How do we get lymph?
OK. So fluid leaks out of blood capillaries because of the net outward filtration pressure. Then what happens to this fluid to cause it to collect in the lymphatic system as lymph?
Fluid starts to accumulate in the body tissues as extracellular fluid. As the amount of extracellular fluid increases, you can imagine that the pressure of this fluid in the body tissues increases-- just a little bit. That little bit of pressure is enough to push the fluid along. When the fluid hits a lymphatic capillary, the tiny bit of pressure in the body tissues pushes the fluid into the capillary. I have drawn a little picture to try to help you understand this...
At the top of the picture is a blood capillary, where the plasma is able to leak out due to filtration pressure (represented by green arrows).
Below it is the lymphatic capillary. Extracellular fluids (green arrows) can enter the lymphatic capillary under the tiny bit of pressure from the body tissues.
Note that the lymphatic capillaries are only open at one end. That is because the lymphatic system is a one-way system. It starts at a dead end (the lymphatic capillary) and progresses on to the lymphatic vessels. I have colored the endothelial cells of the blood capillary and the epithelial cells of the lymphatic capillary in blue... and you are only seeing a thin section through each capillary (so nuclei are not visible in all of the cells).
Notice how the epithelial cells of the lymphatic capillary overlap one another, rather than just connecting at their ends. This overlap allows them to flap open when liquid comes in, but slap back shut if any fluid tries to get out. The flaps are like one-way valves. I have tried to show this to you in this little animation. Only one end of each epithelial cell moves, while the rest of the cell is held in position by filaments that connect to the connective tissue surrounding the lymphatic capillary.
Keep in mind that endothelial cells of blood capillaries do not do anything like this. When plasma escapes from blood capillaries due to filtration pressure (which is a much larger pressure), it squirts out through the tiny, tiny slits that are not obvious visually between endothelial cells.
Once inside the lymphatic capillaries, the fluid is called lymph. Lymph flows into the lymphatic vessels and continues on to eventually reunite with the blood.
What's in lymph?
I pretty much explained the dissolved material in lymph already... mainly plasma proteins and some wastes. So lymph is water plus this other stuff. There are also some cells that travel in the lymphatic system-- white blood cells (and macrophages). I'll get back to the cells in the lymph node page.
How does lymph flow through its vessels?
Lymph is under no pressure to move toward the heart, kind of like blood in veins. You'll be happy to know that all the same methods for getting blood back to the heart in veins apply in lymphatic vessels and trunks. The major method is contraction of skeletal muscles, followed by pressure changes during breathing. The larger lymphatic trunks can even constrict from smooth muscle contraction in their walls, but the smaller lymphatic vessels lack any muscle. In addition, lymphatic vessels and trunks contain one-way valves within them, preventing any backflow of lymph.
Your book points out that during exercise, when skeletal muscles are really working and breathing is rapid, is the best time for getting lymph back into the blood. Now consider when a person is stuck in bed, unable to leave bedrest for extended periods. Can you picture how little lymph is drawn back up toward the heart? As the lymphatic system fills with lymph, more fluid will only enter the lymphatic capillaries if it is under great enough pressure in the body tissues to push its way in. The pressure of fluid from the body tissues only increases if edema begins. So there is a tendency toward edema with extended bedrest, unless a person's body is moved around a bit.
Problems in lymph flow can lead to edema
When a person has certain clinical conditions, edema can occur. This occurs as a result of blockages or disruptions in lymph flow. For example, a large tumor could press on a lymphatic vessel. Or surgery to deal with other problems can cause an interruption in a lymphatic vessel (especially if lymph nodes are removed) or trunk. If the flow of lymph is stopped or slowed dramatically, fluid will accumulate.
I have tried to show you these possibilities here... when a portion of the lymphatic vessels (including lymph nodes) or trunk are removed, edema can occur near the site of removal. It can even occur back at the original areas that supply that vessel or trunk (not shown). If instead, a lymphatic vessel or trunk is blocked in some way, edema can occur back at the sites that supplied that vessel or trunk with lymph. It can even occur near the blockage (not shown).
© 2011 STCC Foundation Press