The functional anatomy and structural integrity of the lymphatic system is, unfortunately, a greatly under appreciated dynamic in the quality of our everyday lives. Frequently the importance of the lymphatic system is nested in conversation of the circulatory system as a whole or in immunity at a cellular level. There are key principles that guide a conversation of the lymphatic system.
- Maintenance: The lymphatics absorb excess fluid, proteins, electrolytes, toxins and foreign debris from the tissues. This cleanses the tissues and thereby maintains functional integrity of the connective tissues.
- Transport: The lymphatics transport substances between the tissue compartment and the blood. Through this function the lymphatics help to localize infections in the body.
- Defense: Immunocompetent cells are generated and distributed by the lymphatic system.
FUNCTIONAL INTEGRATION OF THE SYSTEMS
It is important to note that many authors combine the lymphatic system into the cardiovascular system; the systems are so intricately interwoven that it is certainly fair practice to treat the systems as a single larger system. When thinking of the circulatory system it is absolutely appropriate to view it as a huge circle that starts and ends at the heart.
The lymphatic system is profoundly unique in its unidirectional nature. All other systems in the body are circular from a functional standpoint. This is not the case with lymphatics. This microscopic, blind-ended vasculature begins in the tissue space (interstitial space) and then converges upon itself forming larger and larger vessels. These vessels will specialize into nodes throughout the body.
Lymph nodes can be found in most of the areas of the body where there are ‘pockets’ to provide safety and room for nodes to rest. These areas are referred to as nodal beds. Nodes, for instance, are found under the medial and lateral malleoli under the ankles (malleolar nodes), behind the knees (popliteal nodes), in the hip crease (inguinal nodes), in the connective tissues of the abdomen (mesenteric nodes), under the armpits (axillary nodes), deep to the muscles of the neck and under our jaw bone (cervical chain nodes). There are between 400 and 1000 nodes in the body; over half of the nodes are located in the abdomen. Lymphatics are pervasive throughout the entire body, with one exception. The central nervous system has no lymphatics.
As fluid passes from lymphatic capillaries to vessels to nodes and onto larger vessels into lymphatic trunks, the fluid moves closer to the heart and will eventually converge on the subclavian veins. At the junction of the subclavian vein and the internal jugular vein the lymphatic fluid is returned to the systemic circulation.
The lymphatic system begins functioning around the 5th week of fetal life. Its developmental path closely mimics that of the circulatory system. It is most widely believed that lymphatics develop as outpocketings of the veins in the developing fetus. As such they follow the route of the veins traveling from the periphery toward the heart. The 2 central ‘sacs’ form during the 5th week and allow for the drainage of the head, neck and upper limb. During the 6th week, 4 additional reservoirs form to drain the lower limb and abdomen. When the mature lymphatic system has formed, there is one thoracic duct that drains the entire body, except for the right side of the head and neck, right upper limb right thoracic region. The right lymphatic duct is also part of the final anatomy of the lymphatic system and it serves to drain only this right upper quadrant region of the body.
Blood is ejected from the left ventricle of the heart and travels via a system of arteries and arterioles to vast networks of microscopic capillaries. There are only 2 tissues in the body that do not have blood supply (cartilage and epithelia). The dense mesh of capillaries nutritionally supports all other tissues. Capillaries are the place of exchange. Cellular wastes are exchanged for nutrients in the arterial blood. Carbon dioxide is exchanged for oxygen.
The capillary is literally just one cell thick. Many substances can pass through the cell membrane, while others require transport proteins to carry them across specialized membrane ‘tunnels’ to the other side. The membrane is selectively permeable, meaning that the size and shape of its ‘tunnels’ will only allow certain molecules across.
