by Kerstin Baran
After a long day of sitting in class, you finally get home and take off your shoes. You notice the imprint left on your leg by your crew socks. Throughout the day, fluid collects at the bottom of your legs due to gravity, making your legs slightly swollen and causing the imprint. This extra fluid at the bottom of your legs will be removed by your lymphatic system. Without your lymphatic system, the fluid pooled at the bottom of your feet would remain and result in massive swelling. While often overlooked, the lymphatic system is found almost everywhere in the human body and plays a variety of essential roles in human health like fluid homeostasis, immune cell transportation, and dietary fat absorption.
In order to achieve its many roles, the lymphatic system is made up of a variety of structures such as vasculature, organs such as the spleen and lymph nodes, and specialized structures in the small intestine. Similar to the blood vascular system, the lymphatic vascular system consists of lymphatic capillaries that branch out from larger lymphatic vessels (Figure 1). However, instead of circulating blood, lymphatic vessels are responsible for the transportation of a milky white fluid called lymph. Lymph fluid contains immune cells, proteins, excess cell waste, and fats. Lymphatic capillaries collect the excess lymph fluid from tissues, and the larger lymphatic vessels transport the lymph back into the bloodstream via the thoracic duct. As the lymph is transported back to the bloodstream, the lymph is filtered through small organs that house immune cells called lymph nodes (Figure 1). The immune cells in the lymph node monitor the lymph for harmful substances such as viruses or bacteria. Thus, the lymphatic system plays a role in both collecting and transporting fluid, but also assisting in the detection of harmful substances.
Figure 1. Cartoon of the lymphatic vascular system in males (left) and females (right). Figure made by author in BioRender.
Another fascinating function of the lymphatic system is the absorption of dietary fats. Our dietary fat comes from foods like avocados, fish, nuts, and milk products. Two major roles of dietary fats in our health include absorbing fat-soluble vitamins like vitamin A and D and also to provide us with a source of energy. In the small intestine, there is a specialized lymphatic structure called the lacteal involved in dietary lipid absorption. The lacteal is the center of each villus structure which is in contact with the dietary fats in the small intestine (Figure 2). The lacteal will absorb the fat in the small intestine, and will transport the fat back into the bloodstream through the lymphatic vessels, aiding in the metabolism of fats. The lymphatic system has a variety of roles in human health from aiding in digestion to supporting the immune system.
Figure 2. Cartoon of the lacteal in the villi of the small intestine. Figure made by author in BioRender.
One detriment that occurs with lymphatic dysfunction is when the lymphatic vessels fail to effectively collect lymph from tissues, thus resulting in massive tissue swelling called lymphedema. Lymphedema is a very prevalent disease affecting over 250 million people worldwide. Lymphedema can occur during the treatment of breast cancer as the removal of lymph nodes from the patient near the armpit disconnects the lymphatic vessels in the adjacent arm from the rest of the lymphatic system. This causes the arm to accumulate with lymph and swell (Figure 3). As of now, there are no pharmaceutical drugs to treat lymphedema. The current treatments for lymphedema are manual tissue massages to help drain accumulated lymph from the area of edema, and compression garments to prevent fluid accumulation. The lack of lymphedema treatment options demonstrates the need for more clinical research in the lymphatic system. While not the most well known system in the human body, the lymphatic system has a huge part in maintaining our health.
Figure 3. Photograph of an arm affected (top) with lymphedema compared to an arm unaffected (bottom). Image source.
To learn about the role of lymphatics in the heart check out this article.
