A Brief Description of the Horses's Lymphatic System
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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 connective tissues (‘interstitium’) of the body are fed by nutrients carried on protein molecules dissolved in fluid and released by the local blood circulatory system. For a long time it had been assumed that this fluid ‘filtration’ was balanced by an almost equal ‘resorbtion’ back into the venous capillaries, but it is now known that it is the lymphatic system which is responsible for the removal of this fluid along with cell detritus, waste products, bacteria, inorganic materials and protein molecules. It includes the ‘superficial lymphatic system’ which collects and transports fluid (becoming lymph when it enters the system) from the skin and mucosa, and the deep lymphatic system which drains the muscles, joints, tendon sheaths and nerves. Individual organs also have lymph drainage. During the transportation process the lymph is cleaned, filtered and concentrated by the lymph nodes, where many immune reactions also occur. Unlike the cardiovascular system, there is no central pump moving fluid.
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The vessels start as very fine, blind ended collector vessels, which drain into larger transport vessels made up of tiny units with valves called angions. 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 pulsate rhythmically. The valve in the direction of the flow opens, allowing the lymph to move forward into the next angion. These vessels connect with lymphatic trunks deep in the body which drain into the blood stream via the subclavian veins. Lymph is transported by the intrinsic self contraction of the angions, and by extrinsic factors – movement of skeletal muscles, arterial pulsation, thoracic pressure changes caused by breathing and intestinal peristaltic motion.
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The chief influence on lymph pulsation is stretching from internal filling or external factors, including Manual Lymph Drainage. 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.
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 stretching and relaxation it provides 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.
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 ‘filled legs’ (‘stocking up’) 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 is more responsive to Manual Lymph Drainage than that of people, with whom we know it works well.
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