The administration of intravenous (IV) fluids is a common medical treatment used to correct dehydration, restore electrolyte balance, and manage circulatory volume. These solutions are broadly classified based on their chemical composition and how they interact with the body’s fluid compartments. This classification determines the specific therapeutic use of each fluid type. The properties of these fluids—specifically their molecular size and dissolved contents—dictate their movement within the body, which directly impacts their clinical effectiveness.
The Two Main Types of IV Fluids
The two primary categories of IV fluids are crystalloids and colloids, distinguished by the size of the particles they contain. Crystalloids are aqueous solutions composed of water and small, water-soluble molecules, such as mineral salts or sugar. Because these molecules are tiny, they easily pass through the semipermeable membranes separating fluid compartments. Common crystalloids include normal saline, lactated Ringer’s, and dextrose solutions.
Colloids, in contrast, contain larger, insoluble molecules like proteins or synthetic polymers, which are too big to readily cross capillary walls. This means colloids tend to remain predominantly within the intravascular space (the blood vessels) for a longer period. They are generally less frequently used than crystalloids and are more expensive.
Normal Saline: Composition and Classification
Normal Saline (NS) is classified as a crystalloid fluid, consisting of a simple aqueous solution of sodium chloride (NaCl) in water. It is designated as 0.9% Saline, containing 9 grams of salt per liter. This concentration results in an ionic composition of 154 milliequivalents (mEq) of sodium and 154 mEq of chloride per liter. This chemical makeup gives Normal Saline an osmolarity of approximately 308 milliosmoles per liter (mOsmol/L). Since this value is very close to the osmolarity of human plasma, Normal Saline is classified as an isotonic crystalloid.
How Crystalloids Distribute Fluid in the Body
The functional consequence of Normal Saline being a crystalloid is its rapid and extensive distribution throughout the body’s extracellular fluid space. When an isotonic crystalloid is infused into a vein, it initially enters the intravascular compartment. Because the small ions easily move across the capillary membranes, the fluid quickly shifts out of the blood vessels and into the interstitial space.
This rapid redistribution means that only a small portion of the infused volume remains in the bloodstream to expand plasma volume shortly after the infusion is complete. Studies show that within 30 minutes of a crystalloid infusion ending, only about 15% to 20% of the total volume remains in the intravascular space. This mechanism explains why a much larger volume of crystalloid fluid is often needed compared to colloids when the therapeutic goal is to restore overall body fluid.