Body water, fluid compartments, and third spacing (Proceedings)
Unlike the popular home and garden channel TV shows that boast remodeling of room or house structures, remodeling of fluid compartments within the critical care patient is not the effect a clinician desires. Unfortunately, many of our critical patients have trauma or disease processes that result in an imbalance between fluid compartments.
Total body water and fluid compartments
For the average adult, nonobese dog or cat, total body water is approximately 60% of body weight. Geriatric pets will have less while neonates have considerably more, up to 75% of body weight total body water. Body water is distributed between two compartments known as intracellular fluid (ICF) space and extracellular fluid (ECF) space. The larger of the two is the intracellular compartment comprising two-thirds of total body water and 40% of body weight. The extracelluar compartment holds the remaining one-third of total body water and 20% of body weight. The extracellular compartment is further divided into intravascular fluid (plasma within the vessels) and interstitial fluid (between the cells). The interstitium, simply defined, is the connective structures and lubricating fluids located between cells. The majority of extracellular fluid is contained within the interstitium.Fluid movement
Normal movement of fluid between these compartments is governed by the permeability of the particular tissue membrane and the concentration of molecules located across the membrane barrier. The compartments all contain solutes dissolved in water. Quantity and size of the solutes is an important determinate to the volume of body fluid within the compartment. Body water flows fairly freely in most compartments and moves from areas of lower concentration of solutes into areas of higher solute concentrations. Electrolytes may travel freely between intravascular and interstitial compartments, but they require a transport system to move into cells. Cellular membranes contain ion channels and solute pumps that exchange electrolytes into and out of the cells. The ATPase pump is the most important of these pumps that function to exchange sodium out of the cell in exchange for potassium into the cell. This pump generates the electrochemical gradient across the cell membrane. Macromolecules (>20,000 daltons in size) cannot easily cross the normal vascular membrane, and they attract smaller charged particles. Such naturally occurring macromolecules within the vascular space are albumin, globulin, and fibrinogen.
COP, or colloid osmotic pressure, is the force generated by the concentration of impermeable solutes within the intravascular space that helps to retain fluids. COP is a measurable value of the macromolecular solutes contained in a serum sample. Hydrostatic pressure is an outward pressure within the confines of the vasculature and the natural tendency of water to move out of the fluid compartment. Osmotic pressure is an attractant draw of water into an area of higher concentrations of solutes; thus providing a pull of fluid into a compartment. Explaining these in simple terms would be as follows. Hydrostatic pressure can be described as the pressure of a water hose, full of water, but contained at the end by a trigger sprayer. The water would be pushing out against all sides of the hose trying to move outward. And osmotic pressure can be described as the attraction children feel when standing near a small candy display in a grocery store, but looking at a very large selection of candy on the other side of the store. The larger supply of candy would pull more children to that side of the store, just as a larger concentration of solutes would pull water out of a compartment.
Overall, fluid exchange between the intravascular and extracellular compartments is governed by a balance between hydrostatic pressure and osmotic pressure gradients. During times of good health, the forces balance that favor reabsorption of fluid into the vascular compartments (increased COP or decreased hydrostatic pressure) with forces that favor filtration out of the vascular compartments (decreased COP or increased hydrostatic pressure). During disease states increased fluid losses, decreased fluid intake, and fluid maldistribution (shifting) within compartments will lead to dehydration. When forces do not balance, fluid may accumulate in areas of the body. Edema may form which is an abnormal accumulation of fluid in the interstitium; and "third spacing" which refers to an abnormal accumulation of fluid within body tissue or a body cavity may occur.