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lymph and interstitial fluid

Discussion in 'MCAT Study Question Q&A' started by phospho, Mar 23, 2008.

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  1. phospho

    phospho SDN Lifetime Donor Lifetime Donor 10+ Year Member

    Jun 4, 2006
    Columbus, OH
    I'm having a huge problem distinguishing between the two. I was trying to find a diagram online that had both of them in there, but I couldn't. It's not that I don't know what they do - I'm having trouble forming a mental picture of them in my head. How are they related? are they related? Is interstitial fluid just everywhere in the body? I know it bathes the cells, so it's just flowing everywhere with no certain path? Does lymph have vessels to separate it from interstitial fluid? how can lymph just mix with the blood? they said that the CV system is closed but the lymph is open because it empties into CV system. How can it just "empty" into it? is lymph soluble in blood? where does the new lymph come from? is it eventually separated from the blood it mixes with?


    am i thinking too much about this or am i lost?

    thank you:luck:
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  3. DrMattOglesby

    DrMattOglesby Grand Master Moderator Emeritus 7+ Year Member

    Aug 15, 2007
    whats confusing about these two is that the lymph capillaries can pick up the interstitial fluid. *it does this because lymph capillaries have cell walls with anchoring junctions in the interstitial when there is an increase in the interstitial fluid volume, the lymph capillary will open up to absorb the excess fluid.
    two major functions of the lymphatic system are to provide an immune function as well as to return excess fluid to circulation (making up the plasma of the cardiovascular system).

    (some major vessels associated with the lymphatic system are the thoracic duct, cisterna chyli and right lymphatic duct.)

    i dont know if that answers anything in specific...but i just wanted to spout off some things i happen to know =]
  4. Vihsadas

    Vihsadas No summer Moderator Emeritus Lifetime Donor 5+ Year Member

    Oct 17, 2007
    An Igloo
    Okay, so the blood contains all the proteins, RBCs, platlets, glucose etc. Because of starling forces, when blood flows through capillaries, the blood plasma literally leaks out of the capillaries and into your tissues. This is the 'interstitial space'. No RBCs leak through the capillaries, although WBCs do! When this fluid exits your capillaries it's in the 'interstitial space' and is called 'interstitial fluid'. Note that interstitial fluid is the same thing as 'lymph', although technically this fluid isn't really 'lymph' until it enters the lymphatic system. This is just an issue of nomenclature. When the blood plasma leaks out of your circulatory system it comes in contact with all the cells of your body. This is why you hear that cells are 'bathed' in lymph.

    Now, the lymphatic system is a system of vessels much like our blood vessels except that it is not 'circulatory' in nature. Remember that the blood vessels are continuous. If you traced a line through a blood vessel you could travel round and round the circulatory system ad infinitum. It's never ending, self-contained loop. The lymphatic system doesn't work like that though. It's composed of 'terminal' vessels. If you traced a lymph vessel from beginning to end you'd eventually exit the lymphatic system out into the interstitial space (or into the circulatory system depending which way you're going).

    When there is enough fluid in the 'interstitial' space, the lymphatic system will pick up this fluid and return it to the circulation. I believe that the lymphatic system converges around the level of your neck and then literally feeds back into the circulatory vessels.

    Remember that there is no 'pump' for the lymphatic system (like the heart). It functions via a series of one-way valves. So that means that as your muscles contract or you move around, you're squeezing the lymph vessels in your body and causing the lymph to progress toward your neck. This is why when you have been sitting down for a long time and not moving your feet, your feet might get a little swollen. The swelling is the fluid that leaked from your capillaries into the interstitial space of your feet. When you start wiggling your toes and moving your feet around, you'll eventually push that fluid into your lymphatic system through those one way ducts and up toward your neck.

    Hope this helps. :)
  5. phospho

    phospho SDN Lifetime Donor Lifetime Donor 10+ Year Member

    Jun 4, 2006
    Columbus, OH
    Thank you both very much.

    Vihsadas, I can't begin to tell you how much that helps:love::luck:
  6. Cool Aberration

    Cool Aberration 2+ Year Member

    Mar 23, 2008
    Great explanation, couple follow up questions though:

    You mentioned tangentially that WBCs can leak through capillaries--out of curiosity, do you have a more in depth mechanism for that? I'm looking here at my physio textbook and it mentions that we have "metarterioles" to shuttle WBCs directly from arterioles to venules since WBCs are twice as large as RBCs and have trouble passing through capillaries. In short, what accounts for WBCs ability to leak through?

