So let’s break it down and make this simple . . . what is the difference between hypertonic vs hypotonic . . .and what is isotonic?
What is all This Tonic?
A quick easy way to remember the difference between Hypotonic and Hypertonic is to think of HYPERtonic as someone who is hyper and works out a lot so they get skinny so the cells will shrink but with HYPOtonic think of a hippo and it wants to eat everything and get really big so the cells will swell.
Let’s start with definitions for some of the most important terms:
Solute: a particle, usually a salt
Solvent: liquid, usually water
Solution: solute and solvent mixed together
Osmolality: measurement of the amount of solute mixed per volume of solvent
Tonicity: measurement of osmotic pressure between two solutions
Osmosis: the process of liquid moving across a semi-permeable membrane
Semi-permeable membrane: a membrane that allows a solvent to pass through it but not a solute
Now let’s define what solutions’ tonicity can be:
Hypotonic (definition): low solute, high solvent
Isotonic (definition): equal solute and solvent ratio
Hypertonic (definition): high solute, low solvent
Next, let’s address a few ideas:
Water follows salt
Solutions want to have the same ratio
Semi-permeable membranes allow water (solvent) but not salt (solute) to flow through
So what does this all mean? How do I apply this to nursing?
It’s actually pretty simple, ever see someone who has +3 pitting edema? Well, they have a bunch of solvents in their tissues that need to get back into their veins and then urinated out.
You do this by giving them a hypertonic solution in their veins. Thus in the vein is a solution with a high ratio of solute and outside the vein is a bunch of solution with a low ratio of solute to solvent. The ratio of hypertonic to hypotonic will create pressure and water will pull out of the tissues into the vein to attempt to create a better ratio of water and solute (notice the water followed the salt).
The same is true of people who are dehydrated. We give them an isotonic solution to try to expand the volume of their blood but we don’t want it to necessarily move solvent out of the vein into their tissues.
Try not to overthink the questions that your instructors are asking. Simplify it by following a couple of steps:
Identify what the tonicity of the solution is.
Ask yourself: what is the ratio? Then if there is a different tonicity solution on the other side of the semi-permeable membrane
Ask yourself: how do they equalize the ratio by moving the solution only?
IV Solutions Cheatsheet
Look, we get that this is a complex topic and many students (and nurses) struggle with it. To help keep it straight we have created a Cheatsheet that covers the most common IV fluids and dives a bit deeper into this topic.
What is Osmosis:
Try these examples:
Solution #1: 50 parts salt (solvent) and 100 parts water (solvent)
Fraction 50/100 is reduced to 1/2
Ratio of 1:2
Solution #2: 50 parts salt and 50 parts water
Fraction 50/50 is reduced to 1/1 or just 1
Ratio of 1:1
A semipermeable membrane between the two solutions creates movement of solution number one solvent to solution number twos solution creating:
Modified solution #1: 50 parts salt and 75 parts water
50/75 reduced to 2/3
Ratio of 2:3
Modified solution #2: 50 parts salt and 75 parts water
50/75 reduced to 2/3
Ratio of 2:3
But what if the solvents are different numbers? The concept stays the same!
Solution #1: 75 parts salt (solvent) and 150 parts water (solvent)
75/150 reduced to 1/2
Ratio of 1:2
Solution #2: 150 parts salt and 150 parts water
150/150 reduced to 1/1 or just 1
Ratio of 1:1
The semipermeable membrane between the two creates movement of solution number ones solvent to solution number twos solution creating:
Solution #1: 75 parts salt (solvent) and 100 parts water (solvent)
Lower osmolality is <275 mmol/kg and means blood is hypotonic
Higher osmolality is >295 mmol/kg and means blood is hypertonic
Sodium: 135-145 mEq/L
Hyponatremia is less than 135 mEq/L
Hypovolemia with hyponatremia leaves the blood in a mostly isotonic state
Hypervolemia with hyponatremia leaves the blood in a hypotonic state
Hypernatremia is more than 145 mEq/L
Hypovolemia with hypernatremia leaves the blood in a hypertonic state
Hypervolemia with hypernatremia leaves the blood in a mostly isotonic state
Specific Gravity of urine: 1.005-1.030
Remember “the higher the dryer” If a patients specific gravity is high, they are more dehydrated.
Hypertonic, Hypotonic, Isotonic IV solutions
You want to give your patients a solution that has the tonicity that is opposite their problem most of the time. For example, if your patient is dehydrated their blood is hypertonic. They will need a hypotonic solution to bring their tonicity back within normal ranges.
The following is a list of solutions by their tonicity:
D5 in Lactated ringers
D5 0.45% NaCl
0.9% NaCl (Normal Saline)
D5W (In the bag)
D5W (in the body)
0.45% NaCl (half normal saline)
Why is D5W listed under two categories? Well because it is a trickster. The actual solution in the bad is isotonic, but once you give D5W to patients the body metabolizes the glucose molecules that were once causing the solution to be isotonic. The solution is now missing solute, causing it to become a hypotonic solution.
What’s in your IV bag?
D5 + 0.45NaCl
D5 + 0.9NaCl
D5 + Ringer’s
5% Sodium Bicarb
Should you memorize this chart? NO! I put this chart here to help you visualize why the IV solution is hypertonic, hypotonic or isotonic. The number of particles in the D5W is 50, and this is a hypotonic solution. Whereas D5 + Ringer’s is a hypertonic solution and has 361 particles.
Your patient has had nausea, vomiting, and diarrhea for 4 days. The blood work shows this patient is dehydrated but their electrolytes have managed to stay within normal limits.
What kind of solution is this patient's blood?
What kind of fluid would you give this patient?
Isotonic at first such as 0.9% NaCl (expand their volume and give them more to move or shift around)
Hypotonic second, usually 0.45% NaCl (shift the fluid into the cells)
Your patient comes in with bilateral +2 pitting edema on the lower extremities. The blood work confirms congestive heart failure (CHF).