In the Starling-exchange system, I have said that all the fluid that leaves the capillary at the beginning of the capillary is reabsorbed again at the end of the capillaries.
This is not completely true. The vast majority of the interstitial fluid, more than 95%, is reabsorbed. The remainder is not reabsorbed and is left behind in the interstitial space.
This amount will gradually increase in time and cause swelling.
This fluid therefore has to find another way to go back to the circulation. This other way is the lymph circulation.
We also need the lymphatic system to defend ourselves against bacteria and viruses. This is done by specialized cells, such as lymphocytes and macrophages, who attack and phagocyte (=eat) these intruders.
That function of the lymph system is the discipline of the microbiologist and the immunologist and will not be further discussed here.
The lymph vessels collect in the lymph nodes where potential bacteria and viruses are attacked and removed. About 50% of the lymph is also absorbed in the venous circulation in these nodes.
From these nodes much larger vessels course through the body.
The lower limbs, the abdomen, left arm and left chest and the left side of the head collect their lymph in the thoracic duct that runs through the abdomen and chest and ends at the left subclavian vein.
The right arm and the right side of head and chest drain into the right subclavian vein.
I don’t know why this is asymmetrical but this has undoubtedly something to do with the embryology of the system.
D. How does the lymph system work?
The problem is how does the lymph flow to the lymph nodes and ultimately to the subclavian veins? After all, there is no pump such as the heart in the CVS.
The problem is very similar to the flow of blood in the veins (which also does not have a pump of its own).
The solution is similar. Many of the same systems that work for the veins also work for the lymph system.
The most important system is the muscle pump. As skeletal muscles pump, they push blood (in the veins) and lymph (in the lymph vessels) towards the heart.
Other systems that helped the venous return, such as the respiratory pump, and the arterial pump, also work in the same manner.
The total amount of lymph pumped is very low: about 2-4 litres per day! (Compare this with the cardiac output of 5 litres per minute)
It is also important to realize that the lymph from different parts of the body may be very different. The lymph from skeletal muscles is very different from lymph from the gut (especially after a meal!).
The role of the lymph flow is very clear when nodes in a particular region are removed. This is the case for example in breast cancer,
In that disease, the tumour in the breast is removed and, often, also the lymph nodes in that region (to prevent metastasis).
These nodes are located in the axillaries and they drain the lymph from the arm.
If these nodes are removed surgically, then the lymph can no longer drain away from the arm.
This lymph stays in the arm and causes swelling of that arm.