<P> The lymphatics are responsible for maintaining the balance of the body fluids . Its network of capillaries and collecting lymphatic vessels work to efficiently drain and transport extravasated fluid, along with proteins and antigens, back to the circulatory system . Numerous intraluminal valves in the vessels ensure a unidirectional flow of lymph without reflux . Two valve systems are used to achieve this one directional flow--a primary and a secondary valve system . The capillaries are blind - ended, and the valves at the ends of capillaries use specialised junctions together with anchoring filaments to allow a unidirectional flow to the primary vessels . The collecting lymphatics, however, act to propel the lymph by the combined actions of the intraluminal valves and lymphatic muscle cells . </P> <P> Lymphatic tissues begin to develop by the end of the fifth week of embryonic development . Lymphatic vessels develop from lymph sacs that arise from developing veins, which are derived from mesoderm . </P> <P> The first lymph sacs to appear are the paired jugular lymph sacs at the junction of the internal jugular and subclavian veins . From the jugular lymph sacs, lymphatic capillary plexuses spread to the thorax, upper limbs, neck and head . Some of the plexuses enlarge and form lymphatic vessels in their respective regions . Each jugular lymph sac retains at least one connection with its jugular vein, the left one developing into the superior portion of the thoracic duct . </P> <P> The next lymph sac to appear is the unpaired retroperitoneal lymph sac at the root of the mesentery of the intestine . It develops from the primitive vena cava and mesonephric veins . Capillary plexuses and lymphatic vessels spread from the retroperitoneal lymph sac to the abdominal viscera and diaphragm . The sac establishes connections with the cisterna chyli but loses its connections with neighbouring veins . </P>

Where does the lymphatic system start and end