Hematocrit in hyper osmotic fluid contraction

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zhopv10

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So you are loosing hypotonic solution this is decreasing the ECF volume and increasing its osmolarity. This of course leads to a decrease in ICF. The decrease in volume also leads to an increase is plasma protein concentration but constanzo says the hematocrit is unchanged rather than elevated (due to the hyper osmolarity leading to a decrease in RBC size). Now that makes sense and all but hematocrit is increased in dehydration (which is a loss of hypotonic solution no?) and indeed dehydration is the most common cause of an increase hematocrit. So what gives?? Is it that that effect is overcome in dehydration, or that the shrinking only has an effect at first and then it can't happen anymore and hematocrit goes up or what?


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As a note it also says this but backwards for SIADH I.e the increase in free water reabsorption is redistributed equally in both the ECF and ICF, decreasing the osmolarity of both compartments. The increase in volumes leads to a decrease in protein concentration but no change in hematocrit due to RBC expansion.


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I see what your saying about Constanza saying that hyperosmotic volume contraction (ex: water deprivation) when there is a loss of hypotonic fluid how the ECF decreases but is transiently high osmolarity than ICF so then the water shifts from ICF to ECF (out of cells) then the plasma protein concentration is increased the RBCs shrink proportionately so the HCT stays the same. BUT thats all talking about when there is a loss of more H20 than Na+. but maybe its not as true when the dehydration is so severe that the RBCs become even larger in comparison to the plasma fluid. so that the % RBC goes up faster than the volume of plasma and basically causes the RBCs to be even more concentrated in comparison, raising the HCT.
 
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Ok so I found an NCBI article basically saying that its not that the hematocrit actually increases with dehydration its just that the calculation of hematocrit in a lab appears increased since they use MCV to estimate hematocrit. So basically Constanza is right when it comes to the physiology (obviously i feel like costanza is always right haha). But the reason dehydration causes the hematocrit to appear larger is just a lab calculation issue. I can't copy and paste the article b/c my UWORLD is open but if you google "hemoglobin and hematocrit" NCBI you can see the article.
 
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dorster said:
Ok so I found an NCBI article basically saying that its not that the hematocrit actually increases with dehydration its just that the calculation of hematocrit in a lab appears increased since they use MCV to estimate hematocrit. So basically Constanza is right when it comes to the physiology (obviously i feel like costanza is always right haha). But the reason dehydration causes the hematocrit to appear larger is just a lab calculation issue. I can't copy and paste the article b/c my UWORLD is open but if you google "hemoglobin and hematocrit" NCBI you can see the article.
Click to expand...

So it must just be the calculation algorithm? Because I mean if the RBC's are shrinking with the increased osmolarity of the ECF then that would cause a decrease in the MCV no?


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Directly from BRS

Sweating in a desert—loss of water

■ is also called hyperosmotic volume contraction.

  1. (1) The osmolarity of ECF increases because sweat is hyposmotic (relatively more water than salt is lost).

  2. (2) ECF volume decreases because of the loss of volume in the sweat. Water shifts out of ICF; as a result of the shift, ICF osmolarity increases until it is equal to ECF osmolarity, and ICF volume decreases.

  3. (3) Plasma protein concentration increases because of the decrease in ECF volume. Although hematocrit might also be expected to increase, it remains unchanged because water shifts out of the RBCs, decreasing their volume and offsetting the concentrating effect of the decreased ECF volume.
Click to expand...
 
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jds112 said:
Directly from BRS
Click to expand...

Right, that is what we've been saying with constanzo but in real life when you order the hematocrit on someone that is dehydrated, the hematocrit will be increased and it is indeed the most common cause of an increased hematocrit. I just read the article mentioned above and it's still difficult for me to quite make the connection. So I see that the direct microcentrifugation method would show what constanzo talks about but I fail to see how counting the cells and multiplying by the MCV would really mess things up since it seems that the MCV should be contracted accordingly. W/e too much detailed for step I suppose I'm just worried if they give us arrows and ask us what happens which one to go off, I'd personally go for constanzo but hopefully that is the right way of thinking about it.


