Let's check out the lymphatic system!

    There is an entire system of lymphatic vessels in addition to the blood vessels that you already learned about.  These lymphatic vessels are not circulatory... nothing travels in cycles in them like in blood vessels.  Instead, lymphatic vessels run from body tissue toward the heart.

    One of their main functions is to pick up excess tissue fluids.   Therefore, this one-way system of vessels begins with lymphatic capillaries out in the body tissue. The lymphatic capillaries pick up any excess fluid in the body tissues (mainly fluid lost by filtration from blood capillaries).   This fluid is called lymph once it enters the lymphatic system.   The lymphatic capillaries merge together to make larger vessels called lymphatic vessels.  The lymphatic vessels are rather similar to veins in their structure-- which should make sense because the fluid in them is under no pressure.

    The lymphatic vessels run toward the heart.  You see, the lymph has to be returned to the blood to maintain blood viscosity at its proper level.  So the lymphatic vessels from all over the body join together as they approach the heart into larger and larger lymphatic vessels.  These larger vessels are called lymphatic trunks.  Each lymphatic trunk is large enough to be noted and memorized like the major blood vessels.  However, for our purposes, we will not be doing that.

    Ultimately, there are two major lymphatic vessels that merge with the blood vessel system at the level of the subclavian veins (right and left), to return the lymph into the blood.  The two main lymphatic vessels (the largest of all lymphatic vessels) are called collecting ducts.  All of the lymphatic trunks merge into these two collecting ducts.  Specifically, they are:   the thoracic duct and the right lymphatic duct.   The thoracic duct empties into the left subclavian vein, and drains the lymph from the majority of the body (legs, abdomen, left side of head, left arm, and left thorax).   The right lymphatic duct empties into the right subclavian vein, and drains much less of the body lymph (only the lymph from the right arm, right thorax, and right side of the head).

    Figure 16.6a in your book illustrates how the lymph drains into the collecting ducts as well.  You may notice (faintly) that the right arm and right thoracic area of the guy in the picture is shaded red.  That shading is supposed to represent the regions of the body that drain into the right lymphatic duct.  The rest of the body drains into the thoracic duct.

Where do lymph nodes fit in?

    In the picture to the above, all the yellowish lines are the lymphatic vessels and trunks.  It is not a gorgeous picture, but I found it on the web and thought it might be nice to see a different one from in your book.  All the green circles are meant to be lymph nodes.  You see...

    On the way from the lymphatic capillaries to the lymphatic trunks, as the lymph travels in the lymphatic vessels, it runs through lymph nodes.  A lot happens to the lymph here.  It gets somewhat filtered in here, and then continues on its way to the lymphatic trunks.  I have an entire web page just on lymph nodes.

Overview of lymphatic vessels:

Why bother losing fluid to just regain it elsewhere?

    I figure that you are probably wondering why we lost fluid in the first place from our blood, if we have to have an entire new system to return it.  It seems rather odd, doesn't it?

    Well, back when we were studying the cardiovascular units, you learned that filtration was a necessary means to get material exchanged from the blood to the body tissues.  Without filtration, we can't get many nutrients, hormones, and globulins (with the stuff they carry) out into our body tissues.  It is absolutely vital to spill out these things, especially the nutrients!  Once in the extracellular fluid, these items that spilled out of the blood by filtration can be picked up by the cells of the body.  For example, glucose (a nutrient) in the extracellular fluid is quickly picked up by any cell that comes in contact with it; so the glucose has to be out and about in the extracellular fluid to be available to the cells.

    That's why filtration has to occur in the blood capillaries.   Don't lose sight of that.

    But, as filtration occurs, other things can leak out, too.  For example, blood albumin, which has no function other than to just maintain osmotic balance, can leak out into the body fluids.  Some nitrogenous wastes may spill out on their way to the kidneys for removal from the body.  And, most importantly, lots of water spills out from the plasma as well.  These things should not be out in the body fluids.  If too much water spills out, our body tissues swell.  This is called edema, and is a terribly uncomfortable condition.  But filtration is not specific-- right?   It is just a net pressure on plasma to leave blood capillaries.  So these other things leave, too.  Oops.

    Therefore, we need to get all the stuff that the cells don't pick up back into the blood.  We need to collect the water, albumin, wastes, etc., and put them back into the blood.  This is where the lymphatic system comes in.

    Because we need an entire system to do this, it would seem a waste to use this entire new system solely as a supplement to the blood system.  So, our lymphatic system does more!  (of course)  It is involved in helping maintain our immunity against infection.  And it helps in cleaning out the wastes from our body tissues.

What now?

    OK.  Now it is time to learn about lymph itself.  To get to that, I will describe the lymphatic capillaries and how lymph forms.  This is in your next web page.  After that, I'll describe the lymph nodes, and the other lymphatic organs (thymus & spleen) that are visible in the image above.  Ready?   Good!  Let's go on to learn about lymph.

© 2011 STCC Foundation Press
written by Dawn A. Tamarkin, Ph.D.