The lymphatic system is composed of a network of vessels that collects fluid and plasma proteins that leak out of capillaries and into the interstitial space. Lymphatic vessels return the lymph (fluid and plasma protein) back to the circulatory system through the veins.
The lymphatic system is a secondary system of vessels that is distinct both in anatomy and function from the blood vessel capillaries of the circulatory system. Small lymphatic vessels (or "lymphatics") called lymphatic capillaries are found in almost all organs of the body except superficial layers of the skin, the central nervous system, endomysium of muscles, and the bone. These exceptions have a system of smaller vessels called prelymphatics. Fluid from prelymphatics returns to nearby lymphatic vessels, or the cerebral spinal fluid in the case of the central nervous system.
Lymphatic capillaries are made up of a single layer of endothelial cells. They are anchored to the surrounding connective tissue by special filaments called anchoring filaments. The system begins as a series of sacs. Each sac has a low hydrostatic pressure relative to the outside of the sac. At the end of the lymphatic capillaries there are endothelial valves. The valves form as a result of the slight overlap of the endothelial cells, and the overlapping edge has the ability to open inward. The valves open enough to allow fluid and plasma protein to pass into the lymphatic capillary.
Inside the lymph vessels are valves that prevent the backflow of lymph, a general name for the slightly opalescent fluid picked up by the lymphatics. Surrounding the lymphatics are smooth muscles that contract involuntarily to assist in the movement of lymph through the system. The lymphatic capillaries converge into larger lymph vessels. The larger lymph vessels pass through swellings called lymph nodes and then empty into one of two large lymph ducts. The lymph ducts empty into the venous circulatory system through either the right or left subclavian veins. Lymph from the right side of the head, arm and chest empties into the right subclavian vein. Lymph collected from the lower part of the body, and lymph from the left side of the head, arm and chest empties into the left subclavian vein. Both subclavian veins are located within the thorax underneath the clavicles, the thin bones located on the top part of the chest.
At approximately 600 sites in the human body, lymphatic vessels converge into bundles of tissue called lymph nodes. The shape of a lymph node resembles a kidney bean and ranges in size from a few millimeters to a few centimeters. They are mostly found at the base of extremities such as the arms, legs and head. Many afferent lymphatics or vessels lead the lymph into the node at the larger curve of the bean shape and efferent lymphatics, fewer in number, take the lymph away from the node at the hilum, the depressed region of the bean shape. All nodes have a blood supply from the circulatory system running through them. The blood vessels enter and exit at the hilum. Inside the nodes are a honeycomb of lymphfilled sinuses that have macrophages and groupings of lymphocytes that produce antibodies.
As mentioned, lymph is the fluid flowing through the lymphatic system and originates from the interstitial spaces of the organs and tissues. Another element of the lymph is a type of cell of the immune system called a lymphocyte, which is a type of white blood cell. Lymphocytes mature in either the thymus (T-lymphocytes) or the bone marrow (B-lymphocytes), which are primary lymphoid organs The blood supply transports lymphocytes from their site of maturation (the thymus or bone marrow) to secondary lymphoid organs such as the lymph nodes, spleen, and tonsils. All lymphocytes in the adult originate in the bone marrow.
Sally C. McFarlane-Parrott, The Gale Group Inc., Gale, Detroit,