Body Fluid Compartments – Part 1
Body Composition
E.S.Prakash,
Faculty of Medicine, AIMST University
E-mail: dresprakash@gmail.com
Note to Readers – This is one of the lectures in the Molecular
& Cellular Basis of Medicine course, Element 2 [General
Physiology, Nerve – Muscle Physiology, Basics of
Hematology] in Term 1 of Year 1 of the MBBS programme.
Plasma [about 1/4th of ECF volume]
Interstitial Fluid [ISF];
About 3/4th of ECF volume
Intracellular Fluid [ICF];
2/3 of total body water is inside cells
ECF
Body fluid volumes in a
healthy 70-kg adult male
Compartment
Volume
Subdivisions
Intracellular fluid, ICF (2/3
of TBW)
Extracellular fluid, ECF
(Sucrose space);
1/3 of TBW
28 L
-
14 L
10.5 L
(Interstitial fluid)
Total body water (TBW);
60% of body weight
42 L
3.5 L
(Plasma)
-
Measuring body fluid volumes:

Principle: Indicator-dilution principle

Method:
1.
Administer a known amount (A) of a substance that will
get distributed in a particular body fluid compartment;
2.
Determine the final steady state concentration (C) of this
indicator in blood;
3.
Volume of distribution Vd of the indicator =
Amount of indicator injected (A)
Concentration in blood after mixing (C)
Indicators used for measuring plasma volume,
ECF volume and total body water
Compartment Criterion
Plasma
ECF volume
Total body
water (TBW)
Indicators
Substance should Evans blue dye;
not cross
radioiodinated fibrinogen;
capillaries
radioiodinated albumin
Substance should Isotonic solutions of
cross capillaries sucrose, inulin, mannitol,
but not cross cell NaCl
membranes
Substance
Heavy water, tritiated water,
distributes evenly aminopyrine, antipyrine
in ICF & ECF
Take this problem:

A.
B.
C.
D.
100 mg of sucrose is injected into a 70 kg
man. The plasma sucrose level after mixing
is 0.01 mg/ml. If 5 mg has been metabolized
during this period, then, what is the ECF
volume?
9.5 L
14 L
17.5 L
10 L
Solution:





Amount of sucrose injected = 100 mg
Amount metabolized = 5 mg
Amount remaining in ECF (A) = 95 mg
Concentration in plasma after mixing (C) = 0.01 mg/ml
Thus, volume of distribution (Vd) of sucrose
= A / C = 95 mg / 0.01 mg/ml
= 9500 ml = 9.5 L (= ECF volume)

How would you measure interstitial fluid (ISF)
volume?
 Cannot be measured; it is calculated (estimated)..
 ISF volume = ECF volume – plasma volume

How would you measure ICF volume?
 Cannot be measured; it is calculated (estimated)..
 ICF volume = Total body water – ECF volume
Transcellular fluids






Cerebrospinal fluid
Synovial fluid
Aqueous humor
Pleural fluid
Peritoneal fluid
In health, the total volume of transcellular fluids is <
1 L.
What is the relationship between blood
volume and plasma volume?

Blood = plasma + cells in blood

Volume of cells: packed cell volume
(PCV)

PCV is also called hematocrit (Hct)

Blood volume
= plasma volume × 100 / (100 – Hct)
Image from
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages
/B/Blood.html
Blood volume = plasma volume × (100/100Hct)

A.
B.
C.
D.
In a healthy 70 kg adult male, plasma volume
(measured using the Evans Blue dye) was
found to be 3000 ml. Hematocrit was 40%.
Calculate his blood volume.
5000 ml
5200 ml
5400 ml
5600 ml
Measuring red cell volume
Note: measuring is different from calculating something

Technique & principle:
 Use
Chromium 51 labeled RBC as the indicator;
 Inject a known amount of Cr 51 labeled RBC
intravenously;
 Allow them to mix with RBC in blood;
 Measure the fraction of RBC tagged with Cr 51;
 Principle: same – indicator dilution principle.
Lean body mass (LBM)





Definition: LBM is fat free mass
Total body mass = fat mass + fat free mass
Note: fat is relatively anhydrous
Note: the water content of LBM is constant
Water content of LBM is constant - 70 ml /100 g tissue
Take this problem:

In a healthy adult male weighing 70 kg, total body
water (TBW) was measured to be 42 L. What is his
lean body mass (LBM)? What is his fat mass?

Given TBW = 42 L
 Assume all this water is in LBM & that fat is water free
 We know that water content of LBM is 70 ml/100 g
 Thus, if TBW is 42 L, LBM = 60 kg
 Since he weights 70 kg, his fat mass is 70-60 = 10 kg
Body composition
Lean body mass = fat free mass
Total body weight = fat-free mass + fat mass
Tom
100 kg
Jerry
100 kg
Fat mass
15 % = 15 kg
30 % = 30 kg
Lean body mass
85 % = 85 kg
70 % = 70 kg
Fit
Obese
Body mass
Overall health
Determining body fat:

Technique: bioelectric impedance technique
 Principle:
 Body
fluids conduct electricity well;
 But fat is anhydrous and therefore is a poor
conductor of electricity;
 The resistance to flow of a small current between
points on the body is proportional to fat mass.
Now you should be able to answer these
questions:
1.
Classify body fluid compartments. Mention the volumes
of body fluid compartments in a healthy adult male
weighing 70 kg.
2.
Explain the principle that is used in the measurement of
body fluid volumes and RBC volume.
3.
Mention the indicator that is used to measure the volume
of plasma, ECF, and total body water and explain why
these indicators are chosen for measuring them.
4.
How you would measure interstitial fluid volume?
5.
How you would measure ICF volume?
6.
Mention the relationship between hematocrit and blood
volume.
7.
Describe the meaning of the term ‘lean body mass’.
8.
Mention the relationship between lean body mass and
total body mass.
9.
Briefly explain the principle that is used in the
estimation of body fat.
10.
Briefly explain gender differences in body composition.
Required Reading:

Ganong WF. Chapter 1. The General and Cellular Basis
of Medical Physiology. In: Review of Medical
Physiology. Int’l ed, Mc Graw Hill, 2005; pages 1-6.