Readers know that I love teaching acid base and electrolytes. For years I (and many other educators) have difficulty explaining why edematous states can lead to hyponatremia. We always have talked about ineffective intravascular volume, although when you measure the intravascular volume it measures as increased. Thus, we had a difficult concept to communicate and understand.

This week I read I wonderful piece by Dr. Schrier which allows me to understand the concept and teach it. Decreased Effective Blood Volume in Edematous Disorders: What Does This Mean?

Here is the short story.  These edematous disorders represent an interesting paradox.  Generally 85% of the intravascular volume is venous and 15% arterial.  In these edematous states, the proportion of venous filling increases and the arterial filling decreases, with a net increase in total intravascular volume.  Thus, we should rephrase the concept to decreased intra-arterial volume.

This makes sense.  If intra-arterial volume is decreased we should have increased ADH as well as renin-angiotensin-aldosterone.

This article makes more clear an interesting physiologic mystery.  I hope my revelation helps at least 1 reader!