[Noah930]

Not a veterinarian, but usually, the choice of fluid is based upon whatever condition you're trying to treat.

Normal sodium (NS) is just water with sodium and chloride.

Lactated ringers (LR) has sodium and chloride (though in slightly different concentrations than in NS), potassium, lactate, and calcium. In general, LR is a bit closer to what blood is like (obviously minus the blood cells).

And then there are 1/2 NS and 1/2 LR concentrations, too.

For healthy people (and animals), it probably doesn't matter which fluid is given. Our kidneys and general metabolism will figure it all out. If your cat is at home, it probably won't make a huge difference what fluid gets injected subcutaneously. But for sick creatures (sick enough to be hospitalized), the fluid given usually reflects what we're trying to replace. Fluid loss from dehydration vs blood loss vs excessive urine output vs diarrhea vs gastric suctioning, etc. Does the patient have kidney insufficiency/failure? What are their labs (bloodwork) like? Those are the aspects that go into determining what IV fluid gets selected.