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?
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...
http://www.bio.davidson.edu/Biology/student/Restricted/Lindterm/figure%203.GIF
(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.
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.
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.