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zhopv10 said:
Right, that is what we've been saying with constanzo but in real life when you order the hematocrit on someone that is dehydrated, the hematocrit will be increased and it is indeed the most common cause of an increased hematocrit. I just read the article mentioned above and it's still difficult for me to quite make the connection. So I see that the direct microcentrifugation method would show what constanzo talks about but I fail to see how counting the cells and multiplying by the MCV would really mess things up since it seems that the MCV should be contracted accordingly. W/e too much detailed for step I suppose I'm just worried if they give us arrows and ask us what happens which one to go off, I'd personally go for constanzo but hopefully that is the right way of thinking about it.


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What i've learned about the arrow questions: reason out the factors I do understand which will, in turn, help me eliminate the ones that are incorrect. Most comprehensive question banks (and step 1 in this case) won't give you two identical answer sets with one option being HCT unchanged or HCT increased [eg, inc inc inc unchanged & inc inc inc inc]. So since you can reason why the other variables are increased/decreased/unchanged, you will be able to answer this question. Oh and for these, know the D-Y diagrams
 
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the arrows can always be a bit tricky, in one of our school exams, they asked about the na level in DKA, even though i KNOW you get hyponatremic in DKA, for a second i saw the arrow for Na increased, and i almost went for it bc i thought water is being lost so that would cause increased concentration of sodium, but na is lost in the urine too in dka.
 
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zhopv10 said:
Right, that is what we've been saying with constanzo but in real life when you order the hematocrit on someone that is dehydrated, the hematocrit will be increased and it is indeed the most common cause of an increased hematocrit. I just read the article mentioned above and it's still difficult for me to quite make the connection. So I see that the direct microcentrifugation method would show what constanzo talks about but I fail to see how counting the cells and multiplying by the MCV would really mess things up since it seems that the MCV should be contracted accordingly. W/e too much detailed for step I suppose I'm just worried if they give us arrows and ask us what happens which one to go off, I'd personally go for constanzo but hopefully that is the right way of thinking about it.


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Wait guys I don't think that dehydration is necessarily equivalent to a loss of hypotonic fluid. Profuse sweating without replacement of fluids will not change hematocrit as explained above by two opposing mechanisms: loss of fluid from ECF compartment to the air leads to decrease in plasma volume->increased concentration of RBCs; movement of fluid from ICF (including RBCs) to ECF->decreased volume of RBCs->decreased concentration of RBCs.

However, it is still true that dehydration is the most common cause of increased hematocrit. I think this is because the most common cause of dehydration is diarrhea, not sweat. Diarrhea will cause an isoosmotic loss of fluid, and so ECF will be lost, but ICF volume will remain the same, which means RBC volume will remain the same in the face of decreased plasma volume leading to increased hematocrit. So dehydration can manifest itself in several ways and the physiology all kind of makes sense without having to introduce knowledge of laboratory methodologies.
 
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DerpStore said:
Wait guys I don't think that dehydration is necessarily equivalent to a loss of hypotonic fluid. Profuse sweating without replacement of fluids will not change hematocrit as explained above by two opposing mechanisms: loss of fluid from ECF compartment to the air leads to decrease in plasma volume->increased concentration of RBCs; movement of fluid from ICF (including RBCs) to ECF->decreased volume of RBCs->decreased concentration of RBCs.

However, it is still true that dehydration is the most common cause of increased hematocrit. I think this is because the most common cause of dehydration is diarrhea, not sweat. Diarrhea will cause an isoosmotic loss of fluid, and so ECF will be lost, but ICF volume will remain the same, which means RBC volume will remain the same in the face of decreased plasma volume leading to increased hematocrit. So dehydration can manifest itself in several ways and the physiology all kind of makes sense without having to introduce knowledge of laboratory methodologies.
Click to expand...

Ahh very good point! That makes more sense I think, it requires less leaps of logic.


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Loss of hypotonic fluid: sweating, hypotonic urine (diabetes insipidus, alcohol).

Loss of isotonic fluid: diarrhea (except infant<-hypotonic loss), vomiting, hemorrhage.

Loss of hypertonic fluid: SIADH (inappropriate ↑ in urine osmolarity).

Gain of hypotonic fluid: tap water, hypotonic saline.

Loss of NaCI: e.g. loss of 1 L sweat, drink 1 L of tap water.
 
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