The bodies of mammals have two major circulatory systems, lymphatic and cardiovascular. If you feel that you don’t know as much as you could about your horse’s – or possibly your own – lymphatic system, don’t worry, you’re not alone. Historically it’s been much harder to view and understand the lymphatic system than the cardiovascular and until recently it has been rather neglected. This has led to it not only being overlooked, but to the development of inaccurate and sometimes quite strange ideas about how it functions and its role within the body.
The lymphatic system is a very active, sensitive, highly complex structure which in some ways resembles the cardiovascular system, but there are also important differences. The blood system forms a continuous circuit, carrying oxygen, glucose, vitamins and minerals, hormones and defences against invading pathogens to the tissues, and taking away metabolic wastes such as carbon dioxide, urea and lactic acid. When these nutrients reach the blood capillaries, they are able to move, some on large protein molecules, through the capillary walls and enter the tissues travelling to cells through the interstitium, which is the amorphous connective tissue with which the cells are surrounded. Tissue fluid is essential for this active process, and is continually being refreshed by plasma, the liquid component of blood, also leaving the vessels.
The return of fluid and protein molecules to the blood stream is an essential function of the lymphatic system. It starts with fragile, thin walled lymph capillaries – also called initial lymph vessels – anchored in the interstitium by fibres of collagen and elastin, a protein critical to the elasticity and resilience of many tissues, allowing them to resume their shape after stretching and contracting. As the flow of plasma fluid from the blood capillaries expands the volume of the interstitium, these fibres are moved apart and pull on the capillaries, opening up spaces in their walls which allow them to collect fluid, called lymph once inside the vessels. Lymph typically consists of water, plasma proteins, hormones and enzymes, cellular debris and foreign substances, including bacteria and viruses during infections.
Lymph travels from these capillaries into ‘pre-collector’, and increasingly larger, more substantial ‘collector’ transportation vessels and ducts, which eventually connect with the junction of the jugular and subclavian veins near the heart. With the exception of lymph capillaries, vessels have valves to prevent backflow, which divide them into sections called angions (‘little hearts’) so that together they microscopically resemble a string of pearls. Unlike the blood circulation, the lymphatic vessels don’t need a central heart to propel their contents, as the angions possess ‘pacemaker’ cells and having smooth muscle cells and elastin in their walls, are able to move lymph themselves through rhythmical contractions. These are extremely sensitive to changes in lymph flow and pressures from surrounding organs and tissues, stimulating the angion walls to stretch and contract. Normal lymph vessel contraction is quite leisurely, but can increase at least ten fold when necessary. However, the lymphatic system also has a maximum capacity, and when this is reached oedema may occur because there is more fluid to be removed than the system can handle. This is overcome with time.
The lymphatic system is divided into two parts, the ‘superficial’ which is an interconnected network of initial and collector vessels draining the skin and subcutaneous tissue, found above the fascia separating the skin from deeper tissues, and the ‘deep’ which drains organs, muscle tissue, tendon sheaths, nervous tissue, the periosteum of bones, and joint structures. The two parts are joined by vessels which perforate the fascia. The superficial system is made up of drainage ‘territories’ or lymphosomes, divided by boundaries known as ‘watersheds’, and although these have few connecting vessels, they can be utilised by manual lymph drainage to move lymph from one territory to another.
The lymphatic system is asymmetrical. Lymph draining into the left venous junction comes from the lower (hind) limbs, abdomen, left half of the thorax, head and neck and left upper (fore) limb, that draining into the right from the right halves of thorax, head and neck and right upper (fore) limb.
Transport of proteins and fluid back to the blood circulation is one essential role of the lymphatic system. Another is immune surveillance, ‘lymphoid organs’ – spleen, thymus, bond marrow – create white blood cells essential for fighting infection, and when this does occur, the lymphatic vessels transport pathogen absorbing immune cells to lymph nodes situated along their length, which can recognise bacteria and viruses, and initiate a defensive response.
a significant amount of the water in lymph is also returned to the blood stream through the nodes. Lymph nodes tend to be found in groups, each filtering a specific drainage territory or region of the body’.
The lymphatic system is also the route by which certain essential fatty acids leave the digestive system and are carried into the body.
In recent years, improvements in scientific visualisation techniques have led to greatly increased interest in the lymphatic system. The number of conditions which it is now known to be involved in, and our understanding of its intricate relationships with other systems and tissues, is steadily growing.
For example, it had long been assumed that most of the fluid entering the interstitium was subsequently returned to the blood stream by re-absorption into venous capillaries, with approximately 10% only being returned by the lymphatic system. This is now known to be wrong and that the lymphatic system alone is responsible for fluid return, which means some important rethinking of our traditional medical and therapeutic ideas.
The Horse’s Lymphatic System
Something to note when comparing the equine lymphatic system to our own is that it reflects the horse’s evolution as a creature almost constantly in motion. This may be why elastin makes up 40% of equine lymphatic vessel walls, assisting them to stretch and contract while on the other hand having fewer smooth muscle cells than those of people.
Horses have approx 8000 small lymph nodes, compared to humans’ approx. 600 larger ones and 60 in the dog. Many lymph node groups which are manually accessible in people are not in horses because of our different anatomy. There is a central dorsal and ventral watershed running along the length of the body, in people it’s possible using MLD to move lymph across this, but in the horse the division is absolute.
The absence of a significant muscle layer in the horse’s leg between the hoof and the knee and hock joints means the hoof and fetlock ‘pumps’ gain great importance. The hoof pump is responsible for filling and emptying the lymphatic initial vessels and collectors through the interplay between ground contact and suspension of the moving foot. The hoof wall acts like a pressure container, influencing both lymph and blood vessels, and the lymphatic system’s essential role in fluid balance is especially important in the tightly confined structure of the hoof. The action of the mobile fetlock and the contraction of the elastic fibres in the vessel walls are almost exclusively responsible for the drainage of lymph from the hoof towards the knee and hock joints. The whole of the hoof tissue fluid enters directly into the lymph deep collector system of the leg. In the area of the pastern and fetlock, collector vessels of the skin drain directly into the deep collector vessels, which may increase the risk of infection in the lower limb.
The evolution of the horse’s leg may leave it vulnerable to lymphatic problems. Considering the close role horses have played in our lives for so long, it’s easy to forget that they aren’t really suited for domestication. We want them to be sheltered in nice, comfortable boxes, but their lymphatic systems would much prefer them to be walking many miles a day. This lack of movement can cause problems for the lymphatic system such as ‘filled legs’/’stocking up’, which may eventually lead to lymphangitis. Stable and exercise bandages and even boots, have been shown to adversely affect the blood and lymphatic circulations of the leg.
The Dog’s lymphatic system
Although the lymphatic systems of different mammals show variations reflecting each species evolution, they are fundamentally alike, such that a wide variety of animals are routinely used to explore the human system. The dog’s lymphatic system therefore shares much in common with those of humans and horses, and perhaps the most striking difference is that the dog has approximately 60 larger lymph nodes compared to the horse’s 8000 small ones.