Fluid and Electrolyte Therapy

Osmosis — passive transport of water across a semipermeable membrane. Water tends to move across the membrane until the solute concentration (the amount of solids) on both sides are the same. When water concentration is high in one compartment, there will be a net diffusion of water into the other compartment and also a diffusion of solutes to equalize the osmolarity in the two compartments. The volumes will not change.


Osmotic Pressure — the tendency for water to diffuse to higher solute concentration can be measured as a pressure:

a) Hypertonic Solution — the solution around the cell has a higher [solute] (called osmolarity) than the cytoplasm ==>
water diffuses out of the cell and the volume of the cytoplasm decreases
affects both cells with and without cell walls. cells lose water and shrink



b) Isotonic Solution — solution around the cell has the same osmolarity as the cytoplasm ==> no net movement of water. At osmotic equilibrium, the osmolarity of the intracellular and extracellular fluids are the same. A fluid into which a cell can be placed without causing either swelling or shrinkage is said to be ISOTONIC with the cells. Iso-ionic refers to a similar concentration of a particular electrolyte in a solution compared to extracellular fluid; therefore solutions which are iso-ionic with regard to sodium will have sodium concentrations comparable to the sodium concentration of extracellular fluid and plasma.




c) Hypotonic Solution — the solution around the cell has a lower osmolarity than the cytoplasm ==> water diffuses into the cell; water moves into the cells and they swell

Distribution of Fluids



Water in the body is in a constant state of motion, shifting between the three major fluid compartments of the body. These are:

1. Blood plasma (intravascular volume) (the fluid outside cells or extracellular fluid)

2. Interstitial fluid (the fluid outside cells or extracellular fluid)

3. Intracellular fluid (the fluid inside cells)

Normal Body Water Distribution

Dehydration
The majority (probably >90%) of dehydration cases have sustained isotonic losses of fluids. Gastrointestinal and urinary losses of fluids result in the loss of water AND electrolytes including sodium, potassium, chloride and bicarbonate. The amount and type of electrolytes lost depend on the disease that is causing the abnormal loss. The fluid and electrolytes which are lost originate initially from the extracellular (plasma water + interstitial water – fluids outside of cells) compartment.

Approximately 90% of the body’s water intake comes via the gastro-intestinal tract. The remaining 10% is called metabolic water and is produced as the result of various chemical reactions in the cells of the body’s tissues.

Once an ingested meal enters the GI tract, fluids shift from the plasma, making the contents isotonic. Water movement into and out of the GI tract follows the osmotic gradients created by electrolyte movement. For this reason, providing solid food to a dehydrated animal is contradicted, and could prove fatal.

Rehydration
	1. excess fluids are added to extracellular fluid compartments 2. (interstatial fluid and plasma) 3. Cell membrane 4. and water moves to intercellular compartment by osmosis

Dehydration
	1. water loss from extracellular fluid compartments 2. (interstatial fluid and plasma) 3. Cell membrane 4. water leaves intercellular fluid compartment by osmosis

Estimating the percentage dehydration based upon physical
examination findings.

Fluid Replacement

When designing a fluid therapy plan one needs to decide for each of the three constituents: How much fluid to use, Route of administration, Which replacement fluid to use.