Fluid Management - Part I (water balance, osmolarity, tonicity)

Water Balance :
- The total amount of fluid or water is called the total body water, which accounts for 50% to 70% of body weight.
o Eg. 70 kg man whose total body water is 65% of his body weight has 45.5 L of water.
- Total body water correlates inversely with body fat. Eg. More total body water, lower body fat, vice versa.
- Total body water is distributed between 2 major body fluid compartments: intracellular fluid (ICF) and extracellular fluid (ECF)
o ECF (1/3 of total body fluid)
- Plasma
- Interstitial fluid
o ICF (2/3 of total body fluid)
- Fluid within the cells


Site of water loss :
- Skin
- Urinary tract
- Gastrointestinal
- Respiratory airway

Daily intake of water :
- Ingested in the form of liquids or water in the form of food.
- Synthesized in the body as a result of oxidation of carbohydrate.

Daily loss of water :
• Insensible loss
• Minimal sweat loss
• Faecal loss
• Minimal urine volume to excrete solute load: 500 mls

Osmolarity :
- Osmole – number of particles into which a solute dissociates in solution
- Osmolarity - concentration of osmotically active particles in solution expressed as osmoles per litre.
- It is necessary to know the conc. Of solute and whether the solute dissociates in the solution.
- Eg. Glucose does not dissociate in solution, osmolarity = molarity
- Eg. NaCl dissociates into 2 particles, osmolarity x number of particles in solution
- Osmolarity = g C
o g- number of particles per mole in solution (Osm/mol)
o C – Concentration (mmol/L)

Electroneutrality of body fluid compartments :
- Each body fluid compartment must obey the principle of electroneutrality
o Each compartment must have the same conc. In mEq/L of positive charges (cations) as of negative charges (anions)
o ECF : cation, Na+ / balancing anions, Cl- and HCO3-
o ICF : cation, K+ and Mg2+ / balancing anions, proteins and organic phosphate
o Conc. Differences for individual solutes, the total solute conc. (osmolarity) is the same in ICF and ECF, equality is achieved because water flows freely across cell membranes.
- Water intake in excess of requirements is excreted as urine.
- As daily solute load increases, the maximum urine osmolality decreases until, at high solute loads, it is the same as plasma osmolality.
o This occurs because the increased urine flows necessary to carry the increased solute washes out the medullary osmoles and the time spent in the tubules is decreased.

Tonicity :
- Tonicity refers to the ability of a solution to affect the fluid content and pressure of cells.
- An isotonic solution has the same effective osmolarity of about 300mOsm as intracellular fluid and would not cause any net movement of water in or out of cells.
o A 0.9% solution of sodium chloride (150mM), generally known as normal saline, is an isotonic solution and causes no change in cell volume or shape.
- A hypotonic solution has a lower osmolarity than intracellular fluid, and this would cause water to move into the cells by osmosis.
- A hypertonic solution such as sea water has more non-permeating solutes than intracellular fluid and causes cells to lose water and shrivel or crenate (take on a spikey appearance).
- Water moves so freely through cell membranes by osmosis such that osmotic equilibrium is established within less than a minute after ECF osmolarity first deviates from normal.