The body is composed of fluids that are continually in motion such as water, electrolytes, and other substances. The fluids move in and out of the cell membranes; fluid inside the cells is called intracellular and fluid outside the cell is called extracellular. The maintenance of a proper balance between the intracellular and extracellular fluids is essential to the health of your pet. There are many diseases that disturb this balance, a few examples are heart failure, kidney disease, infections that cause vomiting or diarrhea, and diabetes. It is sometimes necessary to supplement the body with fluids in order to help restore the balance- this is commonly done by subcutaneous (SC) or intravenous (IV) infusion of fluids. The SC method is mainly used in mild cases of fluid loss or dehydration, since a relatively small volume of fluid can be deposited into the subcutaneous space (the space between the skin and the underlying muscle/tissue). For more severe illness or for patients that require blood transfusions, the intravenous (IV) route is used after placement of an IV catheter.