There are two main types of volume expander; crystalloids and colloids. Crystalloids are aqueous solutions of mineral salts or other water-soluble molecules. Colloids contain larger insoluble molecules, such as gelatin. Blood is a colloid.
- Colloids preserve a high colloid osmotic pressure in the blood, while, on the other hand, this parameter is decreased by crystalloids due to hemodilution. However, there is still controversy with regard to the actual difference in efficacy between colloids and crystalloids. Crystalloids generally are much cheaper than colloids.
- The most commonly used crystalloid fluid is normal saline, a solution of sodium chloride at 0.9% concentration, which is close to the concentration in the blood (isotonic). Lactated Ringer's (also known as Ringer's lactate) and the closely related Ringer's acetate, are mildly hypotonic solutions often used for.
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