DETAILED EXPLANATION OF FLUID BALANCE

1. Fluid And Electrolyte Balance
DR.KARTHIK
OMFS

2. Fluids and It’s compartments

3. Introduction
The total Body water is split into 3 major compartments. This constitutes
about 60% of the fluid volume in the body.

4. • The plasma is separated from ISF by highly permeable capillaries.
• Therefore ISF(Interstitial Fluid)+ Plasma= EXTRACELLULAR FLUID. Cell membranes are
highly permeable to water but very selective to certain solutes.
• The fourth compartment contains the Transcellular Fluid compt. It has fluid transit in the lumina
of epithelial organs. Eg: Gall bladder, stomach, intestines, urinary bladder.

5. Extracellular Fluid Compartment
• This is the fluid which is contained in the spaces outside the cell. ECF compt includes Plasma,
Interstitial fluid and Transcellular fluid.
1. Plasma: It is the fluid portion of the blood and represents about 25% of ECF. Its volume can be
calculated from blood volume and Packed cell volume.
Plasma Volume = Blood Volume(L) × 100−PCV
100
Blood volume is the plasma and blood cells which fill the vascular system. It is approx. 80ml/kg of
body weight or 8% of total body weight.

6. Distribution Of ECF in a normal person
Compartment Volume(L) Percent
Body Weight Body Water
Plasma(25% of ECF) 3.5 4.5 8
Interstitial+TCF(75% of ECF) 10.5 15 25

7. Interstitial Fluid: It is that part of that is outside the vascular system.
• It surrounds all cells except blood cells and includes Lymph. It is on
constant motion throughout the body and is transported rapidly in the
circulation.
• Lymph constitutes 2-3% of TBW.
Transcellular Fluid: It is the fluid in the lumen of the structures lined by the
epithelium.
It includes digestive secretions, sweat, CSF, pleural, peritoneal, synovial,
intraocular and pericardial fluids, bile and luminal fluids of the gut, thyroid
and cochlea.
Transcellular fluid volume is 15ml/kg of body weight.
Normal cell function depends upon the constancy of the fluid that forms
the actual immediate environment of the cells. Hence the blood is called
“Milleu Interieur”

8. • The intracellular fluid, also known as cytosol, is all
fluid contained inside the cells.[3] It is the matrix in
which cellular organelles are suspended.
• The cytosol and organelles together compose
the cytoplasm.
• The cell membranes are the outer barrier. In
humans, the intracellular compartment contains on
average about 28 litres of fluid, and under ordinary
circumstances remains in osmotic equilibrium.
• It contains moderate quantities of magnesium and
sulphate ions.
• In the cell nucleus the fluid component of
the nucleoplasm is called the nucleosol.

9. Ionic Composition Of Body Fluids
Characteristic Features
1. Ions constitute approx. 95% of solutes in the body fluids.
2. Distribution of electrolytes in various body compts differ widely.
“The sum of concentration of cation equals the sum of the
concentration of anions in each respective compt making the fluid in
each compt electrically neutral.
Na+, Ca2+, Cl- and HCO3 are lagely extracellular whereas K+, Mg2+,
phosphates and proteins are present in ICF.
Essentially all of the K+ is exchangeable whereas only 65-70%of
body Na+ are exchangeable. (Exchangeable solutes are usually
osmotically Active.
Almost all of the body Ca2+(in bone) and most of body Mg2+ in
bone and cells are non exchangeable.

11. Units For Measuring Concentration Of Solutes
• The no of molecules, electrical charges or particles of a substance per unit volume of a
particular body fluid are expressed in moles, equivalents or osmoles.
Moles: Mole is the std unit for expressing the amt of substances in the SI unit system. It is the
gram molecular weight of a substance(MW in grams).
• Each Mole consists of 6 × 1023molecules. Hence 1 mole of KCL= 39+35.5= 74.5(which is the
sum of atomic masses)
• Always concentration in units of grams of g/L is used since concentration of two dofferent
substances on the basis of no of g/l of solution does not indicate the no of molecules of
each compound.

12. • When the structure of a molecule is known concentrations expressed
as mol/L. This provides a unit of concentration based on no of
molecules of solute in solution.
• Since 1 mole of any molecule will have the same no of solute
molecules per lt as a 1M solution of glucose or any other substance.
Equivalents: It’s the std unit for expressing the solutes in the body
which are in the form of charged particles.
• 1 Eq is 1 mole of an ionized subst divided by its valency. One mole of
KCL dissociates into 1 Eq of K+ and 1 Eq of Cl.
• One Eq of K+: 39/1= 39gm but 1 Eq of Ca2++ 40/2= 20g
• The milliequivalent (mEq) is 1/1000 of 1 Eq.
• The Normality(N) of a solution is no of gEq in 1 Lt.

