Thoroughly confused on hypertonicity hypotonicity-please help

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Phlostein-you there?

Need some help with this anyone...

So goljan defines plasma osmolarity as the number of solutes in the plasma. This, to me, means POsm is different from say concentration of solutes in a fluid.

It also states that hypertonicity is an increase in POsm-thus an increase in solutes in fluid and hypotonicity as a decrease in POsm.

It also states that POsm is directly proportional to concentration of Na. Fine.

But then when it discusses disorders it states under HYPOtonic disorders hypertonic loss of Na.

This is defined as excess Na loss with some H2O loss.

But that doesnt fit the definition of hypertonicity as defined by goljan because hypertonicity should mean an increase in solutes-here there is a decrease.

The only way this would work is if hypertonicity is meant to be defined as a decrease in CONCENTRATION of a solute.

This part (where it says hypertonicy loss of Na) is not a typo, because its repeatedly emphasized.

Therefore, I was thinking Goljan definition of POsm is wrong.

Can someone verify this? Its on chapter 4 of Rapid Review.

I am confused.

Also, while we're at it, why is hypertonic loss of Na under hypotonic disorders?

Also, what is tonicity comparing to? the compartment prior to the change or another compartment(ICF)...

grrr

I think I figured it out.

The key definition here is that POsm can be approximated with Na concentration. The key word being approximated. Thus, hypertonic loss of Na refers to the POsm decreasing because the Na concentration is also decreasing. This decrease is because-obviously- more Na is lost than water. That CONCENTRATION (remember concentration of Na approximates POsm) is what allows one to assume (not directly) that POsm is also decreasing.

Its a semantic issue but can easily trip you up if one does not understand the concept clearly-I think.

Essentially, this is a HYPOtonic disorder because even though a greater concentration of Na is lost (remember this is not the definition of POsm but approximates it), the net effect is that there is now a decrease in Na concentration overall in the compartment-and Na concentration approximates hypotonicity(decreased POsm).

It seems conceptually easy but to me its fairly difficult because in reality if POsm did not approximate Na concentration the above could not possibly happen at all. That is if POsm was only affected by Na LEVELs and not its concentration, the above would not make sense.

Can someone tell me if I'm right?

Your tonicity is directly proportional to the concentration of your solute.

So if you have lots of Na+ and little water --> high osmolarity.

If you have low Na+ and lots of water --> low osmolarity.

Think of three different "compartments": 1) what's actually leaving your body, 2) what that does to your ECF, and 3) what that does to your ICF.

If you're losing a hypertonic solution (e.g. sweat), it means what left your body had high Na+ and a little water, so that means your ECF must have lost a little water and a lot of Na+. So relatively speaking, the ECF now has a lower Na+/H20 ratio, so it is now more hypotonic. So losing a hypertonic solution causes the ECF to become more hypotonic.

The amount of water in the ICF is dependent on the tonicity of the ECF. If there is a relatively hypotonic ECF, that means the Na+/H20 ratio of the ECF is less than that of the ICF, so water shifts to the ICF to equilibrate the system.

The absolute volumes of your ICF and ECF are irrelevant. The Na+/H20 ratio dictates your tonicity. Therefore, volume reduction could actually lead to a hypotonic ECF, for instance, in the event of Addison's disease, since hypertonic urine would be lost due to Na+-wasting. High ECF tonicity (e.g. diabetes insipidus) draws water from the ICF, thereby reducing ICF volume; low ECF tonicity (e.g. SIADH) causes water to shift into your ICF.

One more thing (sorry to make this more complicated): you can think of your erythrocytes as a theoretical ICF. Fluid shifts into and out of them based on the tonicity of the ECF. Therefore, if you've got a hypotonic ECF and volume expansion, the RBCs would get bigger, but because the ECF had also expanded, the [Hct] stays constant. If an ECF-hypotonic volume contraction were to occur (e.g. Addison's), however, fluid still moves into the RBC, but now the [Hct] has increased because the ECF had decreased. Also reason through this with respect to hypertonic solutions, thinking about when [Hct] would remain constant and when it would change.

Also think about these concepts with respect to the different diarrhoea types. Isotonic changes never change ICF volume.

Bottom line: be aware that tonicity is not dependent on absolute volume; it is dependent on relative volume. Absolute volumes only become relevant with regard to [Hct]. The latter is a slightly more abstract element added on top of the fluid-shift concept, but it's actually very easy once you get the basics down.

Hope that helps,

I understand everything you say. My issue was primarily with the way rapid review defines these terms and uses them.

Here is what I mean. I'll construct a practice question that reflects the definitions in RR. Based on that, my question should be tricky... I'm not saying this is board question type. Just that it presents my conundrum easier.

The definition of Plasma Osmolality is the number of solutes in plasma also known as the tonicity of ECF. Suppose a patient has a pure gain of water in their ECF due to SIADH. Using your knowledge from water and electrolyte physiology, what happens to the Plasma Osmolality or POsm?
A. an increase in POsm
B. POsm stays the same
C. POsm decreases
D. POsm constantly changes

Also explain why. Make sure you use the definition I gave for POsm in your explanation (which is word for word same definition as goljan)

Originally Posted by sanj238

I understand everything you say. My issue was primarily with the way rapid review defines these terms and uses them.

Here is what I mean. I'll construct a practice question that reflects the definitions in RR. Based on that, my question should be tricky... I'm not saying this is board question type. Just that it presents my conundrum easier.

The definition of Plasma Osmolality is the number of solutes in plasma also known as the tonicity of ECF. Suppose a patient has a pure gain of water in their ECF due to SIADH. Using your knowledge from water and electrolyte physiology, what happens to the Plasma Osmolality or POsm?
A. an increase in POsm
B. POsm stays the same
C. POsm decreases
D. POsm constantly changes

Also explain why. Make sure you use the definition I gave for POsm in your explanation (which is word for word same definition as goljan)

POsm decreases. Osmolality is solutes per kg of solvent, it is NOT just a raw number of solute particles (this would be meaningless, as a neonate's osmolality would be vastly different than an adult's). A pure gain of water in ECF means that there are the same number of solute particles as before, but now in more solvent. The denominator goes up, the numerator stays the same, and therefore the value (POsm) decreases.

When you have a hypertonic loss of Na, the fluid leaving your body has more Na/kg water than the rest of your body. This is equivalent to a net loss of solutes as compared to solvent, meaning you have less Na/kg water left in your body, hence a hypotonic problem.

YES. Thank you. I wish I had checked the definition of osmolality. I just took Goljan's word.

I mean, I thought it was a typo but didn't bother to check- thanks for the clarification.

Everything makes sense now...stupid waste of time.

Thats what happens when I study the minutest details