The lymphatic system plays a crucial role in protein circulation and in maintaining a healthy fluid balance within the tissues and is part of the horse’s immune defences. Approximately 70% of the horse’s body is water, some of it in circulating blood, but much more contained in and surrounding the cells which make up the tissues. Therefore anything that affects this fluid balance can have a profound effect on the horse’s well being.
In a continuous process the tissues of the body are fed by nutrients carried on protein molecules dissolved in fluid and released by the blood circulatory system. Although most protein molecules and the fluid carrying them are reabsorbed from the tissues into the blood stream some, most notably large 'colloid' protein molecules, can only be removed by the lymphatic system along with inorganic materials, cell detritus, waste products, bacteria and excess water. If for any reason the lymphatic system should fail to do this adequately, swelling of the tissues or ’oedema’ develops. This may be temporary, for example following injury, or it may be because of failure of the lymphatic system itself, which produces the condition lymphoedema.
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The lymphatic system consists of vessels (lymphatics), nodes and some organs. In some ways the lymphatic system resembles that of the blood circulation; a mesh of tiny lymph vessels under the skin collects fluid which becomes ‘lymph’ and connects to larger, deeper vessels which eventually discharge into the blood stream. All but the initial collector vessels have valves which divide them into heart shaped segments – ‘lymphangions’ - and prevent the backflow of lymph. These valves are closed whilst the angion is filling up with lymph, until muscle cells in the vessel walls respond to the rising volume, causing them to pulse. The valve in the direction of the flow opens, allowing the lymph to move forward into the next angion.
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Components of the vessels themselves such as smooth muscle cells and elastic fibres contribute to lymph flow, as do external factors such as respiration, heart pulsation, movement in the intestines and of adjacent skeletal muscles - and Equine Manual Lymph Drainage.
The lymph pathways include lymph nodes which filter lymph, release fluid back into the tissues and play a vital role in the immune defence system.
Over millions of years the horse has evolved from a small, forest living creature with padded feet into a graceful, fast, flight animal, standing on elongated single toes and spending most of its time in constant motion. This continual movement and the support it gives to the lymphatic vessels in the horse’s leg is very important, as these have developed with relatively few of the muscle cells which promote the transport of lymph, but unfortunately, modern methods of horse management - keeping them restricted for hours in boxes, limiting their freedom of movement, and making heavy training demands upon them - severely undermines this fundamental feature of the horse.
One result of this is compromise of a lymphatic system not evolved for these circumstances, and if the lymphatics are not functioning adequately swollen legs will eventually result because the system cannot remove fluid from these tissues quickly enough.
A traditional belief is that high protein feed contributes to horses developing swollen legs, but this is a misunderstanding. A very low protein diet causes oedema, as seen in starvation.
The lymphatic system within the horse’s lower legs is also vulnerable to damage from wounds, infestations and infection, an important factor contributing to the difficulties often encountered with healing in this area.
Fortunately, the horse’s lymphatic system appears to be more responsive to Manual Lymph Drainage than does that of people, with whom we know it works well.