2. INTRODUCTION
1) In an average young male
i. 7% of body weight is mineral
ii. 15% is fat
iii. 18% is protein & related substances, &
iv. 60% is water, called total body water (TBW)
2) TBW is about 10% lower in young females
due to relatively greater amount of adipose
tissue (subcutaneous fat)
3) In infants TBW is 65-75% of body weight.
2
5. EXTRACELLULAR FLUID (ECF)
COMPARTMENT
• This is the fluid present outside the cells.
• It constitutes about 20% of the body weight.
• Normal volume of ECF is 14L
5
6. • ECF compartments includes:
1) Plasma- Is the fluid portion of the blood.
It represents 25% of
the ECF. Its volume can be calculated from blood
volume & PCV (packed cell volume) as under:
Plasma Volume (L)×100-Hematocrit (PCV)
100
Blood Volume i.e. plasma & blood cells which
fill the vasular system.
It is approx 80mL/kg of body weight or 8% of
the total body weight.
6
7. 2) Interstitial Fluid- It is that part of ECF that is
outside the vascular system.
It surrounds all cells except blood cells &
includes lymph.
It is in constant motion throughout the body
& is transported rapidly in the circulating
blood.
Lymph constitutes 2-3% of the total body
weight.
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8. 3) Transcellular Fluid- It represents fluid in the
lumen of structures lined by epithelium.
It includes: digestive secretions, sweat,
cerebrospinal fluid (CSF), pleural, peritoneal,
synovial, intraocular (aqueous & vitreous
humours) & pericardial fluids, bile & luminal
fluids of the gut, thyroid & cochlea.
Transcellular fluid volume is relatively small,
about 1L i.e. 15mL/kg of the body weight
(1.5% of body weight).
8
9. IMPORTANT NOTE
The normal cell function
depends upon the constancy of
the fluid that forms the actual
immediate environment of the
cells.
For this reason blood is called
the internal environment of the
body or Milieu Interieur .
9
10. INTRA CELLULAR FLUID (ICF)
COMPARTMENT
• This is the total amount of fluid present inside
the 75 trillion cells of our body.
• Although the fluid is disributed among the 75
trillion cells of our body, It is considered as
one single unit because the composition of
fluid in all these cells is similar.
• ICF compartment comprises about 40% of the
body weight.
• Normal volume of ICF is 28L.
10
11. COMPOSITION OF BODY FLUIDS
ECF
(mOsm/LH₂O)
Na 142
K 4.2
Ca 1.3
Mg 0.8
Cl 108
Protein 1.2
ICF
(mOsm/LH₂O)
14
140
0
20
4
4
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12. MEASUREMENT OF BODY FLUID
VOLUMES
• The volume of water in each fluid compartment
can be measured by the indicator dilution
principle. This principle is based on the
relationship between:
• The amount of substance injected intravenously
(A)
• The volume in which that substance is distributed
(V), &
• The final concentration attained (C).
• The equation for the relationship is:
• C = A i.e. V = A
V C 12
13. Characteristics Of Indicator (marker) Used
1) It should be relatively easy to measure.
2) It must remain in the compartment being
measured.
3) It must not alter water distribution in the
compartment being measured.
4) It must be non-toxic.
5) It must mix evenly throughout the
compartment being measured.
6) It must be unchanged by the body during the
mixing period or the amount changed must
be known.
13
14. • The indicator may leave the compartment by
excretion or metabolism during the time
allowed for mixing.Then,
• Volume of distribution (V) = (A) administered
minus(A) removed
C
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16. IMPORTANT NOTE
The ECFV/ICFV is larger in
infants & children than it is in
adults, but the absolute
volume of ECF in children is
smaller than it is in adults.
Therefore, dehydration
develops more rapidly & is
frequently more severe in
children than in adults.
16
18. Units For Measuring
Concentration Of Solutes
• The number of molecules, electrical charges,
or particles of a substance per unit volume of
a particular body fluid are frequently
expressed in moles, equivalents or osmoles.
A. MOLES-
The mole is the standard unit for expressing
the amount of substances in the SI unit
system.
A mole is the gram-molecular weight of a
substance, i.e., the molecular weight of the
substance in grams.
18
19. Each mole (mol) consists of approx 6×10²³
molecules.
Thus,1mole of KCl = 39 + 35.5 gm = 74.5 gm
(i.e. sum of atomic masses of all the atoms in
the molecule).
The milimole (mmol) is 1/1000 of a mole,
therefore, 1mmol of KCl = 74.5 mg.
19
20. IMPORTANT NOTE
The concentrations of many solutes
dissolved in the body fluids are much
less than 1M,being in the range of
millimoles per litre (1mM=0.001M).
For ions such as Na⁺,K⁺ & HCO⁻₃,
which have a single positive or
negative charge, the no of mEq/L is
equal to the no of mmol/L.
For ions such as Ca²⁺or phosphate
(HPO²⁻₄), which have two positive or
negative charges, the no of mEq/L is
twice the no of mmol/L. 20
21. B. EQUIVALENTS-
The equivalent is the standard unit for
expressing the solutes in the body which are
in the form of charged particles.
One equivalent (Eq) is 1 mole of an ionized
substance divided by its valency.One mole of
KCl dissociates into 1 Eq of K⁺ & 1 Eq of Cl⁻.
