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Page 1 BODY WATER & OSMOLALITY Ola H. Elgaddar MBChB, MSc, MD, CPHQ, LSSGB Lecturer of Chemical Pathology Medical Research Institute Alexandria University [email protected]

Body water and Osmolality

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Page 1: Body water and Osmolality

Page 1

BODY WATER &

OSMOLALITY

Ola H. Elgaddar

MBChB, MSc, MD, CPHQ, LSSGB

Lecturer of Chemical Pathology

Medical Research Institute

Alexandria University

[email protected]

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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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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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BODY

WATER

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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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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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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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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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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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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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Osmotic Pressure

???

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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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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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Gibbs-Donnan Equilibrium

when a semipermeable membrane separates non-

diffusible 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

the membrane (membrane potential).

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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

and three sodium ions out of the cell

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OSMOLALITY

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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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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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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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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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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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Osmolality and Osmolarity

Which is the more exact expression;

Osmolality or Osmolarity?

Which has higher osmolality; Nacl or

glucose?

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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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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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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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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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Calculation of Plasma Osmolality:

mOsm/Kg =

1.86 [Na+(mmol/L)]+Glucose (mmol/L)

+urea (mmol/L)+9

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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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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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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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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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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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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