The Lymphatic System

The lymphatic system, or lymphoid system, is one of the components of the circulatory system, and it serves a critical role in both immune function and surplus extracellular fluid drainage.  Components of the lymphatic system include lymph, lymphatic vessels and plexuses, lymph nodes, lymphatic cells, and a variety of lymphoid organs. The pattern and form of lymphatic channels are more variable and complex but generally parallel those of the peripheral vascular system. The lymphatic system partly functions to convey lymphatic fluid, or lymph, through a network of lymphatic channels, filter lymphatic fluid through lymph nodes and return lymphatic fluid to the bloodstream, where it is eventually eliminated. Nearly all body organs, regions, and systems have lymphatic channels to collect the various byproducts that require elimination . Liver and intestinal lymphatics produce about 80% of the volume of lymph in the body.

Notable territories of the body that do not appear to contain lymphatics include the bone marrow, epidermis, as well as other tissues where blood vessels are absent. The central nervous system was long considered to be absent of lymphatic vessels until they were recently identified in the cranial meninges. Moreover, a vessel appearing to have lymphatic features was also discovered in the eye. The lymphatic system is critical in a clinical context, particularly given that it is a major route for cancer metastasis and that the inflammation of lymphatic vessels and lymph nodes is an indicator of pathology. 

Structure

The lymphatic system includes numerous structural components, including lymphatic capillaries, afferent lymphatic vessels, lymph nodes, efferent lymphatic vessels, and various lymphoid organs. 

Lymphatic capillaries are tiny, thin-walled vessels that originate blindly within the extracellular space of various tissues. Lymphatic capillaries tend to be larger in diameter than blood capillaries and are interspersed among them to enhance their ability to collect interstitial fluid efficiently. They are critical in the drainage of extracellular fluid and allow this fluid to enter the closed capillaries but not exit due to their unique morphology. Lymphatic capillaries at their blind ends are composed of a thin endothelium without a basement membrane. The endothelial cells at the closed end of the capillary overlap but shift to open the capillary end when interstitial fluid pressure is greater than intra-capillary pressure. This process permits lymphocytes, interstitial fluid, bacteria, cellular debris, plasma proteins, and other cells to enter the lymphatic capillaries. Special lymphatic capillaries called lacteals exist in the small intestine to contribute to the absorption of dietary fats. Lymphatics in the liver contribute to a specialized role in transporting hepatic proteins into the bloodstream. The lymphatic capillaries of the body form large networks of channels called lymphatic plexuses and converge to form larger lymphatic vessels.

Lymphatic vessels convey lymph, or lymphatic fluid, through their channels. Afferent (toward) lymphatic vessels convey unfiltered lymphatic fluid from the body tissues to the lymph nodes, and efferent (away) lymphatic vessels convey filtered lymphatic fluid from lymph nodes to subsequent lymph nodes or into the venous system. The various efferent lymphatic vessels in the body eventually converge to form two major lymphatic channels: the right lymphatic duct and the thoracic duct. 

The right lymphatic duct drains most of the right upper quadrant of the body, including the right upper trunk, right upper extremity, and right head and neck. The right lymphatic trunk is a visible channel in the right cervical region just anterior to the anterior scalene muscle. Its origin and termination are variable in morphology, typically forming as the convergence of the right bronchomediastinal, jugular, and subclavian trunks, extending 1 to 2 centimeters in length before returning its contents to the systemic circulation at the junction of the right internal jugular, subclavian, and/or brachiocephalic veins. 

The thoracic duct, also known as the left lymphatic duct or van Hoorne's canal, is the largest of the body's lymphatic channels. It drains most of the body except for the territory of the right superior thorax, head, neck, and upper extremity served by the right lymphatic duct. The thoracic duct is a thin-walled tubular vessel measuring 2 to 6 mm in diameter. The length of the duct ranges from 36 to 45 cm. The thoracic duct is highly variable in form but typically arises in the abdomen at the superior aspect of the cisterna chyli, around the level of the twelfth thoracic vertebra (T12). The cisterna chyli, from which it extends, is an expanded lymphatic