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 shrinkDistribution 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
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.
Estimating the percentage dehydration based upon physical examination findings.
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.