    You also mention how the squeezing of skeletal muscle helps circulate lymph. I am recalling the term "skeletal pump" to describe the driving force for venous return. Would you say that the the lymph system is very similar to venous flow in that both are pushed by surrounding muscle and follow through one-way valves? I'm asking because I think I've heard the same exact explanation for swelling of feet attributed to poor venous circulation. If you sit still for a long time, blood pools in the veins of your lower extremities and you need to contract your leg muscles to help push the blood back through to the heart.

    Thanks for your help.
  7. Vihsadas

    Vihsadas No summer Moderator Emeritus Lifetime Donor 5+ Year Member

    Oct 17, 2007
    An Igloo
    I might be wrong about the WBC leak, but I think I remember reading that somewhere. If they do leak, I'm sure it would happen in a similar fashion to how monocytes extravasate into the tissue (like in atherosclerosis, or immune response). The monocyte would interact with the blood vessel walls and then eventually bind to it via adhesion molecules. The WBCs can then squeeze through the endothelial cells, going out of the vascular lumen and into the tissues. Let me see if I can find a cartoon...
    (This cartoon is specific to atherosclerosis)
    Again ^^this is out of scope for the MCAT (I think).

    Even if I'm wrong and they don't leak, remember that some WBCs are housed in the lymph nodes which are connected to the lymph.

    Yes that's exactly right. In fact, the same exact mechanism I mentioned for lymphatic return happens for venous return in the lower extremities. So while my original explanation of swelling feet is contributed to by lack of lymphatic return, it's also contributed to by lack of venous return. If you think about it though, the two would be related, right?
    If you aren't moving your feet, your venous skeletal pump isn't returning blood to the heart, and, the heart is still pumping blood out to your capillaries. This would cause more fluid to accumulate on the walls of the capillaries. Thus, pressure on the capillary wall would increase, and more fluid would leak out into the interstitium and become lymph.

    So yeah, you're exactly right. I probably should have been more explicit in saying that both contribute to the 'swollen foot' symptom. There are other factors at play too (such as compliance of veins) but I don't think it's MCAT relevant.
  8. eikenhein

    eikenhein 10+ Year Member

    Apr 9, 2006
    Everything sounds good. WBCs leak out of capillary endothelium typically in an inflammatory response. Histamines make the tissues more permeable, and there is interaction of leukocyte integrins and endothelial selectins under stimulation from cytokines. The process proceeds as interaction, rolling, tight binding, and finally diapedesis.

    Yep, but the nodes are not associated with the starting point of lymph vessels. Rather, they can hold on to some WBCs that have passed into the lymph vessels from extravasation.
  9. Rofeh20

    Rofeh20 Kaplan MCAT Instructor 5+ Year Member

    Feb 1, 2007
  10. spreebee

    spreebee Junior Member 5+ Year Member

    Aug 7, 2006
    I'll share with you some brief basic science knowledge...

    1) Extracellular Fluid = Blood Plasma & Interstitial Fluid (fluid between cells)

    2) Intracellular Fluid = Fluid inside the cells

    The Cardiovascular system filtration made extremely simple:

    oncotic pressure --> pull fluid inward, constant pressure from arteriole to venule end
    hydrostatic pressure --> push fluid outward, higher pressure at arteriole than venule end

    1)Blood at the capillaries undergoes filtration-->

    A) Fluid flows from the capillaries into the interstitium (area between cells) at the arteriole end of the capillaries (high pressure) via-->

    *capillary hydrostatic pressure (push fluid out of capillaries),
    *interstitial oncotic pressure (pull fluid into interstitium),
    *negative interstitial hydrostatic pressure (caused by interstial fluid being taken up by lymph vessels connected to interstitium so pull fluid out of capillaries like a straw)

    B) Fluid flows from the interstitium back into the capillaries at the venule (lower pressure than arteriole end) end of the capillaries via capillary oncotic pressure (pull fluid into the capillaries)

    When filtration into the interstitium exceeds lymph flow from the interstitum = edema or swelling-->can tie in with various cardiovascular disorders, seen in swollen ankles, etc...
  11. Area4


    Dec 16, 2007
    Thanks for all the great explanations, I was about to ask the same question.

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