Importantly the large red blood cells are retained inside the capillary. They are simply too big to cross over into the tissue space. Only a clear filtrate of the blood is permitted to pass into the tissue space. When this fluid was part of whole blood it would have been termed blood plasma. Now that this fluid has filtered out of the capillary and into the tissue space, it will be called tissue fluid, or interstitial fluid. In the space between all of the tissues in the body there is this fluid. It acts as part of the interface for the transfer of nutrients and wastes between cells.
Interstitial fluid can be considered the ‘clear phase’ of the circulatory system. ‘Clear’ refers to the relative color of the fluid itself. When erythrocytes, or red blood cells, are added to the fluid it takes on a dark red appearance that we usually think of as blood. When the red blood cells are sieved out of the fluid, a clear(ish) colored fluid is left behind. This fluid bathes the cells and acts as a medium for the exchange. This fluid will come to contain a relative sampling of the state of the tissue space. For example, if there were bacteria present the tissue fluid would contain indicators of a potential infection.
Each circulatory cycle, the tissue fluid is filtered out of the capillary, acts as a medium for exchange in the tissue space and then the tissue fluid re-enters the capillaries, becomes part of the blood plasma again and returns to the heart. Fluid returning in the veins will travel through a system of venules and veins before entering a vena cavae and returning to the heart and closing the loop of the circulatory system. In the systemic circulation:
heart > arteries > capillaries > veins > heart
But … not all of the tissue fluid returns to the circulation in this manner, some fluid will take a different route through the structures of the lymphatic system. About 10% to 20% of the tissue fluid is taken up from the tissue space not by the venous side of the capillary, but by a blind-ended lymphatic capillary. Once the tissue fluid enters the lymphatic capillary it is appropriately termed lymphatic fluid, or lymph. This ‘sampling’ of tissue fluid could be thought of similar to quality control in a factory setting. If a factory makes shoes they do not carefully inspect each shoe that comes off the line, rather they take a sample (maybe 10 to 20%) of the shoes and inspect them as representative of all the shoes. Our lymphatic system exists for this function, to act as ‘surveillance’ for the body. The proportion of tissue fluid that enters the lymphatics will take a different journey, through immunological checkpoints, called lymph nodes, before reentering the blood plasma of the veins.
capillaries > vessels > nodes > trunks > right lymphatic duct/thoracic duct > veins
Most of the body will drain ultimately its lymphatics through the thoracic duct. The upper right quadrant of the body has its own unique drainage pattern. We will see later that the right side of the thorax, right head/neck and right upper limb all have unique circulation compared with the left. Bilaterally, lymph is dumped from the main trunks into the subclavian veins that run under the clavicle [sub, under; clavian, clavicle). These veins join the superior vena cava and thereby return to the fluid as a component of whole blood to the heart.
THE LYMPH NODE
Lymph travels through the lymphatic vessels to nodes where a sort of ‘percolation’ takes place. Think of a coffeepot. It has a hole in the top to dump a good deal of fluid into. For our conversation here this is analogous to the lymph coming in the afferent ducts of the lymph node from the periphery. The coffeepot has a chamber wherein the fluid is slowed down and altered. This represents the node. Inside the node the fluid will be surveyed for potential situations that will require an immune response. Fluid leaves through a very small opening and continues in the same direction onto its next fate. The fluid may pass through multiple nodes before reentering the blood plasma at the subclavian veins. Lymph nodes function specifically to:
- Filter and purify the lymph
- Capture and destroy toxins
- Concentrate lymph
- Produce lymphocytes (which increases as flow through the node increases)
FUNCTIONAL INTEGRATION OF THE SYSTEMS
So, the circulatory cycle begins with a nutrient and oxygen rich phase that brings these elements to the tissues (the ‘red’ phase). At the smallest level of the single celled vessel, the capillary allows for exchange (the ‘clear phase’). Most of the waste and carbon dioxide is carried back to the heart through the venous system (the ‘blue’ phase). A small amount of tissue fluid (‘clear phase’) is carried into the lymphatic system where it acts as a biological sample for surveillance of the status and needs of the body before returning to the veins just before they enter the heart.