IV fluids can be classified as hypo, hyper, or isotonic… but to what? The blood cells? Or other body cells? How do IV fluids even work in the body, and under what conditions would someone need a specific IV solution?

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IV fluids can be classified as hypo, hyper, or isotonic… but to what? The blood cells? Or other body cells? How do IV fluids even work in the body, and under what conditions would someone need a specific IV solution?

In: Biology

Tonicity is relative to the physiological osmolarity. As cell membranes are permeable to water amongst other things, the intra and extracellular compartments are approximately equal in osmolarity. So blood, blood cells, interstitial fluid, and other cells are usually all of the same osmolarity. There are exceptions, like urine.

IV is just a route. You deliver a solution intravenously, via veins. There’s intraperitoneal, subcutaneous, epicutaneous, intradermal, transdermal, intramuscular, transcorneal, intraocular, intraventricular, intrathecal, intraoasseous, intranasal, intratracheal, etc… Etc.. Why IV? Well if what you want to deliver is unstable outside blood, needs to be delivered quickly, need the dose to be precise in amount or in time, etc. then IV is your optimal approach. Once something is in the blood, it has entered circulation, it will move all around the body very quickly, depends on what’s in the solution, the injected things either get taken up by specific cells or in general any cells or just help fluid or ion balance in the body.

You’re mostly made of water, but it’s not pure water, it’s a little salty. Adding pure water is generally bad, which is why you get a “saline drip.”

Isotonic is the same saltiness as the rest of you, hypertonic is more salty than you, and hypotonic is less salty than you. They can pick what to give you to change how salty you are for therapeutic reasons.

Hypo- means below, or less than. A hypotonic solution has a lower concentration of dissolved substances (such as salt) than the inside of the cells it is in contact with.

Hyper- means above or more than. A hypertonic solution has a higher concentration of dissolved substances (such as salt) than the insides of the cells it is in contact with.

Iso- means equal or the same as. So, an isotonic solution has the same concentration of solutes (the same salinity, for example) as the insides of the cells it contacts.

Water, through osmosis, will travel from the area of greater concentration of water to the area of lesser concentration of water. Therefore, blood cells in a hypertonic solution will shrivel up, blood cells in a hypotonic solution will explode, and blood cells in an isotonic solution will be fine.