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INTRODUCTION
The lymphatic system develops from four cystic spaces that appear on either side of the neck and in both groins. These large cisterns develop communications (lymphatic vessels or lymphangioles) which allow most of the lymph from the lower limbs and abdomen to be channelled through the cisterna chylae into the thoracic duct, which passes up on the left side of the bodies of the thoracic vertebrae before entering the internal jugular vein in the left hand side of the neck. A separate lymphatic trunk which drains lymph from the right upper limb and right side of the head and neck enters the right internal jugular vein. Lymph nodes develop as condensations along the course of these lymphatic pathways.
Abnormalities of development result in lymphatic aplasia, cystic hygroma, lymphatic and nodal hypo- and hyperplasia, and lymphangiomas.
FUNCTION
The lymphatic system has two main functions. The first is to remove macromolecules and excessive fluid from the interstitial space. Large molecules that escape into the tissue fluid have considerable difficulty in re-entering the vascular compartment. Proteins such as albumin, globulins, and fibrinogen that enter the interstitial fluid are usually returned to the plasma through the lymphatics. A number of coagulation factors and fibrinolytic activators also enter the lymph. Between 2 and 4 litres of interstitial fluid are returned to the vascular compartment each day by the lymphatics 
The second major function of the lymphatics is to allow the circulation of lymphocytes from the lymph nodes into the bloodstream. Most exogenous antigens are presented to the central lymphoid system for the first time via the lymphatics. Recognition of antigens, with subsequent proliferation of specific clones of lymphocytes, takes place in the lymph nodes. Activated lymphocytes then pass into the circulation and thus to the other lymphatic tissues throughout the body.
PHYSIOLOGY
The interstitial space has a negative pressure and this, in combination with the hydrostatic pressures of the capillaries, encourages fluid to escape from the vascular compartment, overcoming the oncotic pressure of the plasma proteins which acts to suck fluid back into the circulation. The intraluminal pressure of the lymph system is similar to that of the interstitial fluid, and lymph capillaries must therefore actively absorb proteins through their pores. The mechanism by which this is achieved is unknown, but appears to require energy.
The lymphatic capillaries have large pores to allow large molecules to enter the lumen, and many valves that prevent the reflux of lymph and encourage its onward passage. Lymphatics have some circular smooth muscle in their wall and are capable of contraction. The combination of inherent contractility and valves ensure that lymph is propelled along the lymphatics and into the veins. Other factors that may influence lymphatic drainage include compression from surrounding arteries, and the negative pressure of the thoracic cavity sucking lymph upwards into the thorax from the abdomen.
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