Inulin and extracellular volume measurement

[ChessMaster3000]

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I just read in FA that inulin is used to measure ECV, as opposed to albumin which is used for intravascular volume. I was wondering if anyone knew why inulin was used--are there particular properties that makes inulin suitable for this measurement?

 

[ulikedaggers]

Albumin doesn't leave the vascular space, so it can't tell you anything about the interstitial fluid.

Inulin can cross capillaries, but not cellular plasma membranes (isn't filtered or reabsorbed at the glomerulus), so it can be used to measure the entire ECV.

 

[ChessMaster3000]

Albumin doesn't leave the vascular space, so it can't tell you anything about the interstitial fluid.

Inulin can cross capillaries, but not cellular plasma membranes (isn't filtered or reabsorbed at the glomerulus), so it can be used to measure the entire ECV.

great, just to clarify, I think you meant inulin is not secreted or reabsorbed in the tubules, as it is freely filtered at the glomerulus. I am guessing the lack of secretion/absorption in the tubules mimics its fate at cells throughout the entire body (no absorption, so nothing intracellular)

 

[Myxedema]

great, just to clarify, I think you meant inulin is not secreted or reabsorbed in the tubules, as it is freely filtered at the glomerulus. I am guessing the lack of secretion/absorption in the tubules mimics its fate at cells throughout the entire body (no absorption, so nothing intracellular)

Inulin, like mannitol, is a large sugar. Because of this chemical property, it can be freely distributed throughout throughout ECF, but it cannot pass the cellular membrane. That's why it can be used, like mannitol, as a marker for ECF.
As for inulin being a marker for GFR, it again has to do with its chemical properties: Correlate with being able to freely pass into the urine (freely distributed throughout ECF) and not being reabsorbed (does not cross cell membrane). It is also not secreted, making it an ideal marker to measure GFR.

 

[ulikedaggers]

great, just to clarify, I think you meant inulin is not secreted or reabsorbed in the tubules, as it is freely filtered at the glomerulus. I am guessing the lack of secretion/absorption in the tubules mimics its fate at cells throughout the entire body (no absorption, so nothing intracellular)

Right, thanks for the correction!