Chapter 1 What is the lymphatic system?

Chapter 1

What is the lymphatic system?

The circulatory system in the body of an average adult is a network of about 60,000 miles of arteries, veins, vessels and capillaries, each reducing in size the farther they extend from the heart and increasing as they return to heart. Blood flows to the organs through the arterial vessels, supplying oxygen to the cells of our body. It also delivers nutrients to them so that the cells can eat, work and interact with the rest of the body. Unnecessary products are removed from the cells by the lymphatic system and transported back to the circulatory system. The walls of the arterial capillaries, to put it simply, are permeable, like a “sieve”, with many holes through which only particles of a corresponding size can pass. Molecules of substances pass into the intercellular space from the arterial capillaries and, with them goes 100% of the volume of liquid.

As the blood gives off oxygen, the arterial capillaries become venous, i.e., oxygen depleted. Arterial capillaries differ from venous capillaries in the amount of oxygen present. In the first, there is a high volume of oxygen because it is coming ‘from’ the heart. In the second, the oxygen has been nearly depleted and the platelets of the blood become more charged with carbon dioxide. This is the junction at which the arterial and venous capillaries unite and the exchange is made. The latter feed back into the veins and then the arteries through which the blood is returned to the heart, passes through the lungs, is re-oxygenated and, once again, sent to nourish the cells. (Fig. 1)

The venous (deoxygenated) side of the lymphatic capillary has the same permeable ‘sieve-like’ structure as the arterial (oxygenated) side. Therefore, particles of only a certain (small) size can return from the intercellular space, and larger molecules (e.g., large, dispersed proteins) are not permitted to pass pass through. Ninety percent (90%) of the fluid that entered the intercellular

Fig.1 Arterial and venous capillaries. 1 - arteriole, 2 – capillaries, 3 - body cells, 4 - venule, 5 arteriole sphincter

space, together with the substances that were able to pass through to the venous side of the capillary, is returned in the direction of the heart. The remaining ten percent (10%) of its volume, together with large proteins, remains in the intercellular space which acts like a ‘water magnet’. If everything were to stop here, in other words if the function of the lymphatic system were somehow compromised or would fail completely, the intercellular space would begin to accumulate liquid, large proteins, and various other substances and structures. In addition, the volume of the intercellular space would increase accordingly. As a result, our body mass would have the potential to increase by as much as 3-4 kilograms per day. But, this typically doesn’t happen thanks to the efficient work of our lymphatic system.

This system is an amazing example of nature’s complex engineering. The lymphatic system begins with the formation of the initial lymphatic vessels (lymphatic capillaries) in the intercellular space almost out of nowhere. (Fig.

2, Fig. 3)

Fig. 2 Circulatory and lymphatic system. 1- small circle of blood circulation, 2 - large circle of blood circulation, 3 - arterial bed, 4 - venous bed, 5 - lymphatic system, 6 – heart, 7 – pulmonary veins, 8 - lungs, 9 – initial lymphatic vessels, 10 – anchoring filaments. The arrows indicate the direction of flow.

Fig. 3 Lymphatic capillary. 1 - lymphatic capillary, 2 - venule, 3 - arteriole, 4 – body cells, 5 arterial section of the blood capillary, 6 - venous section of the blood capillary, 7 - sphincter of the arteriole. The arrows indicate the direction of the fluid flow

What are the lymphatic vessels and what are their distinctive features?

As we have already established, the wall of the blood vessel is permeable and resembles a sieve. Conversely, the wall of the initial lymphatic vessel or lymphatic capillary looks much like roofing shingles on a house or, perhaps better, the scales of a fish: one cell slightly overlapping another. Anchor filaments (draw strings) are attached to each cell of the lymphatic capillary and, at the opposite end these filaments are attached to the framework of the intercellular space. (Fig. 4)

Arterial capillaries not only deliver oxygen and water to the intercellular space but, also, all the necessary nutrients and building materials (glucose, amino acids, etc.) for our body’s cells. Nutrients enter the cells and provide fuel for various processes of metabolism such as cell repair, building new components, etc. Then, the byproducts of metabolism (that is to say, those substances that the cell no longer needs, and those that it, itself, has synthesized) fall back into the intercellular space. These substances may be the same size, or slightly larger, depending upon the substances produced by this type of cell.

