When healthy, every component of the human body works in harmony, continually making adjustments through complex feedback systems to enable the body to balance its ever-changing internal environment and external demands. When things go wrong and the body, or any of its components come under threat, there are multiple mechanisms to protect, defend and enable the body to heal.
Despite its highly specialised features enabling it to serve multiple functions, the lymphatic system is neglected in textbooks and given little attention until it goes wrong. However, greater understanding of the lymphatic system and how it works makes it possible to help it to work more effectively and reduce the risk of problems such as lymphoedema and cellulitis. In this way, nurses can help educate patients.
This article briefly summarises the role and importance of the lymphatic system and explains how it can be supported, based on understanding how it works, particularly as a transport system and in regulating volume homeostasis.
What is the lymphatic system and what does it do?
The lymphatic system comprises lymphatic organs, lymphoid tissue and vessels (Figure 1) working synergistically to fulfil its three main functions:
Immune surveillance
The thymus produces T-lymphocytes, and the spleen provides a reservoir for maturation and storage of lymphocytes. Other lymphoid tissue such as the tonsils, Peyer's patches in the gut and lymph nodes are lymphoid tissue with high concentrations of lymphocytes ready and waiting to address any threat from invading organisms. The ubiquitous system of distribution of lymphatic vessels ensures that harmful substances are collected, filtered, neutralised and excreted.
Nutrition
Most fat absorption from the gut is undertaken by the lymphatic system. It therefore also plays a key part in the transport of fat-soluble vitamins including vitamin K, important for clotting mechanisms.
Fluid homeostasis
By providing constant drainage of fluid and particulate matter filtered from the cardiovascular system into the interstitial spaces, lymphatic vessels provide essential rather than ancillary support for the cardiovascular system and ensure a stable internal fluid environment (Rossitto et al, 2019). It is only in recent years that physiology research has demonstrated the vital role played by the lymphatic system in maintaining homeostasis and regulating fluid balance by collecting virtually 100% of excess fluid and the particles within it, from interstitial spaces throughout the body (Mortimer and Rockson, 2014).
This article focuses mainly on the lymphatic transport system and how it works in relation to other body systems to support each of its three main functions.
The cardiovascular-lymphatic systems synergy
The cardiovascular system enables transport of fluid, gases and almost all nutrients to every organ and every cell in the body to enable it to live, grow, reproduce and carry out its essential functions. The pumping of the heart maintains the flow of blood into the aorta, the increasingly smaller arterial vessels and via the microcirculation of capillary beds where all the fluid and gas exchange takes place, then into venules to begin its journey back to the heart (Figure 2). When threatened by infection or inflammation, lymphocytes and other specialised cells, including macrophages, are delivered by the circulatory system to enable destruction of the harmful organisms.
These processes themselves result in debris and toxins being released. Basic cellular metabolism produces waste products and a constant turnover of cells results in dead or dying cells within the body. All of these unwanted particles are released into interstitial fluid and need to be eliminated. The unique characteristics of the lymphatic system enables the collection of all this fluid and particulate matter that would be harmful to the internal environment of the body if they were retained. Only when it is within the lymphatic system is this fluid and particulate matter called lymph.
The lymph journey
The lymphatic system transports lymph via a hierarchical series of vessels. Entry into the system is the first step. Fluid and particulate matter enter the system via microscopic blind-ended lymphatic capillaries (initial lymphatics) throughout the body, including a fine mesh immediately under the skin. These comprise a single layer of overlapping endothelium and are loosely connected to surrounding structures by anchoring filaments.
The large spaces between the endothelial cells are more permeable than the blood capillary membranes and so permit passage of larger particles such as used proteins. However, the lack of musculature means that the initial lymphatics are passive, collapsed structures; it is only when there is movement of surrounding structures of the skin, muscle and fascia pulling on the anchoring filaments that the junctions between the overlapping cells open and allow entry of fluid.
Filling of the initial lymphatic chamber and pre-collecting vessels stimulates a peristaltic-type reaction in the subsequent collecting and increasingly larger, lymphatic vessels, which possess an increasingly more developed musculature. This, together with a series of one-way valves, creating chambers called lymphangions, ensures a uni-directional flow channelling the lymph through many superficial and deep lymph nodes, then into larger vessels that return the lymph to the cardiovascular system for elimination via the kidneys.
