It is necessary to clarify certain terms which are relevant for the discussion of compartment specific losses. The type of fluid that is lost from the body, as well as the fluid compartment from which it is lost is important for determining a strategy for treating the patient. When fluid losses exceed fluid intake a fluid imbalance develops. These imbalances may be evident on physical examination, laboratory data, or assumed based on signalment and history. Disruption in the balance between fluid intake and loss can result in disorders of hydration and perfusion. Disease states can lead to fluid losses from additional sources such as pyrexia, cutaneous injury, gastrointestinal disease, respiratory disease, third spacing of fluid, and hemorrhage. Fluid is lost from the body under normal conditions from the respiratory system, the feces, and the urine. Some water is also produced via metabolic processes. Fluid enters the body in the form of water consumed by the animal. In addition to maintaining a balance between the fluid compartments within the body, the animal must also maintain fluid balance with the outside world. ![]() The activity of this pump maintains a high extracellular sodium concentration and a low extracellular potassium concentration, which is evident in the electrolyte composition of plasma in the healthy patient. The ECF and ICF have very distinct ionic compositions which are maintained primarily through the activity of the sodium / potassium ATPase pump. Fluid movement across these membranes depends on the characteristics of the membrane, hydrostatic pressure, colloid oncotic pressure, and osmolarity. The endothelial membrane and endothelial glycocalyx separate the intravascular from the interstitial space and the cell membranes separate the ICF from the ECF. These compartments are separated by membranes. Twenty percent of body weight is extracellular fluid. Forty percent of body weight is intracellular fluid. The easiest way to remember these proportions is the 60:40:20 rule. The extracellular fluid is further compartmentalized into the interstitial fluid which makes up ¾ of the ECF and the intravascular fluid which comprises the remaining ¼ of ECF. The remaining third of total body water is contained in the extracellular fluid (ECF) compartment which makes up roughly 15-30% of body weight. ![]() The intracellular fluid (ICF) makes up 40% of body weight, or two-thirds of total body water. It is the major component of all body fluids and is distributed into distinct compartments within the body. Water has a multitude of roles within an animal’s body including transport of red blood cells, electrolytes, and nutrients, carrying substrates across cellular membranes, facilitating evaporative cooling, acting as a solvent, and participating in metabolic functions and reactions. This composition will vary with age, sex, species, and body condition score. Sixty percent of an adult animal’s body weight is water (referred to as total body water). Having a greater understanding of the fluid balance within the body, the effect of different disease states on this fluid balance, and the fluid choices available for treating our patients is vital to maximizing patient benefit, while minimizing side effects of therapy. Though it has great ability to help our patients, like any medical intervention, it has the potential to do harm as well. Massachusetts Veterinary Referral Hospital, Woburn, MAįluid therapy is a crucial part of the treatment of patients in the emergency room and the critical care unit.
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