Lymph System and Interstitual Fluid Pressure

[sniderwes]

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So this is my current understanding of the lymph system and how it recycles fluid:

1) You have blood flowing through blood vessels
2) Some fluid+proteins leak out into the interstitial tissue
3) Interstitial fluid flows into lymph vessels
4) Lymph vessels have lymph valves that stop the backwards flow of lymph and keep the fluid flow one-directional

However, Passage 58 from the TPR Science Workbook says "the lymph valves create a negative pressure in the interstitial tissue through what is essentially a sucking action, facilitating forward fluid flow into the lymphatic capillary." If the valve somehow makes the interstitial fluid pressure more negative, then why would this drive interstitial fluid into the lymph vessel? Fluid flows from high pressure to low pressure.

Furthermore, I thought the purpose of the valves was simply to prevent backflow, not to drive fluid into the lymph vessel. Thanks for any clarification

 

[SuperSneaky24]

I think you're confusing the definition of interstitial fluid. Interstitial fluid is the fluid that isn't in the blood vessel. So it totally makes sense that fluid from the blood (plasma) moves into the more negative pressured interstitial fluid.

So skeletal muscle contraction drives most of the lymph fluid flow in the lymphatic system. When a muscle contracts, the lymph vessels are also compressed, forcing fluid to flow in the forward direction due to the valve preventing any backflow. When the muscle relaxes, the valves makes the fluid that was previously in the vessel gone, so there is a sort of mini-vacuum created that creates this negative pressure. As fluid flows from high to low pressure, it makes sene that fluid from blood flows from the blood vessel (which has higher pressure) to the lymphatic capillaries (which now has the more negative pressure).

 

[sniderwes]

I think you're confusing the definition of interstitial fluid. Interstitial fluid is the fluid that isn't in the blood vessel. So it totally makes sense that fluid from the blood (plasma) moves into the more negative pressured interstitial fluid.

So skeletal muscle contraction drives most of the lymph fluid flow in the lymphatic system. When a muscle contracts, the lymph vessels are also compressed, forcing fluid to flow in the forward direction due to the valve preventing any backflow. When the muscle relaxes, the valves makes the fluid that was previously in the vessel gone, so there is a sort of mini-vacuum created that creates this negative pressure. As fluid flows from high to low pressure, it makes sene that fluid from blood flows from the blood vessel (which has higher pressure) to the lymphatic capillaries (which now has the more negative pressure).

So I understand that fluid flows from the high pressure blood vessels to the more negatively pressured interstitual fluid. What I don't get is this: If "the lymph valves create a negative pressure in the interstitial tissue", how does fluid flow from the negative pressured interstitual tissue into the lymph vessel, which I presume is more positively pressured than the interstitial tissue.

 

[SuperSneaky24]

So you have to look at the lymphatic system differently than the circulatory system. The lymphatic system is open, while the circulatory system is closed. The lymphatics also only go in a one-way direction. This means that when the lymphatic valves close, they pull the interstitial fluid into the lymph vessels, causing any remaining interstitial fluid to see a reduced pressure. Basically, when the interstitial fluid goes into the lymphatics, it causes this negative pressure in the interstitial fluid.

 

[sniderwes]

So you have to look at the lymphatic system differently than the circulatory system. The lymphatic system is open, while the circulatory system is closed. The lymphatics also only go in a one-way direction. This means that when the lymphatic valves close, they pull the interstitial fluid into the lymph vessels, causing any remaining interstitial fluid to see a reduced pressure. Basically, when the interstitial fluid goes into the lymphatics, it causes this negative pressure in the interstitial fluid.

Ah, so the lymph system doesn't rely on high pressure ==> low pressure to get fluid into lymph vessels? It is more of a "this fluid randomly came into the lymph vessel, and once it is in it can't get out" type of thing? thanks for the help btw

 

[SuperSneaky24]

Well sort of but it still does rely on high -> low pressure. It's just the valves create the low pressure environment by "sucking" in the interstitial fluid, making the interstitial fluid now have low pressure, allowing fluid from the blood to flow in, and so on and so on.

And no prob!