The lymphatic system comprises a network of conduits called lymphatic vessels that carry lymph unidirectionally towards the heart.
- Describe the structure of the lymphatic system and its role in the immune system and blood circulation
- The lymph system is not a closed system. Lymph flows in one direction toward the heart.
- Lymph nodes are most densely distributed toward the center of the body, particularly around the neck, intestines, and armpits.
- Lymph vessels and nodes are not found within bone or nervous system tissue.
- Afferent lymph vessels flow into lymph nodes, while efferent lymph vessels flow out of them.
- Lymphatic capillaries are the sites of lymph fluid collection, and are distributed throughout most tissues of the body, particularly connective tissue.
- lymph: A colorless, watery, bodily fluid carried by the lymphatic system, consisting mainly of white blood cells.
- plasma: The straw-colored/pale-yellow liquid component of blood that normally holds the blood cells of whole blood in suspension.
- Efferent: A type of vessel that flows out of a structure, such as lymph vessels that leave the spleen or lymph nodes and arterioles that leave the kidney.
The lymphatic system is a circulatory system for lymphatic fluid, comprising a network of conduits called lymphatic vessels that carry the fluid in one direction toward the heart. Its functions include providing sites for certain immune system functions and facilitating plasma circulation in the cardiovascular system. The lymphatic system is composed of many different types of lymph vessels over a wide distribution throughout the body.
Lymph Node Distribution
Lymphatic System: The lymph nodes and lymph vessels in human beings. (Public Domain; NIH via Wikipedia)
Lymphatic vessels are most densely distributed near lymph nodes: bundles of lymphoid tissue that filter the lymph fluid of pathogens and abnormal molecules. Adaptive immune responses usually develop within lymphatic vessels. Large lymphatic vessels can be broadly characterized into two categories based on lymph node distribution.
- Afferent lymphatic vessels flow into a lymph node and carry unfiltered lymph fluid.
- Efferent lymphatic vessels flow out of a lymph node and carry filtered lymph fluid. Lymph vessels that leave the thymus or spleen (which lack afferent vessels) also fall into this category.
Lymph nodes are most densely distributed around the pharynx and neck, chest, armpits, groin, and around the intestines. Afferent and efferent lymph vessels are also most concentrated in these areas so they can filter lymph fluid close to the end of the lymphatic system, where fluid is returned into the cardiovascular system. Conversely, lymph nodes are not found in the areas of the upper central nervous system, where tissue drains into cerebrospinal fluid instead of lymph, though there are some lymph vessels in the meninges. There are few lymph nodes at the ends of the limbs. The efferent lymph vessels in the left and lower side of the body drain into the left subclavian vein through the thoracic duct, while the efferent lymph vessels of the right side of the body drain into the right subclavian vein through the right lymphatic duct.
Flow Through Lymph Vessels
The lymphatic vessels start with the collection of lymph fluid from the interstitial fluid. This fluid is mainly water from plasma that leaks into the intersitial space in the tissues due to pressure forces exerted by capillaries (hydrostatic pressure) or through osmotic forces from proteins (osmotic pressure). When the pressure for interstitial fluid in the interstitial space becomes large enough it leaks into lymph capillaries, which are the site for lymph fluid collection.
Like cardiovascular capillaries, lymph capillaries are well distributed throughout most of the body’s tissues, though they are mostly absent in bone or nervous system tissue. In comparison to cardiovascular capillaries, lymphatic capillaries are larger, distributed throughout connective tissues, and have a dead end that completely prevents backflow of lymph. That means the lymphatic system is an open system with linear flow, while the cardiovascular system is a closed system with true circular flow.
Lymph flows in one direction toward the heart. Lymph vessels become larger, with better developed smooth muscle and valves to keep lymph moving forward despite the low pressure and adventia to support the lymph vessels. As the lymph vessels become larger, their function changes from collecting fluid from the tissues to propelling fluid forward. Lymph nodes found closer to the heart filter lymph fluid before it is returned to venous circulation through one of the two lymph ducts.