Lymphatic System

The lymphatic system is made up of lymph (the fluid itself), lymphatic vessels, and a set of associated organs and tissues. Lymph, literally 'clear water', is a fluid connective tissue whose composition is very close to blood plasma, except that it lacks red blood cells, platelets and some of the larger plasma proteins. It forms when fluid from the intercellular (interstitial) spaces of body tissue seeps into blind-ended lymphatic capillaries; from there it travels through progressively larger lymphatic vessels — picking up filtering along the way at lymph nodes — until it is finally emptied back into the blood via the thoracic duct and the right lymphatic duct, both of which drain into the subclavian vein.

Lymph nodes and lymphoid tissue are distributed widely through the body rather than concentrated in one place: examples include the thymus gland, tonsils and cervical lymph nodes near the head and neck, axillary lymph nodes under the arms, the appendix, the spleen and red bone marrow, and inguinal and popliteal lymph nodes in the groin and behind the knee.

A lymphatic capillary works because of how its wall is built: a single layer of overlapping endothelial cells held in place by anchoring filaments, creating flap-like openings that let interstitial fluid (and larger particles and proteins that have leaked from blood capillaries) enter easily but close up to stop that fluid leaking back out once inside. This lets lymphatic capillaries running alongside the systemic and pulmonary blood capillaries continually drain away excess interstitial fluid, route it through the chain of lymph nodes (where it is filtered and checked for pathogens) and larger valved lymphatic vessels, and finally return it to general circulation via the subclavian vein — completing the fluid-balance role that keeps the volume of interstitial fluid around the body's cells stable, and closing the loop this chapter opened in section 8.7 on how a complex body moves material between its parts.