13. Molality: Is a property of a solution and is defined as the number of moles of solute per kilogram of solvent.
The SI unit for molality is mol/kg. A solution with a molality of 3 mol/kg is often described as “3 molal” or “3
m.” However, following the SI system of units, mol/kg or a related SI unit is now preferred.

14. Molar concentration (also called molarity, amount concentration or substance concentration) is a
measure of the concentration of a chemical species, in particular of a solute in a solution, in terms
of amount of substance per unit volume of solution.
In chemistry, the most commonly used unit for molarity is the number of moles per litre, having the unit
symbol mol/L or mol⋅dm−3 in SI unit.
A solution with a concentration of 1 mol/L is said to be 1 molar, commonly designated as 1 M.

15. • Osmotic concentration, formerly known as osmolarity is the measure
of solute concentration, defined as the number of osmoles (Osm) of solute per litre (L)
of solution (osmol/L or Osm/L).
• The osmolarity of a solution is usually expressed as Osm/L (pronounced "osmolar"), ie
mole of solute particles.
• Whereas molarity measures the number of moles of solute per unit volume of solution,
measures the number of osmoles of solute particles per unit volume of solution.[2]
• allows the measurement of the osmotic pressure of a solution and the determination
solvent will diffuse across a semipermeable membrane (osmosis) separating two
different osmotic concentration.

16. • Osmolality is a measure of how much one substance has dissolved
in another substance. The greater the concentration of the substance
dissolved, the higher the osmolality. Very salty water has
higher osmolality than water with just a hint of salt.
• Plasma osmolality measures the body's electrolyte-water balance.
• Osmotically active substances in the body are dissolved in water as
the density of eater is 1. Therefore osmolal concentration Is
expressed in osmoles per litre(Osm/L).

17. Concept Of pH And H+ Concentration:
H+ concentration of various body fluids is expressed in two different ways, either directly(H+) or
indirectly as pH.
pH refers to –ve logarithm of the H+. The relation b/w H+ & pH can be expressed.
i. pH= log10 1/[H+]
ii. pH= -log[H+]
2. pH and H+ are inversely related: When the pk of a buffer system is known the effective pH range
of buffer is known.
Here in pK….K= Ionisation constant. ∴ pK= -logK and is equal to the pH at which half of acid
molecules are dissociated and half are undissociated.

18. • Optimal pH range for blood(for the body to function properly) is 7.35 to
7.45
• Clinically pH of blood is <7.35 is referred as Acidosis.
• pH> 7.45 as Alkalosis.
Buffer System: A buffer is a substance that has the ability to bind or release H+ in solution.
• A buffer in solution consists of a weak acid and a salt of its conjugate base.
• Thereby keeps the pH of the solution constant inspite of addition of acids or bases.
• Buffering is the means by which large changes in H+ ae minimised.

19. Buffer System In Body Fluids

20. Bicarbonate ion (HCO3
–)- weak base,
Carbonic acid (H2CO3)- weak acid.
The pH falls, the HCO3
– removes excess H+:
Carbonic acid-bicarbonate buffer system

21. If the pH rises, H2CO3 can provide H+:

22. This system acts similarly to the carbonic acid-bicarbonate buffer
system. Dihydrogen phosphate (H2PO4
–) and monohydrogen
phosphate (HPO4
2–) are the ions used in this system.
H2PO4
– acts as a weak acid:
Phosphate buffer system

23. HPO4
2– acts as a weak base:

24. • Protein buffer system: most abundant in intracellular fluid
and blood plasma. When pH rises, the COOH group
dissociates to act like an acid.

25. PROTEIN BUFFER SYSTEM
• When pH falls, the free amino group dissociates to act like a base.

26. Dynamics Of Equilibrium
• The general equation for buffer system: Henderson-Hasselbalch Equation
pH: Acidty of a buffer solution
pKa: negative log of Ka(ionisation constant)
Ka: Acid dissociation constant
[A-]: concn of acid
[HA]: concn of conjugate base
• can be used to estimate the pH of a buffer solution. The numerical
value of the acid dissociation constant, Ka, of the acid is known or
assumed.
• The equilibrium between the weak acid and its conjugate base
allows the solution to resist changes to pH when small amounts of
strong acid or base are added.

27. • The Henderson–Hasselbalch equation relates the pH of a solution
containing a mixture of the two components to the acid
dissociation constant, Ka, and the concentrations of the species in
solution.[3] To derive the equation a number of simplifying
assumptions have to be made.