One Eq of K⁺=39gm/1=39gm, but 1Eq of
Ca²⁺= 40gm/2= 20gm.The milliequivalent
(mEq) is 1/1000 of 1Eq.
The normality (N) of a solution is the no of
gram equivalents in 1 litre. Therefore,1N
solution of hydrochloric acid contains
1+35.5 gm/L= 36.5gm/L. 21
22. C. OSMOLES-
The amount of concentrations of osmotically
active particles are usually expressed in
osmoles (osm).
1 osmoles equals the gram molecular weight
(i.e. one mole) of the substance divided by the
no of freely moving particles each molecule
liberates in solution.
The miliosmole (mosm) is 1/1000 of 1osm.
As 1osmole = Gram molecular weight (i.e.
1mole) of a substance
No of freely movable particles, each molecule
liberates in solution
22
23. i. If a solute is a non-ionizing compound (eg,
glucose), then 1osmole is equal to 1mole of
solute particle. Thus, 1molar solution of
glucose has a concentration of 1osm
(1osmole per litre).
ii. If the solute ionizes & forms an ideal solution
each ion is an osmotically active particle
thus in a 1molar solution of NaCl, NaCl
would dissociate into Na⁺ & Cl⁻ ions, so that
each mole in solution would supply
2osmoles of solute per litre of solution.
Similarly, 1 mole of CaCl₂ would dissociate
into Ca²⁺, Cl⁻ supplying 3 osmoles.
23
24. The no of osmoles per litre of solution is called
osmolarity, e.g. plasma, whereas the no of
osmoles per kilogram of solvent is osmolality.
IMPORTANT NOTE
Osmolarity is affected by the
volume of the various solutes in
the solution & the temperature,
while the osmolality is not.
24
25. D. CONCEPT OF pH & H⁺ CONCENTRATION
1. H⁺ concentration [H⁺] of various body fluids is
expressed in two different ways, either
directly as [H⁺] or indirectly as pH (pH stands
for power of hydrogen)
pH refers to the negative logarithm of the
[H⁺].
The relation between [H⁺] & pH can be
expressed as:
i. pH = log₁₀ 1/ [H⁺]
ii. pH = -log₁₀ [H⁺]
25
26. IMPORTANT NOTE
For decrease in each pH
unit (e.g. from 7.0 to
6.0), the [H⁺] is increased
10 fold.
For each pH unit
increase (e.g. 7 to 8), it is
decreased 10 fold.
26
27. 2. pH & [H⁺] are inversely related: Another
advantage of the pH concept is that when the
pK of a buffer system is known, it is
immidiately possible to determine the
effective pH range of the buffer, where
K= the ionization or dissociation constant.
Therefore, pK = negative log of K (-log K) & is
equal to the pH at which half of the acid
molecules are dissociated & half are
undissociated.
27
28. 3. Blood pH always refers to plasma pH (7.4),
the range of [H⁺] that is compatible with life is
20-126mEq/L i.e pH of 7.7 to 6.9.
Optimal pH range for blood at which human
body functions properly is 7.35 to 7.45
(7.4 ± 0.05).
Clinically, blood pH <7.35 is referred as
acidosis, & blood pH > 7.45, as alkalosis
28
30. E. CONCEPT OF BUFFER SYSTEM
A buffer is a substance that has the ability to
bind or release H⁺ in solution.
A buffer in a solution consists of a weak acid
& its conjugate base, thus keeping the pH of
the solution relatively constant despite the
addition of considerable quantities of acid or
base.
Buffering is the primary means by which large
changes in [H⁺] are minimized within fraction
of seconds.
30
31. DYNAMICS OF BUFFERING:
The Henderson-Hasselbalch Equation
The general equation for a buffer system is
HA⇌ H⁺ + A⁻
Where A⁻ represents any anion & HA the
undissociated acid.
If an acid stronger than HA is added to a
solution containing this buffer system, the
equilibrium is shifted to the left.
H⁺ are ‘tied up’ in the formation of more
undissociated HA, so increase in H⁺
concentration is much less than it would
otherwise be. 31
32. Conversely, if a base is added to the solution,
H⁺ & OH⁻ react to form H₂O, but more HA
dissociates, limiting the decrease in H⁺
concentrtion.
By the laws of mass action, the product of the
concentrations of the products in a chemical
reaction divided by the product of the
concentration of the reactants at equilibrium
is a constant:
[H⁺] [A⁻] = K
[HA]
32
33. If this equation is solved for H⁺ & put in pH
notation (pH is the negative log of [H⁺]), the
resulting equation is that originally derived by
Henderson & Hasselbalch to describe the pH
changes resulting from addition of H⁺ or OH⁻
to any buffer system
(Henderson-Hasselbalch equation):
pH = pK + log [A⁻]
[HA]
33
34. It is apparent from these equations that the
buffering capacity of a system is greatest
when the amount of free anion is equal to the
amount of undissociated HA i.e. When
[A⁻]/[HA] = 1, so that log [A⁻]/[HA] = 0
& pH= pK.
This is why the most effective buffers in the
body would be expected to be those with pKs
close to the pH in which they operate.
The pH of the blood is normally 7.4, that of
the cells is probably about 7.2, & that of urine
varies from 4.5 to 8.0
34
