ECF Fluid Movement - vascular and intravascular spaces

[BlondeCookie]

The spaces within the ECF (extracellular fluid) compartment are:
- vascular space (ie, plasma)
- intravascular space (ie, interstitial)

I don’t really understand the movement of fluid within the ECF compartment. For example, if you have an infusion of saline into the ECF, why does most of it enter the interstitial space? Also, if you had an infusion of plasma into the ECF, why does most of it remain in the vascular space?

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[BarronBlack]

Answer #1
B/C saline diffuse evenly through out the ECF. Now since ECF is made out of approximately 65% intravascular and 35% vascular. You will now have 65% of saline in intravascular and 35% of saline in vascualar.

Answer #2
Plasma remain in the vascular space b/c it cannot diffuse through the blood vesssel into the intravascular compartment.

Hope this help.

 

[Tristy]

When you infuse normal saline (which has the same osmolarity as plasma if I recall correctly), you are infusing osmotically active particles into the ECF. And those particles (Na and Cl) cannot freely diffuse across membranes, like urea for example.

And since you are not changing the osmolarity of the compartments and therefore not creating an osmotic gradient between ICF & ECF, the water will stay in the intravascular compartment of the ECF and expand it (the ICF stays the same).

And actually (somebody please correct me if I'm wrong) intravascular doesn't equal interstitial. The ECF is composed of interstitial fluid (3/4) and plasma (1/4). Plasma is intravascular.

I think that when you infuse plasma it remains in the intravascular compartment because of the proteins in the plasma infusion that contribute to the oncotic pressure. And the oncotic pressure oposses fluid "leaking "/filtration out of the blood vessels.

It is when you infuse half saline or hypertonic saline for example, that you get fluid shifts from the ICF and ECF to equilibrate the osmolarities.

Hope that my rambling made sense, is correct, and it helps!

 

[BlondeCookie]

And actually (somebody please correct me if I'm wrong) intravascular doesn't equal interstitial. The ECF is composed of interstitial fluid (3/4) and plasma (1/4). Plasma is intravascular.

Hope that my rambling made sense, is correct, and it helps!

OMG! I was wrong. You are right Tristy. Intravascular space IS plasma. I messed up in my initial posting when I posted that intravascular space was the same thing as interstitial. It isn't. I should have delineated between intravascular (aka vascular) space and extravascular space.

There are two sub-compartments that make up ECF. They are (correctly, I hope):
- intravascular space, aka vascular space (ie, plasma)
- extravascular space (ie, interstitial)

OK, so I get that plasma stays in the vascular space on an infusion of plasma because of the plasma proteins. Confused a tiny bit about an isotonic saline infusion. Can someone just check this next statement for me? From what I gather from you guys... isotonic saline that is infused into the ECF, will distribute mostly into the extravascular (ie interstitial) space because there is MORE extravascular space than there is intravascular (aka vascular) space. Is this correct?

 

[agranulocytosis]

The way I look at it is that the ECF is always affected first, no matter what. Then, it's the osmolarity that determines movement of water into or out of the ICF. A high osmolarity in the ECF, despite the volume (for example a 9.5% saline infusion) will draw water out of the ICF and a low osmolarity in the ECF will drive water into the ICF (a pure water infusion).