The lymphatic network of vessels begins with the microscopic lymphatic capillaries. Lymphatic capillaries are closed-ended tubes that form vast networks in the interstitial spaces within most vascular tissues. Notably, these capillaries are not found in the CNS. Instead the CNS relies on the flow of CSF to removes excess fluid from nervous tissue. Because the walls of lymphatic capillaries are composed of endothelial cells with unique junctions, interstitial fluid, proteins, and microorganisms can easily enter the vessels but cannot leave and reenter the interstitial space. Once fluid enters the lymphatic capillaries, it becomes a fluid connective tissue referred to as lymph (limf). Adequate lymphatic drainage is needed to prevent the accumulation of interstitial fluid, a condition called edema (e-de ‘ma). Additionally, within the villi of the small intestine, lymphatic capillaries called lacteals (lak’te-alz) transport absorbed lipids and lipid-soluble vitamins away from the digestive tract.
From merging lymphatic capillaries, the lymph is carried into lymphatic vessels. These lymphatic vessels merge into even larger vessels called lymphatic trunks that are named after large body regions. The walls of lymphatic vessels and trunks are much like those of veins. They have the same three layers and also contain valves to prevent backflow. The pressure that keeps the lymph moving comes from the massaging action produced by skeletal muscle contractions, intestinal movements, respiratory pressure changes and from peristaltic contractions of some lymphatic vessels. Interconnecting lymphatic trunks eventually empty into one of the two principal vessels: the thoracic duct and the right lymphatic duct. The larger thoracic duct drains lymph from the left thoracic region, left upper limb, left side of the head and neck, and all areas inferior to the diaphragm. The thoracic duct begins in the abdominal cavity as a saclike enlargement called the cis- terna chyli, which collects lymph from the lower limbs and the intestinal region. The thoracic duct then ascends along the vertebral column and drains into the left subclavian vein near the left internal jugular vein. The smaller right lymphatic duct receives lymph from the right upper limb, right thoracic region, and right side of the head and neck. The right lymphatic duct empties into the right subclavian vein near the right internal jugular vein.