Fluid replacement or fluid resuscitation is the medical practice of replenishing bodily fluid lost through sweating, bleeding, fluid shifts or other pathologic processes. Fluids can be replaced via oral administration (drinking), intravenous administration, or hypodermoclysis, the direct injection of fluid into the subcutaneous tissue. Fluids administered by the oral and hypodermic routes are absorbed more slowly than those given intravenously.

In severe dehydration, intravenous fluid replacement is preferred, and may be lifesaving. Physiologic normal saline, or 0.9% sodium chloride solution, is often used because it is isotonic, and therefore will not cause potentially dangerous fluid shifts. If given intravenously, normal saline remains in the circulation, boosting blood pressure and preventing the complications of inadequate circulation.

Fluid shifts occur when the body's fluids move from the intracellular space into the intravascular space, or vice versa. Many medical conditions can cause fluid shifts. When fluid moves out of the intravascular space (the blood vessels), blood pressure can drop to dangerously low levels, endangering critical organs such as the brain, heart and kidneys. When fluid shifts out of the cells (the intracellular space), cellular processes slow down or cease from intracellular dehydration.

The third space where bodily fluid resides is the interstitial space, or the space between the cells within the tissues. Depletion of this space is not life-threatening, and usually accompanies depletion of the other spaces. Extensive tissue swelling occurs when the third space fills with excess fluid. If this process robs the intracellular and intravascular spaces of fluid volume, a type of shock called distributive shock ensues. Severe allergic reactions are an example of this type of shock. Sepsis and some neurologic conditions can also cause distributive shock, although by other mechanisms.

Bleeding, extensive burns and excessive sweating (as from a prolonged fever) deplete both the intracellular and the vascular spaces. Immediate swift replenishment of fluid via an intravenous line (or several) is required. Initial treatment of trauma and burn victims places high priority on aggressive fluid replacement to maintain organ perfusion.