Crystalloids: Definition & Examples

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  • 0:04 Crystalloids
  • 1:41 Types Of Crystalloid Solutions
  • 2:05 Isotonic Solutions
  • 2:43 Hypertonic Solutions
  • 3:22 Hypotonic Solutions
  • 4:01 Lesson Summary
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Lesson Transcript
Instructor: Sheila Bouie
This lesson focuses on defining crystalloids and fluid tonicity. Examples of IV solutions used in clinical practice are also discussed. By the end of this lesson, you'll also be able to explain the categories of crystalloids.


There are quite a number of intravenous (IV) fluids used in clinical therapy, and the type of fluid is selected based on the patient's condition. Crystalloids are one type of fluid used often. In clinical practice we use crystalloid solutions for fluid replacement, to maintain a steady state, and to help the body achieve different outcomes. To do this, crystalloid solutions contain electrolytes (namely sodium and chloride) and non-electrolytes (namely dextrose). Understanding how crystalloid solutions work in the body is an important part of therapy.

Crystalloids are solutions in water that dissolve easily and may form crystals. Think of adding a couple spoonfuls of sugar to your tea. When you stir your tea, the sugar dissolves, and you can't actually see it in the tea. However, if you add too much sugar and the tea evaporates, you will be left with sugar crystals at the bottom of your glass.

A crystalloid solution has particles of molecules dissolved in water; typically these molecules are salts or sugars. The molecules in crystalloids are small, and they can pass through semipermeable membranes with relative ease. Semipermeable means that some molecules, usually the very small molecules, easily cross the membranes, but larger ones do not. Just imagine you squeeze some fresh lemon into your properly sweetened tea for extra flavor. Now imagine your tea being poured through a paper coffee filter. Nothing stops the flow of tea, right? This simulates how easily the small molecules in a crystalloid move across the semipermeable membranes. It also simulates how the larger molecules like the pulp from the lemons do not cross the membrane.

Types of Crystalloid Solutions

Crystalloids are known by their composition and/or tonicity. Tonicity refers to the relative concentration of particles in a solution. There are three tonic states: isotonic, hypertonic, and hypotonic. The terms isotonic, hypertonic, and hypotonic indicate the concentration of molecules dissolved in water.

Isotonic Solutions

If a crystalloid solution is very close to the normal body fluid composition, this is known as an isotonic solution. Isotonic solutions are those that mimic the body's fluid composition in electrolytes and water. An isotonic solution may be used for fluid replacement or to help maintain a steady body state. Isotonic solutions of 0.9% sodium chloride (also known as normal saline) and Lactated Ringer's solution are common. Advantages of using isotonic solutions for fluid replacement are that they are readily available, less likely to cause adverse reactions, and inexpensive.

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