The important role of the lymphatic system

The lymphatic system consists of a network of lymphatic vessels and various lymphatic organs such as lymph nodes, thymus, spleen, tonsils and lymphatic tissue in mucous membranes. Lymph is the fluid that flow in the lymphatic system. Blood moves via the arteries to the capillaries and there, excess plasma leaks into the interstitial space, and there is oxygen and dissolved nutrients, with destination to the cells.

Now this fluid is called interstitial fluid or tissue fluid, which is then absorbed by the lymph capillaries and converted into lymph. In simplified terms, the blood plasma, tissue fluid and lymph are the same basic fluid but in different places and with different molecules, nutrients, waste, cells and more

The lymph is formed when the blood plasma is “squeezed” out at the beginning of the smallest blood capillaries, to provide nutrition and oxygen to the cells. The plasma now enters the tissue and fluid flow between the cells, what we also call the ground substance in the extracellular space of the fascia or the interstitial flow. After exchange with the cells, most of the fluid gets reabsorbed into the post-capillary venules, back to the blood plasma, with carbon dioxide and other waste products. These transports, in and out between blood capillaries and interstitium, occur according to hydrostatic and osmotic pressure gradients.

However, a fraction of the fluid is convected through the tissue and enters the lymph capillaries and travel through the lymph system, later to be drained back into the blood system. Now the fluid is called lymph. This lymph fluid can also contain white blood cells, apoptotic cells, tumor cells, proteins, pathogens and other antigens that exist in the interstitial fluid.

Some fluid remains in the interstitium and there must always be a fluid flow between the cells, but in order for it not to remain too much and become an increased pressure in the tissue, the lymphatic system is an extra drainage system. The smallest vessel of the lymphatic system, the lymph capillaries, begin blindly in the tissue. The lymph is transported away to larger and larger collecting vessels, much like a sucking root system to a tree, and eventually on to the heart to be drained into two veins, before the right atrium of the heart. Now the fluid is back in the blood circulation as blood plasma.

In this way, the lymphatic system maintains the homeostasis in the tissue outside the vessels, the extravascular homeostasis, through a unidirectional flow transport from the interstitial space (the ground substance) back to the blood circulation.

The body produces two to three liters of lymph per day, more when we are exposed to stress such as exercise, heat or inflammatory processes. If there is a stop in the lymph flow, for example due to injury, so that the lymph is not transported away at the same rate as it is produced, the pressure in the tissue increases and swelling and edema occur.

The lymphatic organ and other lymphatic tissue are an important part of the body’s immune system, as the lymphocytes, cells of the specific and adaptive immune system, mature in different lymphatic organs.

In addition to water, the lymph may contain white blood cells, tumor cells, dead cells, proteins, fat molecules, pathogens, and other antigens found in the interstitial fluid. On their way back to the venous system and blood, the lymph passes through lymph nodes. The lymph nodes may occur individually or together in groups. Such large accumulations of nodes are found in axillary, fossa jugularis and groins.

The lymphatic system is an open (starts blindly in the tissue), and has no actual pump like the heart in the blood circulation. It is therefore a slower system than the blood system, the blood moves about 600 times faster than the lymph. The lymph vessels have a certain ability to contract, the vessel walls contain smooth muscle, and thus push the lymph fluid upwards towards the heart. They also have a series of valves that will prevent reflux, such as reflux valves, to ensure the unidirectional flow of lymph. The valves are missing in the smallest capillaries. The skeletal muscle contractions contribute to press the lymph vessels and also push the lymph upwards, past the valves. Also breathing, the pulse of the arteries and the peristalsis of the intestines help to increase lymph flow.

If we summarize the tasks of the lymphatic system, this is how it is;

  • Drain off excess fluid in the tissue and thus help with fluid homeostasis.
  • An important part of the immune system as white blood cells of various kinds are found in the lymph and lymph nodes where foreign invaders, dead or damaged cells are taken care of.
  • Helps transport nutrients and waste, to and from, tissues and blood.