Lymphatic vessels, or simply lymph vessels, are tubular structures responsible for carrying lymph throughout the human body, much as blood vessels convey blood. These vessels are a vital part of the lymphatic system, which supports the human immune and circulatory systems, filters toxins and waste products from somatic cells and tissues, and aids in the production of certain protein molecules. The lymphatic system has a one-way cycle that moves lymph upwards through the human body, and it includes two kinds of vessels: afferent and efferent. The afferent vessels are similar to veins in that they transport fluid into the lymph nodes, while the efferent vessels are similar to arteries, carrying filtered lymph away from the lymph nodes.
The solution that runs through these vessels, called lymph, begins in the circulatory system as blood plasma and then is converted to a fluid known as an interstitial or intercellular fluid. This fluid is responsible for delivering hormones, oxygen, and various nutrients to cells throughout the body. As it leaves the cells, it bathes them, taking waste products away with it. About 90% of this fluid is then reintroduced to the circulatory system. What remains in the interstitial fluid is the lymph.
The lymph is conveyed into the lymphatic system through vessels called lymph capillaries. These are comparable to the capillaries found within the circulatory system, but they are larger in circumference and have a unique one-way structure, meaning that lymph cannot pass out of the vessels the same way it entered. The lymph capillaries eventually form a complex network of larger vessels that are the lymphatic vessels.
These vessels are also one-way structures; like veins in the vascular system, they have valves that prevent lymph from flowing backward. After passing through the lymph capillaries, the fluid is introduced into the afferent lymphatic vessels and then flows into the lymph nodes, which are small, round organs located throughout the body. The nodes filter waste from the fluid, while cells called lymphocytes kill any viruses or bacteria found within it. Lymphocyte activity is what causes the lymph nodes to become swollen when a person is sick, a condition commonly referred to as "swollen glands."
After the lymph has been filtered by the lymph nodes, it passes into the efferent vessels. These lymph vessels come together in large groups sometimes referred to as a lymph trunk. They drain the filtered fluid into one of two lymph ducts, which are very large lymphatic vessels. The thoracic duct is the largest of these vessels in the human body and is responsible for draining the majority of the body's lymph, while the smaller right lymphatic duct only drains lymph from the upper right side of the body. After passing through the lymphatic ducts, the fluid enters vessels known as the subclavian veins, which transport it back to the circulatory system.