Lymphatic vessels are structures of the lymphatic system that transport fluid away from tissues. Lymphatic vessels are similar to blood vessels, but they don't carry blood. The fluid transported by lymphatic vessels is called lymph. Lymph is a clear fluid that comes from blood plasma that exits blood vessels at capillary beds. This fluid becomes the interstitial fluid that surrounds cells. Lymph vessels collect and filter this fluid before directing it toward blood vessels near the heart. It is here that lymph re-enters blood circulation. Returning lymph to the blood helps to maintain normal blood volume and pressure. It also prevents edema, the excess accumulation of fluid around tissues.
Large lymphatic vessels are composed of three layers. Similar in structure to veins, lymph vessel walls consist of the tunica intima, tunica media, and tunica adventitia.
- Tunica Intima - lymph vessel inner layer composed of smooth endothelium (a type of epithelial tissue). This layer contains valves in some lymph vessels to prevent fluid backflow.
- Tunica Media - lymph vessel middle layer composed of smooth muscle and elastic fibers.
- Tunica Adventitia - lymph vessel strong outer covering composed of connective tissue as well as collagen and elastic fibers. The adventitia attaches lymphatic vessels to other underlying tissues.
The smallest lymphatic vessels are called lymph capillaries. These vessels are closed at their ends and have very thin walls that allow interstitial fluid to flow into the capillary vessel. Once the fluid enters the lymph capillaries, it is called lymph. Lymph capillaries can be found in most areas of the body with the exceptions of the central nervous system, bone marrow, and non-vascular tissue.
Lymphatic capillaries join to form lymphatic vessels. Lymphatic vessels transport lymph to lymph nodes. These structures filter lymph of pathogens, such as bacteria and viruses. Lymph nodes house immune cells called lymphocytes. These white blood cells protect against foreign organisms and damaged or cancerous cells. Lymph enters a lymph node through afferent lymphatic vessels and leaves via efferent lymphatic vessels.
Lymphatic vessels from various regions of the body merge to form larger vessels called lymphatic trunks. The major lymphatic trunks are the jugular, subclavian, bronchomediastinal, lumbar, and intestinal trunks. Each trunk is named for the region in which they drain lymph. Lymphatic trunks merge to form two larger lymphatic ducts. Lymphatic ducts return lymph to the blood circulation by draining lymph into the subclavian veins in the neck. The thoracic duct is responsible for draining lymph from the left side of the body and from all regions below the chest. The thoracic duct is formed as the right and left lumbar trunks merge with the intestinal trunk to form the larger cisterna chyli lymphatic vessel. As the cisterna chyli runs up the chest, it becomes the thoracic duct. The right lymphatic duct drains lymph from the right subclavian, right jugular, right bronchomediastinal, and right lymphatic trunks. This area covers the right arm and right side of the head, neck, and thorax.
Lymphatic Vessels and Lymph Flow
Although lymphatic vessels are similar in structure to and generally found alongside blood vessels, they are also different from blood vessels. Lymph vessels are larger than blood vessels. Unlike blood, lymph within lymphatic vessels is not circulated in the body. While cardiovascular system structures pump and circulate blood, lymph flows in one direction and is ushered along by muscle contractions within lymph vessels, valves that prevent fluid backflow, skeletal muscle movement, and changes in pressure. Lymph is first taken up by lymphatic capillaries and flows to lymphatic vessels. Lymphatic vessels direct lymph to lymph nodes and along to lymphatic trunks. Lymphatic trunks drain into one of two lymphatic ducts, which return lymph to the blood via the subclavian veins.
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