Sodium chloride and saline are related concepts, but they are not the same thing. Sodium chloride (NaCl) is a pure chemical compound, the salt itself, while saline is a specific type of solution where that compound has been dissolved in water. The distinction lies in the difference between an ingredient and a carefully prepared mixture, where the concentration of the ingredient defines the mixture’s properties and application. Understanding this relationship is fundamental to comprehending the role of these substances, especially in medical and biological contexts.
Defining the Chemical Compound and the Solution
Sodium chloride is an ionic compound composed of sodium ions (\(\text{Na}^{+}\)) and chloride ions (\(\text{Cl}^{-}\)) in a 1:1 ratio. It exists as a colorless, crystalline solid and is commonly known as table salt. In the human body, these ions are electrolytes that play a central role in maintaining fluid balance, nerve conductivity, and muscle function.
Saline is an aqueous solution containing sodium chloride, or salt water. For medical use, the solvent is highly purified water, and the sodium chloride solute must be sterile and free of contaminants. Common table salt often contains anti-caking agents or iodine, making it unsuitable for injection or delicate medical procedures. Therefore, preparing medical saline requires strict control over the purity of both the salt and the water.
Understanding Concentration: The Isotonic Standard
The most common form of saline used in medicine is known as “normal saline,” which is defined by a precise concentration of 0.9% sodium chloride. This means that 9 grams of NaCl are dissolved in every liter of water. This specific concentration is the reason normal saline is considered “isotonic.”
Isotonic refers to a solution with an osmotic pressure comparable to the body’s fluids, such as human blood plasma. The osmolality of 0.9% saline is approximately 308 milliosmoles per liter (\(\text{mOsm/L}\)), closely matching plasma osmolality (around 288 \(\text{mOsm/kg}\)). This match is biologically important because it prevents the movement of water across cell membranes.
The mechanism behind this is osmosis, the movement of water across a semipermeable membrane to balance solute concentrations. When an isotonic solution is administered intravenously, it minimizes the net flow of water into or out of the body’s cells, particularly red blood cells. If a solution is too dilute, red blood cells could swell and rupture; conversely, if too concentrated, the cells would shrink. The isotonic standard ensures the fluid maintains cell integrity while replacing lost volume and electrolytes.
Practical Applications of Varying Saline Solutions
Not all saline solutions are isotonic; variations in concentration allow them to serve different therapeutic purposes by manipulating the osmotic gradient. These varying solutions are classified based on their tonicity relative to human plasma. Isotonic 0.9% saline is widely used for routine intravenous hydration, volume expansion in cases of hypovolemia, and as a solvent for dissolving medications. It is also the standard for wound irrigation and is often used in contact lens solutions.
Hypotonic solutions contain a lower concentration of sodium chloride than 0.9%, such as 0.45% saline, often called half-normal saline. Since they have a lower solute concentration than the blood, they cause water to move out of the vessels and into the cells and surrounding tissues. These solutions are used to help rehydrate cells following conditions that cause water loss exceeding salt loss, such as hypernatremia.
Conversely, hypertonic solutions have a salt concentration greater than 0.9%, such as 3% or 7% sodium chloride solutions. These solutions possess a higher concentration of solutes than the body’s fluids, drawing water out of the cells and into the blood vessels. Hypertonic saline is administered to reduce swelling in specific tissues, such as the brain (cerebral edema), by pulling excess fluid out of the cells. Using these concentrated solutions requires careful monitoring due to the potential for rapid fluid shifts.