When the volume of the intercellular space increases, the anchoring filaments are activated, they shorten, and the cells of the lymphatic capillaries divert (shift) slightly. Openings are created from this shift which allow large proteins; the remaining 10% of the liquid; viruses; bacteria; fungi, and dead and tumorous cells to enter. So, anything that cannot be removed from the intercellular space with the help of blood vessels is collected in the lymphatic capillary. (Fig. 5)

Fig. 4 Structure of the lymphatic capillary. 1 - endothelial cell, 2 - supporting anchor filaments, 3 - direction of lymph flow, 4 - inlet valves, 5 – prelymphatic channels.

Fig. 5 Lymph formation. 1 - lymphatic capillary, 2 - venule, 3 - arteriole, 4 - body cells, 5 arterial section of the blood capillary, 6 - venous section of the blood capillary, 7 - sphincter of the arteriole, 8 - tumor cells, 9 - bacteria, 10 - metabolic products, 11 - nutrients.

What is the function of lymph nodes?

The content of the lymphatic vessels is called lymph. Lymph returns proteins and excess interstitial fluid to the bloodstream. The lymph moves through the lymphatic vessels and, after some time, arrives at the lymph node. (Fig. 6)

Lymph nodes are not unlike a common water filter that we use in our kitchen. They are biological, active filters which contain the cells of our

Fig. 6 Lymph node. 1 - capsule, 2 - cortical substance, 3 - brain substance, 4 - follicle, 5 lymphatic vessel (in), 6 - lymphatic vessel (out), 7 - lymphangion.

immune system. These cells have the ability to fight viruses, bacteria, fungi and tumor cells that are transported here by the flow of lymph. In other words, the function of the lymph nodes is to clear the lymph. The immunity cells shred these pathogens into smaller pieces so that they are no longer able to damage our body. Proteins are permitted to safely pass through these filters and enter back into the circulatory system. They are the main building material of the cells of the entire human body. Also, proteins are the main component of all enzymes. Proteins are involved in the blood transport of oxygen (hemoglobin), lipids (lipoproteins), carbohydrates (glycoproteins), and some vitamins, hormones, medications, etc. Since the protein is necessary for the body, the lymph must be cleaned so that the result is a sterile protein solution that is returned the blood system.

The lymph of most of the body (the intestines, lower extremities, the left half of the chest, the left arm, and part of the face) is collected in the thoracic lymphatic duct, which flows into the left venous angle. There is also a short right lymphatic duct that collects lymph from the right side of the chest, right arm, and part of the face. But, the main load still diverted to the thoracic duct.

(Fig. 7)

The average person may be aware of the large groups of lymph nodes in the body — the popliteal, inguinal, axillary, and cervical; but, there are

Fig. 7 Right and left venous angle. 1 - internal jugular veins, 2 - subclavian veins, 3 - superior vena cava, 4 - thoracic duct, 5 - right lymphatic duct.

many more in the chest and abdomen. A huge amount of lymph (pathogens, bacteria, etc.), which require cleaning, comes from the intestine. The lymph is directed to lymph nodes for filtering while fats from our food are absorbed and are passed to the blood circulatory system. That is why the largest number of lymph nodes is concentrated here. (Fig. 8)

Fig. 8 Thoracic duct and lymph nodes of the abdominal and thoracic cavities.

Fig. 9 Superficial lymphatic vessels and nodes (front view). Lymphatic anastomoses (in red): 1 – axillary (armpit), 2 – axillary-inguinal, 3 - inguinal (groin). Watersheds - regions of the body that receive dual blood supply from the distal branches of 2 large arteries - (in blue): 1 - upper horizontal, 2 - lower horizontal, 3 – sagittal (center – vertical).

Fig. 9 Superficial lymphatic vessels and nodes (back view).

How is lymph transported?

When we speak about the circulatory system, we are speaking about the roadway of arteries, veins and capillaries through which blood is circulated. The circulatory system is a closed system. When we speak about the lymphatic system, we are speaking about the transport of fluid. The lymph moves from one point to another, from the periphery (the extremities and organs) to the center, and, normally, never returns back.

Fig. 10. Лимфообразование (см. обозначения на 5-м рисунке)

There are five main mechanisms of lymph transport:

1. Lymph formation. As new lymph enters the lymphatic capillary, the old moves and gives way. This mechanism works only in the smallest lymphatic vessels, the lymphatic capillaries. In larger vessels, other mechanisms operate. In order to understand them, let’s draw a section of a lymphatic vessel, located between two adjacent valves - lymphagion. (Fig. 9) In lymphatic vessels larger than capillaries, there are folds of the inner wall that form valves. They, like gates in the wind, can open, allowing a portion of lymph to pass. But, when the valve is closed, it does not allow reverse current, making the movement of lymph one-directional.

