This document discusses body water, osmolality, and related topics. It begins by explaining that total body water decreases from around 90% of body weight in fetuses to around 65% in adult males and 55% in adult females. It then covers Starling forces that govern fluid movement between compartments, osmotic pressure, electrolyte composition differences between intracellular and extracellular fluid, Gibbs-Donnan equilibrium, and active transport mechanisms. The document defines osmolality and osmolarity, explains colligative properties, and discusses clinical significance and methods for measuring plasma and urine osmolality.

1. BODY WATER
&
OSMOLALITY
Ola H. Elgaddar
MBChB, MSc, MD, CPHQ, LSSGB
Lecturer of Chemical Pathology
Medical Research Institute
Alexandria University
Ola.elgaddar@alexu.edu.eg
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2. ILOs:
After this lecture, you should be able to:
 Understand the volume and distribution of
body
water
among
different
body
compartments.
 Explain the reasons for composition
differences of body fluids.
 Understand Gibbs – Donnan Euilibrium.
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3. ILOs:
After this lecture, you should be able to:
 Explain what osmotic pressure is.
 Understand the colligative properties of a
solution.
 Calculate teh osmolality of a solution.
 Know the different methods used in
measuring osmolality
 Recognize the significance of Osmolal gap.
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4. BODY
WATER
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5. Total Body Water (TBW)
• During gestation~ 90 % of fetal body wt
• Decreases gradually
• Adult male ~ 65 % of body weight
• Adult females ~ 55 % of body weigth
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8. Starling forces
 Interstitial fluid is an ultrafiltrate from
plasma and both are separated by
caillary endothelial lining which acts as
a semipermeable membrane.
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10. Semipermeable membrane
 A membrane that allows certain
types of molecules to pass through but
blocks others, based on characteristics
such as the molecules size, chemistry,
solubility, or other specific properties.
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13. Starling forces
 Starling
forces
formula
demonstrates that the net movement
of fluid across a capillary membrane is
a function of membrane permeability
and differences in hydrostatic and
oncotic pressure on the two sides of
the membrane.
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14. Hydrostatic Pressure
The pressure exerted by a fluid at
equilibrium at a given point within the
fluid, due to the force of gravity.
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15. Oncotic pressure
(Colloid osmotic pressure)
A form of osmotic pressure exerted
by proteins in a blood vessel, that
usually tends to pull water into the
circulatory system.
It is the opposing force to hydrostatic
pressure.
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17. Osmotic Pressure
???
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19. Electrolytes composition among
Different body compartements
The composition of ICF differs
markedly from that of ECF because of
the
separation
of
these
compartements by the cell membrane
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21. Electrolytes composition among
Different body compartements
The composition differences are a
consequence of both Gibbs-Donnan
Equilibrium and active transport of
ions.
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22. Gibbs-Donnan Equilibrium
when a semipermeable membrane separates nondiffusible substance (ex:protein), from diffusible
substances (ex: electrolytes), the diffusible
substances are distributed on the two sides of the
membrane so that 1) the products of their
concentrations are equal, and 2) the sum of the
diffusible and non-diffusible anions on either side of
the membrane is equal to the sum of the
concentrations of diffusible and non-diffusible
cations; the unequal distribution of diffusible ions
thus produced creates a potential difference across
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the membrane (membrane potential).

24. Active ions transportation
The sodium-potassium pump, also known as
the Na, K-ATPase, is a critical protein found
in the membranes of all animal cells.
It functions in the active transport of sodium
and potassium ions across the cell
membrane against their concentration
gradients.
For each ATP the pump breaks down, two
potassium ions are transported into the cell
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and three sodium ions out of the cell

26. OSMOLALITY
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27. Osmotic pressure and osmosis
 Osmotic pressure governs the movement
of solvents (water) across membranes that
separate two solutions.
 Different membranes vary in pore size
and shape (e.g: glomerular and capillary
vessels) They are permeable to water,
small molecules, and ions, but not
permeable to macromolecules e.g. proteins.
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29. Osmotic pressure and osmosis
Differences in concentration of molecule
that cannot cross membranes will cause
those that can cross to move, thus
establishing an osmotic equilibrium. This
movement of solute and permeable ions
exerts what is known as osmotic pressure
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31. Osmotic pressure and osmosis
Osmosis is the process that constitutes
the movement of solvent across a
membrane is response to differences in
osmotic pressure across the 2 sides of the
membrane.
 Water migrates across the membrane
toward the side containg more concentrated
solute.
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34. Osmolality and Osmolarity
 Osmolality is a physical property of a
solution that is based on the number of
particles of the solute relative to mass of
the solvent (expressed as mmols) / kg of
solvent (w/w). (? Molality)
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35. Osmolality and Osmolarity
 Osmolarity is the no of particles of solute
per liter of the solution, its units of
measurement is mosmol/Liter or mmol/Liter.
(? Molar conc.)
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36. Osmolality and Osmolarity
Which is the more exact expression;
Osmolality or Osmolarity?
Which has higher osmolality; Nacl or
glucose?
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37. Colligative properties of solutions
When a solute is added to a solvent the
following occurs:
Increased osmotic pressure.
Lowered vapour pressure.
Increased boiling point.
Decreased freezing point.
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38. Colligative properties of solutions
 Colligative properties are all directly
related to the total number of solute
particles per mass of solvent.
1 osmolal solution is defined to contain 1
osmol/k.g H2O.
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39. Colligative properties of solutions
 An electrolyte in solution dissociates into
2 (e.g. NaCl) or 3 (CaCl2) particles. The
colligative effects of such solutions are
multiplied by the no of dissociated ions
formed/molecule.
 Incomplete dissociation??
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40. Plasma and Urine Osmolality
Clinical significance:
 Assessment of acid-base disorders
 Assessment of electrolyte disorders e.g.
in diabetes inspidus or in syndrome of
inappropriate secretion of anti-diuretic
hormone (SIADH).
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42. Calculation of Plasma Osmolality:
mOsm/Kg =
1.86 [Na+(mmol/L)]+Glucose
+urea (mmol/L)+9
(mmol/L)
1.86= Na+ and Cl- (incomplete dissociation)
9: The contribution of other osmotically
active substances in plasma such as K+,
Ca2+, and proteins.
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43. Calculation of Plasma Osmolality:
Ref values for osmolality:
Plasma osmolality = 275-300 mosm/Kg
Urine (24- hours) = 300-900 mosm/Kg
N.B: Urine osmolality cannot be calculated
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44. Measuring Plasma Osmolality:
Comparison of measured osmolality to the
calculated value can help identify the
presence of an osmolal gap which can be
important in determining the presence of
exogenous osmotic substances that can
lead to acid-base disturbances, ex: ethanol
intoxication
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45. Measuring Plasma Osmolality:
The methods for determining osmolality are
based on the collegative properties which
are properties of a solution related to the
number of molecules of solute per kilogram
of solvent, such as changes in freezing
point and vapor pressure.
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46. Measuring Plasma Osmolality:
 An increase in osmolality decreases the
freezing point temperature and the vapor
pressure.
 Measurment of freezing point depression
and vapor pressure decrease (Dew point)
are the 2 mostly used methods of analysis
 Freezing point depression is better??
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47. Freezing point depression osmometer:
(Cryoscope)
The components of an osmometer:
1. A thermostatically controlled cooling bath
maintained at -7°C.
2. Stirring rod to initiate freezing of the sample.
3. Thermistor probe connected to a circuit to
measure the temp of the sample.
4. Galvanometer that displays the freezing curve.
5. Potentiometer with direct read out.
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