B.5.5. The Lymph Circulation



A. What is the problem?
1.
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.

2.
This is not completely true. The vast majority of the interstitial fluid, approx. 95%, is indeed reabsorbed by the capillaries. However, a small amount (5%) is not reabsorbed and is left behind in the interstitial space.
3.
This amount will gradually increase in time and cause swelling (=oedema!).

4.
This fluid therefore has to find another way to go back to the circulation. This other way is the lymph circulation.
5.
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.
6.
That defence function of the lymph system is the discipline of the microbiologist and the immunologist and will not be further discussed here. We will (only) talk about the drainage of the lymph.


B. Micro structure of the lymphatic system:

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Local Lymph capillary
1.
Lymph vessels look and behave very much like venules (=small veins) and veins.
2.
They start in the tissues, as blind-ended vessels.
3.
As shown in this diagram, there is a lot of space between the vessels cells so that the interstitial fluid can easily flow, drift, into the lymph vessels.

4.
These spaces and large holes are important because large molecules (proteins, fat) and even fragments of cells (in the case of an inflammation) can easily flow into those lymph vessels.
5.
Remember that these large particles cannot pass the capillary wall into the blood circulation. The only way that they can be transported is through the lymph.

6.
The cells of the lymph vessels are arranged in such a way that they overlap. These overlaps work as valves that will allow fluid to go into the lymph but hinder the fluid to go out again.
7.
There are also, as in the veins, valves inside the lymph vessels so that the fluid can only flow in one direction, away from the tissue towards the heart.
8.
In fact, in lymph vessels, they are many more valves, large and small.


9.
Finally, there are numerous anchoring filaments that attach the lymph wall to neighbouring cells.
10.
These filaments (collagen) keep the lymph vessels open in case of an increase in interstitial fluid.
11.
In fact, as the interstitial fluid increases, this increased amount of fluid will push the cells away from each other and this will pull the lymph vessels even more open! And this in turn will push more interstitial fluid into the lymph vessels.
A lymph vessel in oedema:
Local Lymph capillary in oedema

C. The lymph nodes:

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1.
The lymph nodes are small kidney-shaped structures into which the lymph flows from peripheral tissues. As explained above, this lymph is filled with extracellular fluid, large molecules, cell fragments and possibly bacteria and viruses.
2.
Inside the medulla of the lymph node, nodules filter the lymph. These nodules contain numerous lymphocytes and macrophages. These can ‘eat’ and destroy large molecules etc.

diagram of a lymph node
3.
Inside the medulla, there is also an elaborate capillary supply of blood, which also absorbs a lot of the lymph fluid and streams it back in to the venous system.
4.
As stated before, approx. 50% of the lymph flows back into the venous system in all the lymph nodes together.


D. The lymph tree (both right and left!):

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1.
The lymph vessels, running away from the tissues, gradually merge together and form larger vessels.
2.
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 back into the venous circulation in these nodes.
3.
From these nodes larger (lymph) vessels course through the body. They are actually very similar to the veins (with valves etc).
4.
In several areas, nodes congregate into clusters where they drain from large parts of the body. For example, the axillary lymph nodes drain and filter the lymph from the arms. The inguinal does do the same for the legs.
5.
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. The thoracic duct drains into the left subclavian vein.
6.
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.





diagram of the Lymphe Tree

E. Finally: How does the lymph flow?

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1.
The problem is how does the lymph flow to the lymph nodes and ultimately to the subclavian veins and back into the circulation? After all, there is no lymph ‘pump’ such as a heart.
2.
The problem is very similar to the flow of blood in the veins (which also does not have a pump of its own).


3.
The solution is also similar. Many of the same mechanisms that work for the veins also work for the lymph system. For starter, they also have valves, just like the veins.

4.
The most important mechanism is the muscle pump. As skeletal muscles pump, they push blood (in the veins) and lymph (in the lymph vessels) and thus towards the heart.
5.
Other systems that helped the venous return, such as the respiratory pump, and the arterial pump, also work in the same manner for the lymph system.
6.
It is important to realize that 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!).
7.
It is also important to realize that the lymph from different parts of the body may vary a lot. The lymph from skeletal muscles for example is very different from the lymph that comes from the gut (especially after a meal!).
8.
The role of the lymph flow becomes very visible when nodes in a particular region are removed. This is the case for example in the case of a breast cancer.


9.
In that disease, the tumour in the breast is removed and, often, also the lymph nodes in that region (to prevent metastasis).
10.
These nodes are located in the axillaries and they drain the lymph from the arm.

11.
If these nodes are removed surgically, then the lymph can no longer drain away from the arm.
12.
This lymph stays in the arm and causes swelling (=oedema) of that arm.


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