The Lymphatic System: Part 1
What is lymph?
Today we are going to learn a bit more about lymph - I know, it doesn’t sound overly exciting, but it actually plays a vital role in your horse’s body. Many of us are highly aware of our circulatory system which transports blood and nutrients around the body, however, the Lymphatic system is effectively the 2nd circulatory system and vital for your horse’s wellbeing.
Lymph is found in lymphatic vessels and is an almost colourless (slightly yellow) watery fluid. In fact, lymph actually originates from blood plasma which, along with white blood cells, leaks from the capillaries of the circulatory system and becomes interstitial fluid, filling the space between individual tissue cells and bathing the tissue cells of the body. The plasma and white blood cells are forced out of capillaries by hydrostatic pressure (simply translated to pressure on an object submerged in fluid) and as it mixes with the interstitial fluid the volume slowly increases. Because the cells are bathed in tissue fluid they maintain a constant temperature and osmotic pressure which is vital for healthy cells and is known as homeostasis, which you may have heard of. Approximately 90% of this fluid is returned to the circulatory system with around 10% being diffused into lymph capillaries to join the lymphatic system – it is only at this point when the tissue fluid enters the lymphatic capillaries does it become lymph.
Not too complicated, no? Take a look at the diagram below that shows you the key lymph nodes on a horse (and please allow for my poor artistic skills!).
Diagram: The main equine lymph glands/nodes.
The lymphatic system works very much as a secondary circulatory system, however, it has no pump (unlike the vascular circulatory system which has the heart). Lymph, therefore, moves slowly and under low pressure which is caused by an involuntary movement called peristalsis. Always draining towards the heart, the movement of muscles and tendons has a massaging effect to help propel the lymph along. Gravity, however, isn’t lymph’s best friend and can cause it to accumulate in the lower parts of the limbs if stationary for long periods of time (you have probably noticed your horse’s lower legs are slightly puffier in the morning after being stood in a stable overnight). This is why equine massage is so beneficial to horses on box rest or those with limited access to grazing, meaning they are stabled or confined for long periods of time.
The lymphatic system relies on movement of muscles and tendons to “pump” the lymph around the body.
Horses stood still for long periods of time will often display signs of swollen lower limbs.
Movement is essential to help keep lymph flowing, so when movement is not possible (when on box rest for example), equine massage therapy can help.
What are the functions of lymph?
“Okay, okay, but what does lymph actually do?”, I hear you ask, “other than travel very slowly?” Well to summarise, the lymphatic system is responsible for transporting lymph fluid from tissues to the circulatory system and is a major factor in the immune system.
The equine lymphatic system has three main functions:
Removal of excess fluid
Absorption of fatty acids & subsequent transportation of fat to the circulatory system
Production of immune cells e.g. lymphocytes, monocytes, and antibody producing cells called plasma cells
Let’s look at each of these in a little more detail…
The removal of excess fluid: The main function of the lymphatic system is to drain excess fluid from the body with an aim to drain fluid from tissues of the limbs so that they do not become waterlogged. Changes to the balance of blood can cause waterlogging as it can affect the osmotic pressure resulting in a lessened ability for fluid to pass through the vessel walls leaving excess fluid around the tissue cells.
The absorption of fatty acids & transportation into the circulatory system: The lymph vessels from the intestines absorb fat from the intestinal villi and transport it to the Vena Cava where it joins the circulatory system.
The production of immune cells: Any foreign particles along with lymphocytes are carried by the lymph from infected tissue to the nearest lymph node. From here, large cells surround the foreign particles and pass on a message to the lymphocytes warning them of the threat. The lymphocytes then respond by producing antibodies to neutralise particles of a similar nature.
So, that’s the technical bit - we now understand what lymph is and the role it plays in our horses' bodies. In Part 2 of the Lymphatic System Series, we learn more about conditions of the lymphatic system, symptoms and treatment/management. To make sure you don’t miss out, please “Like” my Facebook page Equilibriagold to get an alert when Part 2 is released (very soon!).
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