Fluid replacement
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.
'third space' is also a term used to refer to, for example, the bowel with an ileus, and the collection of fluid therein, usually post-operatively.  The interstitial space could be considered to be a sub-section of the extracellular compartment.  The distinction one could make is that fluid in the interstitium is still readily available to either the intravascular or the intracellular compartments, in response to ionic changes, whereas that which is pooled in the bowel is not so readily available.  Hopefully someone can clarify this point.
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.

# Solutions
The choice of fluids for patients with sepsis has been reviewed with network meta-analysis which concluded that balanced crystalloids (lactate and acetate solutions) or albumin might be best at reducing mortaltiy. PlasmaLyte is an example of an acetate solution and Hartmann's and Lactated Ringer's are examples of lactate solutions. Solutions lower in chlorides such as Hartmann's solution and Lactated Ringer's solution, may cause less acute kidney injury when administered to adults in intensive care. Likewise hydroxyethyl Starch should be avoided. There were no direct comparisons between balanced crystaloids and saline for the outcome of mortality.
A more recent systematic review that focused on patients with sepsis, found that crystaloids are better than starch, but no clear difference between balanced crystaloids and saline regarding reduction in renal replacement therapy.. There were no direct comparisons between balanced crystaloids and saline for the outcome of renal replacement therapy.
More recent randomized controlled trials did not find a benefit from balanced solutions:
- SPLIT Randomized Trial
- SALT Randomized Trial