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How much is total body water?

Total body water is approximately 60% of body weight


In who is total body water largest?

TBW is highest in newborns and adult males and lowest in adult females and in adults with a large amount of adipose tissue


What are the body fluid compartments?

TBW is divided into intracellular fluid (ICF) and extracellular fluid (ECF).

ICF is two thirds of TBW or 40% of body weight.
ECF is one third of TBW or 20% of body weight, and is composed of interstitial fluid and plasma.


How is the extracellular fluid further divided?

ECF is further divided into interstitial fluid (3/4 ECF) and plasma (1/4).

The composition of the interstitial fluid is the same as that of plasma, except that it has less protein. Thus interstitial fluid is an ultrafiltrate of plasma.

The major plasma proteins are albumin and globulins.


What are the major cations and anions of the ECF and ICF?

ECF - Na+, Cl- and bicarbonate
ICF - K+, Mg+, protein and organic phosphate


What is the 60-40-20 rule?

This helps to remember the relative sizes of each fluid compartment relative to body weight.

TBW is 60% of total body weight.
ICF is 40% of total body weight
ECF is 20% of total body weight (3/4 interstitial, 1/4 plasma)


What is osmosis?

Osmosis is the passive movement of solvent (water) across a semi-permeable membrane to a region of higher concentration of solute.

The ECF and ICF have different solute of import. Osmosis is the process by which fluid equilibrates between the body fluid compartments.

The rate of solvent movement depends on the number of particles of solute rather than the molecular mass.


What is effective osmotic pressure?

Osmotic pressure is the hydrostatic pressure that must be applied to the solution of greater concentration to prevent water movement across a semi-permeable membrane separating the two aqueous solutions of unequal concentration.

So, the movement of solvent by osmosis can be considered as moving down an osmotic pressure gradient.


What is the difference between osmolaRity and osmolaLity?

Osmolarity = the number of osmoles per litre
Osmolality = the number of osmoles per Kg

These two terms are often used interchangeably because 1L of water weighs 1Kg. However, they do not mean the same thing when discussing solvents other than water.

Osmotic activity is expressed in osmoles - 1 osmole of solute dissolved in 1Kg of solvent has an osmolality of 1 osmol/L


How is plasma osmolality calculated? What is a normal value? Why is it important?

Plasma osmolality = 2Na + Glucose + Urea = 290 mosmol/Kg

At a steady state, ECF osmolality = ICF osmolality. To achieve this, water moves by osmosis between the ECF and ICF compartments. It is assumed that solutes such as NaCl and mannitol do not cross cell membranes and are confined to the ECF.

Notice that protein concentration does not determine plasma osmolality. That is because we are interested in osmotic pressure not colloid oncotic pressure.


What is tonicity?

Tonicity refers to the osmolality of a solution relative to plasma. There are 3 different solutions under this definition.
1) Hypotonic - e.g. distilled water
- lower osmolality compared to plasma
- water moves from plasma into cells (causing red cell lysis)

2) Hypertonic - e.g. twice normal saline
- causes red cells to shrink

3) Isotonic - e.g. 0.9% normal saline
- identical osmolality to plasma so no fluid shift


What are isotonic fluid disorders?

Disorders where there is isotonic loss or addition of fluid.
1) Isotonic fluid loss:
= net isotonic loss of Na+ and water
- POsm and serum Na+ are normal (hypovolaemic normonatraemia)
- No osmotic gradient or fluid shift exists between compartments; ECF volume contracts, ICF unchanged
- signs of volume depletion are present
- e.g. adult diarrhoea, secretory type
- Rx: infusion of normal saline

2) Isotonic fluid gain
= net isotonic gain of Na+ and water
- POsm and serum Na+ are normal (hypervolaemic normonatraemia)
- No osmotic gradient or fluid shift; ECF volume expands, ICF unchanged
- pitting oedema and body cavity effusions may be present
- e.g. excess infusion of normal saline


What are hypotonic fluid disorders?

These are disorders where the underlying pathology makes the plasma hypotonic (decreased osmolarity). As a result, an osmotic gradient is present so fluid shifts into the ICF compartment (expands). HYPOnatraemia is always present:
- hypertonic loss of sodium (more sodium is lost relative to water so osmolarity decreases)
- gain of pure water
- hypotonic gain of sodium (more water is gained relative to sodium)


What causes a hypertonic loss of sodium?

This is net loss of sodium in excess of water (more sodium is lost than water) - i.e. a hypotonic fluid disorder
POsm and serum Na+ are decreased (hypovolaemic hyponatraemia)
ECF volume cotracts; ICF volume expands
Signs of volume depletion are present
e.g. thiazide diuretics, Addison's disease, 21 hydroxylase deficiency (loss of mineralocorticoids)
Rx: infuse normal saline


What is central pontine myelinolysis?

In an alcoholic, rapid i.v. correction of hyponatraemia with saline may result in central pontine myelinolysis, an irreversible demyelinating disorder. However, as a general rule, all i.v. replacement of sodium containing fluids should be given slowly over the first 24 hours regardless of the underlying cause.