Lymph nodes process antigens and are rich in lymphocytes. This is where lymph is filtered, cleansed, concentrated and then released. After this process the lymph is channelled into two main routes before being released into the circulatory system. Lymph from the right side of the head, neck, right arm and thorax travels via the right lymphatic duct. Lymph from the rest of the body travels via the thoracic duct, both emptying into the right subclavian vein and then the superior vena cava. The thoracic duct is a large lymph vessel running alongside the spinal column, at the bottom of which is the cisterna chyli, a chamber collecting the fat-rich lymph from all around the gut (Figure 1).
The lymphatic system therefore not only supports the body's immune response by producing cells to combat infection, it also removes fluid and potentially harmful substances, neutralising them and enabling elimination. It is also very responsive to additional demands. If there is an increase in interstitial fluid, for example, as a consequence of an acute or chronic inflammatory response increasing the permeability of the capillary membranes, or chronic venous insufficiency increasing capillary hydrostatic pressure and greater net outflow of fluid into the tissue spaces, the peristaltic action of the lymphatic vessels increases. This compensatory mechanism enables the lymphatic system to manage up to 20 times its normal load, but this is not sustainable.
Maintaining a healthy lymphatic transport system
Fortunately, there are simple actions people can all take to support a healthy lymphatic system. These actions are also effective in improving function in an already compromised system.
Be active
Entry of fluid and particles into the lymphatic system is dependent on movement opening up the initial lymphatic channels. Onward propulsion of lymph is encouraged by muscular activity. It does not need to be a workout but it does need to be regular and frequent. Sitting or lying inactive for long periods means that the passive initial lymphatics remain completely collapsed and empty. The British Lymphology Campaign EveryBodyCan aims to encourage people to be more active to improve lymphatic function and has developed guidance and resources to support this no matter how limited the starting point. Its message is that ‘EveryBodyCan’ be a little more active than they are, and every little helps (https://thebls.com/pages/everybodycan).
Breathe deeply
The largest lymphatic vessel passes through the abdominal and thoracic cavities. Deep breathing and different types of breathing alter the pressures within these cavities and stimulate upward movement and propulsion. Deeper breathing may be a natural consequence of activity but even on its own, can encourage movement of lymph.
Protect the skin
The skin is the largest organ and a barrier to injury and infection. Loss of integrity of the skin damages initial lymphatics and may initiate an inflammatory response. In a healthy individual with a competent lymphatic system, resolution is likely to be swift. However, even in such individuals, failure to resolve quickly should be considered a warning sign and help sought to manage infection or non-healing wounds.
As we age, skin is drier, less elastic, more prone to breaks in the skin and may take longer to resolve (Woods, 2019). There are also many people at risk of lymphatic failure (lymphoedema) who should take extra care, including those whose lymph nodes may have been removed or damaged, for example in cancer treatment, joint surgery, burns, those with chronic venous insufficiency, and those who are obese and/or sedentary (Brown, 2019). Keeping skin clean and supple through regular moisturising is key in maintaining it as an effective barrier.
Maintain a healthy diet and weight
Being overweight increases the load and pressure on the lymphatic system and will ultimately cause parts of it to fail.
Never ignore oedema
Seek to determine and resolve the cause of any oedema. If it lasts more than 3 months it becomes a chronic problem and is indicative of a degree of lymphatic failure.
Over time, a sustained additional burden on the lymph transport system will lead to dilation of lymph vessels, leakage of lymph into the vessel walls, valve incompetence, lymphangiosclerosis and vessel failure, ie lymphoedema. Lymphoedema does not merely result in failure to remove fluid from the interstitium. There is also an inflammatory response due to the inability to remove fats and various toxins, increasing vulnerability to infection, particularly cellulitis. Indeed, cellulitis may be the first indication that an individual's lymphatic system has failed, or is failing, and should always be treated promptly and robustly and followed up.
Other changes occur in the skin and subcutaneous tissues due to overgrowth of the cellular matrix, causing dry, scaly skin, thickened tissues, hyperkeratosis or lymphorrhoea (wet legs).
A common example is seen in chronic venous insufficiency, when pathology in the venous system results not only in an increase in interstitial fluid overburdening the lymphatic system, but also leaking blood vessels, the cells of which add to the interstitial debris not collected by the failing lymphatic system and causing characteristic haemosiderin staining and ankle flare (Figure 3).
Summary
The lymphatic system works constantly to keep the body healthy but, unlike many other body systems, most people are unaware of its functioning until something goes wrong. Yet it is an essential support for the cardiovascular system, vital for fluid homeostasis and the immune response, it has a role in nutrition and maintaining gut health and is responsive to demands increased as a result of other pathologies or lifestyle. It deserves more attention and effort to maintain its effective function.