2. Reduction of lymphangions. In the wall of the lymphangion, there are muscle cells that are able to contract independently. Sometimes the lymphangion is called the lymphatic heart. We have one heart in our circulatory system, but

Fig.11. Лимфангион («лимфатическое сердце»): 1 - клапаны; 2 - мышечный компонент стенки

there are hundreds or thousands of «hearts» in the lymphatic system. They are, of course, not so strong, but each contributes to the movement of lymph. Lymphangions have a much slower pulse rate than a normal heart (it is a very lazy structures) - from 6 to 20 times a minute. (Fig. 10)

3. The contraction of skeletal muscles. (Fig. 12) This is the strongest of the lymph transport mechanisms, but it is not as permanent as the structure of the lymphangion. When we strain our muscles, they increase in volume. If the strained muscle is located near the lymphatic vessel, it compresses it and the valves will not allow the lymph to go down — only up, which means that it improves the transport of lymph. So, any of our movement contributes to the movement of lymph. When we stand in one place or sit, the transport of the lymph is carried out only by the constriction of the walls of the lymphangion. When we start moving, the skeletal muscles come to the rescue. Each of our muscles is a very powerful pump.

Fig. 12 Lymphatic vessel and skeletal muscle. 1 - bone, 2 - muscle, 3 - vein, 4 - lymphatic vessels, 5 - adipose tissue, 6 - skin.

4. Pulse wave. There is another mechanism at work in the deep lymphatic vessels that lie almost next to the bones. It is a fact that in a closed space, arterial, venous and lymphatic vessels lie close. When our heart contracts, a portion of blood is ejected from it and a so-called ‘pulse wave’ spreads through the arteries causing them to dilate. (Fig. 13) Since this occurs in a confined space, this expansion is transmitted to the lymphatic vessels as well. Accordingly, the arterial vessel expands and the lymphatic vessel narrows. The pressure between the valves increases and creates an additional opportunity for the transport of lymph in the desired direction.

5. Breath. When we breathe, the pressure in the chest periodically becomes very low. There is a sucking effect in all of the vessels in the chest as if the liquid is being sucked from the underlying compartments. This applies to both veins and lymphatic vessels. Also, the movements of the diaphragm are transmitted to the area of the ‘milky cistern’ - the place where the thoracic

Fig. 13 Pulse wave as a mechanism of lymph transport. 1 - vein, 2 - fascia, 3 - lymphatic vessel, 4 - artery.

lymphatic duct begins. With each movement, the cistern is emptied, and a significant volume of lymph rushes through the largest lymphatic vessel and into the circulatory system. Therefore, in the treatment of lymphedema, respiratory gymnastics is used, which has a direct effect on the work of the lymphatic vessels, located in the thoracic and abdominal cavities.

Fig. 14. Дыхание как механизм транспорта лимфы:

A - повышение давления в грудной клетке (выдох); В - снижение давления в грудной клетке (вдох)

Superficial and deep lymphatic systems

Under our skin is subcutaneous fat (SCF) and, under this, muscles and bones. (Fig. 14)

In the extremities, the lymphatic system is arranged as follows: there are deep lymphatic vessels that are between the muscles and bones, and there are superficial vessels that pass through the subcutaneous fat. These are not two separate systems – they are connected to each other, and interact with each other. In a healthy body, the load that falls on the superficial lymphatic vessels of the hands and feet is greater than on the deep vessels. As we have discussed, the valves in the vessels of the lymphatic system permit flow in

Fig. 15 Cross-section of forearm. 1-bone, 2 muscles, superficial lymphatic vessel (SLV), deep lymphatic vessel (DLV)

only one direction, which means that flow is only permitted from the deep lymphatic vessels to the superficial lymphatic vessels. Why is this important? Because bones, muscles and fascia are dense environments, they are unable to store lymph. If, for any reason, lymphatic flow is disrupted, lymph is flushed to the surface portion of the lymphatic system.

At the same time, the skin is pliable and stretches very well, the adipose (fat storage) tissue is somewhat similar to a sponge - it is loose and soft. This leads to the accumulation of fluid in the area of the skin and subcutaneous tissue in the event that the function of the lymphatic system is compromised.

Fig. 16. Схема среза конечности в норме (А) и с лимфатическим отеком (В): 1 - кожа; 2 - подкожная жировая клетчатка; 3 - мышцы