What causes gain of pure water?

This is a net gain in water.
POsm and serum Na+ are decreased (euvolaemic hyponatraemia)
Expansion of both ICF and ECF compartments
Normal skin turgor, because total body Na+ is normal
e.g. SIADH, psychogenic polydipsia
Rx: water restriction


What causes hypotonic gain of sodium?

This is a net gain of water in excess of sodium (more water is gained than sodium)
POsm and serum Na+ decrease (hypervolaemic hyponatraemia)
Expansion of both compartments
This type of fluid gain produces pitting oedema and body effusions associated with Starling force alteration, e.g.
- RHF with an increase in venous hydrostatic pressure
- Cirrhosis and nephrotic syndrome with a decrease in plasma oncotic pressure
Rx: restrict water and sodium, diuretics


What are hypertonic fluid disorders?

An increase in POsm is most often due to hypernatraemia or hyperglycaemia. In these disorders and osmotic gradient exists so water shifts from the ICF to the ECF compartment causing expansion. Hypernatraemia is always a feature.


What causes a hypotonic loss of sodium?

Definition - net loss of water in excess of Na+ (more water is lost than Na+)
Both POsm and serum Na+ increase (hypovolaemic hypernatraemia)
Both compartments contract
Signs of volume depletion are present
e.g. sweating, osmotic diuresis (e.g. glucose, mannitol), diarrhoea (osmotic type - laxatives), diuretics and vomiting
Rx: isotonic saline if hypotension is present and then switch to oral replacement or more hypotonic Na+ containing i.v. fluids


What causes loss of pure water?

There is net loss of water.
Both POsm and serum Na+ are increased (euvolaemic hypernatraemia)
Both compartments are contracted
- ECF contraction is mild because there is no loss of Na+
e.g. diabetes insipidus (loss of ADH or refractory ADH), insensible water loss (e.g. fever)


What is hypertonic gain of sodium?

Definition - net gain of Na+ in excess of water (most sodium is gained than water)
Both POsm and serum Na+ increase (hypervolaemic hypernatraemia)
ECF compartment expands; ICF contracts
Pitting oedema and body cavity effusions may be present
e.g. infusion of sodium bicarbonate or Na+ containing antibiotics


What are the normal maintenance fluid requirements of a nil by mouth patient?

Depends on the patients weight...
Water = 1.5ml/kg/hour
Na+ = 1-2mmol/kg/24 hour
K+ = 0.5-1mmol/kg/24 hour

Urine output should be >0.5ml/kg/hour

These values are important to know in order to work out what a patient needs for maintenance and how much of the available fluids to give to match this.

Overall requirements = MAINTENANCE + REPLACEMENT


What is Hartmann's solution? When is it best used?

Hartmann's solution is the most physiological (i.e. similar to plasma) meaning it is very good for replacing plasma loss e.g. during surgery or GI losses. However, this does not mean that it is good for maintenance fluids, as 3L of Hartmann's solution over 24 hours would give 3 times too much Na+ (1L contains 131mmol Na+) and not enough K+ (1L contains only 5mmol when the 24 hour requirement is 35-70mmol).


What is the composition of normal saline?

Saline is much more physiological than dextrose as it contains salts, but not as physiological as Hartmann's solution. Also be warned, too much chlorine will give a hyperchloraemic acidosis and also cause renal vascoconstriction. 0.9% NaCl contains 154mmol of Na and Cl.


What is 5% dextrose?

5% dextrose contains no salt but 50g of glucose. It is given instead of pure water (the glucose is used up) to maintain osmolarity. The glucose content plays no role whatsoever. It is used for maintenance to give water when needed with no electrolytes. It has to place for replacing plasma/ blood loss because it is not physiological. Too much too quickly can cause hyponatraemia.


What id dextrose-saline?

This contains some dextrose and some sodium chloride. It is a good solution for maintenance fluids because, given at the correct rate for the patients weight, it contains approximately the correct requirements of sodium. But it should not be used to replace plasma/ blood loss because it is not physiological.


How should hydration be assessed?

Hydration status should always be assessed prior to prescribing fluids. It is important to assess blood pressure, capillary refill time, fluid balance charts, skin turgor and weight.


How does 0.9% NaCl distribute between the body fluid compartments?

The fluid distributes between the intravascular and extravascular compartments according to the ratio of their relative concentrations in the extravascular fluid:
- 25% of the volume will go to the intravascular compartment
- 75% will go to the interstitial compartment


If a patient lost 1L of blood, what is the equivalent volume of normal saline that would be required to replace it?

As only 25% of administered normal saline enters the vascular compartment 4L will be required to expand the plasma sufficiently to replace the fluid lost.


How does 5% dextrose distribute between the body fluid compartments?

Dextrose is rapidly metabolised by the liver to produce pure water. It is isotonic with plasma. As such 2/3 goes to the intracellular fluid, 1/3 goes to the extracellular fluid, and of this 1/3 approximately 80ml (of a 1000 given) will stay in the vascular compartment.