Hypotonic IV Solutions

If you’re looking for a list of IV solutions to memorize, then you’re in the wrong place.

But if you want to understand WHY and HOW IV solutions work the way that they do so that you can become a better nurse…here you go!

Osmosis Results Part 1 (from left to right):
Hypotonic solution, Isotonic solution,
Hypertonic solution

If you haven’t already seen the results of my osmosis experiment from last week, make sure to check that out, because I’ll be referring to it in all of my posts this week.

Hypotonic solutions contain less solute then blood does, which causes water to want to leave the hypotonic solution and enter an area that has a higher concentration of solute via osmosis.  When discussing IV fluids, that usually means that the water will want to leave the intravascular space and enter Red Blood Cells (RBCs).

The results of my osmosis experiment on the egg placed in the hypotonic solution there, but were not dramatic.  Definitely not as dramatic as the results for the egg placed in the hypertonic solution.  You can see in the picture that the hypotonic egg is only barely larger than the isotonic egg.  Unfortunately, this difference can be much more serious in RBCs.

What happens to a normal Red Blood Cell when it is placed in a Hypotonic solution?

 Notice how the normal disc-like shape of an RBC blows up like a balloon when placed in a hypotonic solution.  Remember, this occurs because the extra water in the blood vessels wants to move into the RBC via osmosis in an attempt to dilute the higher concentrate of solute found there.  Of course, that is not what’s best for the body in this case! If allowed to progress too far, the RBC can actually fill up so much with water that it “pops” and dies. (Side note: that’s a fun way to remember that the term “apoptosis” means cell death, by the way!)

The list of Hypotonic solutions is easy to remember, because it really only includes one true hypotonic solution (plus one “faker”):

  • 0.45% Saline
  • *5% Dextrose in Water (this is technically isotonic, but once the dextrose is absorbed then it acts on the body as if it were hypotonic)

The 5% Dextrose in Water is really isotonic (see more about that in my next article).  However, once the dextrose (aka sugar) has been absorbed by the body, then only plain water is left in the intravascular space.  And plain water is clearly hypotonic, so it can have the same effect on the body as 0.45% Saline.

Typically though, if a patient needs a hypotonic solution, then they will receive 0.45% Saline.  Probably the most common reason to give 0.45% Saline is for true dehydration, which is when the body has lost water only, without losing any electrolytes (this is different from fluid volume deficit, when the body loses BOTH water AND electrolytes).  In dehydration, the body already has a normal amount of electrolytes, so there is no need to provide more in the IV solution.  The patient simply needs some of their water replaced!

I hope this gives you a better understanding of hypotonic IV solutions.  Please leave a comment below to share one new thing that you learned about them!

Comments

  1. Jack says

    Thank you so much for this article! This is the first time I actually understand this topic. You should become a teacher, if you aren’t already.

    • says

      Thank you so much for the compliment! I love teaching, but much prefer to teach through Your Nursing Tutor rather than in a University setting…gives me much more flexibility to focus on the trickiest topics for nursing school, and helps me keep my “finger on the pulse” of nursing students across the country.

  2. Kathy says

    Thank you so much for this. You make it so much easier to understand. I just have one question. When someone is dehydrated, why do we have them drink Gatorade and children Pedialyte? That’s confusing to me:-(

    • says

      Great question. Most often, the dehydration involves loss of water AND electrolytes. And example would be vomiting or diarrhea (especially in kids). Vomiting and diarrhea cause the body to lose water (which can lead to the dehydration), but they ALSO cause the body to lose electrolytes. So we need to use an intervention that will replace both, which Pedialyte or Gatorade can do.

  3. Dory says

    Hello Nicole,
    Your post very nice!!!
    Still, I don’t understand something:
    You wrote: “The 5% Dextrose in Water is really isotonic (see more about that in my next article). However, once the dextrose (aka sugar) has been absorbed by the body, then only plain water is left in the intravascular space. And plain water is clearly hypotonic, so it can have the same effect on the body as 0.45% Saline.”

    I would like to understand why we can not say that about the normal saline for example?
    the electrolytes absorbed and the water is left as hypotonic.

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