF = plasma and ISF = 12 L).
Slow (24 h): DCT/BW plus some of TC water (~ 5 L).

17. Plasma Fluid Tracers
Anything that binds to albumin when flux is large:
lots of drugs (a lot do that…);
some vital dyes like Evan’s blue.
Problem: over time, albumin extravasates into ISF... correction required.

18. Tracers for Other Compartments
ICF: no tracer. Indirectly via ICF = TBW - ECF.
ISF: no tracer; indirectly via ISF = fECF - PLF.
TCF: no tracer; sub-compartments can often be determined individually.
Special case: Blood volume
Consists of PLF and part of ICF (erythrocyte volume).
Determined via autotransfusion of 51Cr-labelled RBCs.
Problems using Hct to obtain PLF:
Hct in veins > arteries: Cl--shift.
Some volume trapped between RBCs (“true” Hct < 4-8%).

19. Determining Compartment Size
We need to know total amount of tracer applied and determine concentration in fluid.
Practically, this is not necessarily simple (volume distribution; see also pharmacokinetics by K. Saliba).

20. Composition of ECF Major cation: Na+ (>120 mM)
Minor cations: ~ 2 mM
Major anion: Cl- (~100 mM)
Minor anions: < 2 mM
Major buffer: HCO3-
Why a difference between PLF and ISF? Plasma protein.
Mechanism? Gibbs-Donnan equilibrium (see osmosis lecture).
after Schmidt/Thews 1977

21. Composition of ICF Major cation: K+ (>120 mM)
Minor cations: < 1 mM, Mg2+ “high”
Major anion: HPO42- (<100 mM)
Minor anions: protein, SO42- and HCO3- (Cl- ~4 - 8 mM)
Major buffer: HPO42-
Positive and negative charges need to be matched: electro-neutrality.
after Schmidt/Thews 1977

22. Comparison
Other ions are negligible (trace elements like Fe, Cu, Se, Zn, etc.).
Why is total intracellular [anion & cation] higher than the extracellular? Protein.
What maintains ionic gradients between inside and outside? Transporters using ATP.
after Schmidt/Thews 1977

23. “Free and Bound…”
All monovalent cations/anions are fully ionized (fully dissolved, i.e. “free”).
Special role of Ca2+ and Mg2+: both are about 50% ionized (free) and 50% protein/organic acids bound.
Most other divalent/trivalent cations are bound: 100% of Fe3+, Zn2+, Cu2+, etc. to specialized plasma proteins/albumin.

24. Take-Home Messages 60% of total body weight is H2O, of which
60% is intracellular and
40% extracellular.
A number of tracers is used to measure sizes of compartments.
Some compartments cannot be measured directly.
Major extracellular ions are: Na+, Cl-, Ca2+.
Major intracellular ions are: K+, HPO42-, Mg2+.
~50% of Ca2+ and Mg2+ are bound and free.

25. MCQ
To determine volume distribution of a new drug, a 70 kg male volunteer receives an i.v. injection of 150 µmol dissolved in 20 mL solute. Extensive monitoring reveals that plasma concentration at time “zero” was 12.5 µM. In which of the following body fluid compartments is this drug most likely distributed?
Total body water
Plasma fluid compartment
Interstitial fluid compartment
Intracellular fluid compartment
Functional extracellular fluid compartment

26. That’s it folks…

27. MCQ
To determine volume distribution of a new drug, a 70 kg male volunteer receives an i.v. injection of 150 µmol dissolved in 20 mL solute. Extensive monitoring reveals that plasma concentration at time “zero” was 12.5 µM. In which of the following body fluid compartments is this drug most likely distributed?
Total body water
Plasma fluid compartment
Interstitial fluid compartment
Intracellular fluid compartment
Functional extracellular fluid compartment