General Edema isosmotic volume expansion question

[Bathrover]

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In general edema where does the extra (isosmotic) fluid come from to cause an expansion of the ECW? I mean it is usually said that the plasma (which is considered part of ECW) fluid flows into the interstitium (also ECW) which results in peripheral edema. However why does ECW increase in total volume if there is a transfer of fluid between plasma and interstitium? Is this extra fluid simply coming from water intake that is being collected over time in the interstitium and therefore not being excreted in urine?

 

[theseeker4]

In general edema where does the extra (isosmotic) fluid come from to cause an expansion of the ECW? I mean it is usually said that the plasma (which is considered part of ECW) fluid flows into the interstitium (also ECW) which results in peripheral edema. However why does ECW increase in total volume if there is a transfer of fluid between plasma and interstitium? Is this extra fluid simply coming from water intake that is being collected over time in the interstitium and therefore not being excreted in urine?

The "extra" water is retained water plus solutes, usually sodium chloride. Simply moving water and solute from the plasma to the interstitial space, or vice versa, doesn't change the total amount of extracellular water, so there either has to be an increase in intake, a decrease in excretion, or both. Third spacing is what occurs when you have, for example, a decrease in plasma protein, reducing the oncotic pressure of the intravascular space and allowing the interstitial space to expand, but this also has a component of increased retention; the water in the interstitial space is not subject to excretion via the kidneys, so more is retained. There is also the fact that a decrease in intravascular volume causes a decrease in blood pressure, which sets off the RAA system to retain salt and water, further increasing edema (since a proportion of the newly retained water and solute will itself be third-spaced).

Remember that normally, there is a constant balance between water entering the interstitium from the intravascular space by filtering out of the capillaries, and the water re-entering the vasculature mainly through the lymphatic system.

Essentially, you are correct in that there is an increase in fluid being collected, either over the short term (such as an IV bolus) or long term (third spacing due to decreased protein causing peripheral edema and fluid retention).