hydrostatic, interstitial, osmotic pressure, blood pressure differences?

[sangria1986]

whats the difference between these pressures? I get confused by the nomenclature

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[cwfergus]

whats the difference between these pressures? I get confused by the nomenclature

I think what these pressures are referring to are in the capillaries right?

Well.. if so, hydrostatic pressure is the pressure at the arteriole end of the capillary bed. It is the fluid pressure that pushes fluid out of the capillaries into the interstitial space.

Interstitial pressure would be the pressure in the tissues. As you can tell, the difference between these pressures drives fluid out of the capillaries. Towards the end of the capillaries, on the venule side, there is a high osmotic pressure (which is the pressure do to the plasma proteins --> albumin, etc). This high osmotic pressure drives fluid back into the capillaries.

Again... the difference between the interstitial pressure and osmotic pressure here would drive blood back in.

You should note that not all fluid is brought back into the capillaries (due to not as big of a pressure difference i believe) and therefore, the lymphatic system picks up excess fluid. When this fails, edema and what not will result

 

[sangria1986]

I thought albumin indicates high osmolarity...? Not osmotic pressure. Thus hi osmolarity in capillaries means more fluid flows into cappilaries. Am I wrong?

Thanks!

 

[cwfergus]

I thought albumin indicates high osmolarity...? Not osmotic pressure. Thus hi osmolarity in capillaries means more fluid flows into cappilaries. Am I wrong?

Thanks!

Hey. So think about it this way: High albumin in the capillaries on teh venus side (because after fluid leaves, the [albumin] increases ---> High Osmolarity). This high osmolarity causes a high osmotic pressure (or in other words, oncotic pressure). This osmotic (oncotic pressure) 'pulls' water from the interstitial space into the capillaries.

So you are right , but it's due to the osmotic pressure in the capillaries... which causes more fluuid to flow into capillaries

Im pretty sure for MCAT, we just need to know--> Hydrostatic pressure causes fluid to leave capillaries in arteriole side, while osmotic or oncotic pressure pulls fluid back into capillaries on venus side.