Major confusion about oncotic pressure

[Derkha]

Hi there! This forum seems excellent and I hope I can get such a detailed answer that I may pass on without questions nagging on my head. The more I read about this, the more confused I get. Right now, every piece of knowledge is just falling apart since my lecturer on this subject didn't understand my question and told me to simply accept the state of things. Personally, I can't get satisfied with it or just move on without a resolution.

I've been reading about osmotic pressure, hydrostatic pressure and oncotic pressure (colloidal osmotic pressure) in Guyton 12th ed.

I believe I understand that the osmotic pressure is the force needed to, potentially, oppress the flow of water through a semipermeable membrane. It's a gradient that evens water out between one side with a solution and the other with water, and when enough water has gone to the side with the solution (e.g. the osmosis moves from water side to solution side), the hydrostatic pressure on the solution side will stop more water from entering through the membrane. This is osmotic pressure defined.

I believe I understand that the oncotic pressure simply refers to the molecules involved, in this case albumin in the blood plasma, which is too big to go through the lumen of capillaries.

What I'm not sure I understand though is this (and these questions are in fact one big, I've just divided them up so that answering one might make sense and maybe resolve the big one once I read the answer):

Why is the oncotic pressure inside the lumen of capillaries responsible for reabsorbing water, when logically it's supposed to keep water OUT with it's osmotic pressure mechanism (conversely, why isn't the interstitial oncotic pressure defined as the force needed to keep too much water from flowing OUT of the capillary lumen?
Is water reabsorbed in the capillaries or the venoles?
Where is there more water; in the capillies or the interstitia?

In short: the books and the teachers say that the oncotic pressure inside the capillaries will make water prone to flow inside to the lumen and thus is the opposing force to the hydrostatic pressure (blood pressure) in the capillary, AND the interstitial oncotic pressure which forces fluid out. I can stop here and accept this and probe no further because the exam questions will not go further than this simple concept, except that I can't, because I don't feel like I understand it well enough when my head tells me it should be the other way around.

(A cookie to the person who can connect this to the Starling net flow equation/principle).

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[sazerac]

Hydrostatic pressure is just pressure like normal people think of fluid pressure.

Osmotic pressure is a little different. Osmotic pressure means that water with crap in it will naturally try to suck more water into itself. The crap can be salt, or the crap in the water can be protein. If it's protein we call it on "oncotic" pressure, but it is still osmotic pressure.

So in any given situation with a membrane, like the blood vessel wall between the interstitium and the blood, there are four pressures at work.

Hydrostatic in the vessel pushes water out to the interstitium. Osmotic in the vessel sucks water into the vessel from the interstitium. Hydrostatic in the interstitium pushes water into the blood vessel. Osmotic pressure in the interstitium sucks water from the vessel to the interstitium. You can just add these four pressures together to determine the net pressure and therefore the direction the water will flow.

I'm not sure if I answered everything you wanted, but once you get that osmotic pressure is a sucking pressure, everything is pretty easy to figure out.

 

[the_fella]

It has to do with concentration. Water with more "stuff" in it (say 0.5 M NaCl) has less water (or conversely, more stuff) than say another bit of water with 0.3 M NaCl (less stuff, but more water). As such, the water will flow from an area of high concentration to an area of low concentration. Generally, "stuff" doesn't flow through membranes, so the water does.

It helps me to relate this to tonicity, which refers to the solute concentration outside of a membrane (for instance, the membrane of a blood vessel). A hypotonic solution has less solute outside the membrane than it does inside. That means there's more water outside, so it crosses the membrane into the lumen. You could view the water as being "sucked" in. A hypertonic solution has more solute outside the membrane than inside. That means there is less water outside, so water rushes out of the vessel. Water is going to flow from an area of high concentration to an area of low concentration.

Fwiw, "sucking" doesn't really exist. It's just a pressure differential. You don't "suck" air into your lungs. You expand your lungs in order to create an area of pressure less than atmospheric pressure, and air rushes in to fill this void.

I believe Starling Forces is just another term for hydrostatic and oncotic forces.

Hope this helps.