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Formation of Tissue Fluid

So I know that at the arteriole end of a capillary bed the hydrostatic pressure is high due to ventricular systole so plasma is forced out to form tissue fluid. I also know that at the venule end the hydrostatic pressure is low and tissue fluid moves into the capillaries, but I'm really confused about the whole osmotic pressure thing....

In my booklet it says that the osmotic pressure stays the same and the hydrostatic pressure is just lower, but doesn't water leave the capillaries and lower their water potential and raise the water potential of the tissue fluid?

Basically I'm so confused about what osmotic pressure actually is and whether it stays the same and just how it's involved? :colondollar:

Thaaaanks

(edited 9 years ago)

Reply 1

Chronokeeper

It was my understanding that osmotic pressure increased on the capillary because most of the water had been forced out, while the large molecules stayed in the capillary. Then at the venous end, the osmotic pressure adds to the hydrostatic pressure, which is why most of the tissue fluid re-enters.

Reply 2

Democracy

Original post by Qaiys

Thaaaanks

Starling's forces is what you're referring to. Here are a few definitions you need to grasp first:

Hydrostatic pressure - the pressure driving fluid out of the capillary. This is 30mmHg at the arterial end of the capillary and 20mmHg at the venous end.

Oncotic pressure (also known as colloid osmotic pressure) - the pressure drawing fluid back into the capillary. This is 25mmHg.

So at the arterial end, the hydrostatic pressure is greater than the oncotic pressure (30mmHg vs 25mmHg) so there is net loss of fluid.

At the venous end, the oncotic pressure is greater than the hydrostatic pressure (25mmHg vs 20mmHg) so there is net reabsorption of fluid.

Personally, I find Starling's forces much more understandable with a simple diagram, so hopefully this will help:

Reply 3

white_o

Original post by Democracy

Thanks so much! So the osmotic pressure doesn't actually change? So any water that enters/leaves the capillaries doesn't effect the osmotic pressure?

Reply 4

iwantoknowhy

Original post by Democracy

but why doesn't oncotic pressure change if the water potential of the capillaries is decreasing and the water potential of the tissue fluid is increasing?

Reply 5

Jpw1097

Original post by white_o

Thanks so much! So the osmotic pressure doesn't actually change? So any water that enters/leaves the capillaries doesn't effect the osmotic pressure?

The water leaving the arteriole end of the capillary will have a negligible effect on the water potential of the tissue fluid (as the tissue fluid volume is much larger than the volume leaving the capillary), however, it will have an effect on the osmotic pressure in the vessel (i.e. the oncotic pressure) as the concentration of plasma proteins within the venule end of the capillary is now greater as the volume is less. This means that the venule end of the capillary will have lower hydrostatic pressure and higher oncotic pressure, causing fluid to move back in.

However, less water comes back into the venule end than what left the arteriole side. The remaining tissue fluid re-enters the circulation via the lymphatic system, where it is now known as lymph.