How Drugs Work

The Lymphatic System

Incredibly, the human circulatory system's average vasculature can span over 60,000 miles, thus being able to circulate the circumference of the earth at least two times. ²⁵ This vast and intricate system not only regulates the flow of blood and nourishes body tissues, but it plays a key role in defense. This is because the circulatory system comprises not just the cardiovascular system but also includes the lymphatic system. The lymphatic system aides the circulatory system by re-circulating lymph fluid back into the heart. Lymph fluid (recycled blood plasma) accumulates as a result of high blood pressure forcing fluid out of the capillary into surrounding interstitial space and tissue cells. Out of all the fluid leaving the arterioles and returning to the veins, about 15% is lost between the arteriole end of the capillary bed and the venous end. Lymphatic vessels surrounding capillary vessels catch this fluid, now called lymph, diffused from the capillaries.

The lymphatic system is network of afferent and efferent lymph vessels that regulates about 3 liters of lymph fluid daily. Like veins, lymphatic vessels have valves that prevent backflow of lymph flow. Unlike veins, lymphatic vessels require skeletal muscle contraction to move lymph fluid into its draining ducts at the heart. Afferent vessels carry lymph fluid into the lymph nodules while efferent vessels carry lymph fluid away. ²⁶

One of the major functions of the lymphatic system is to defend the body against foreign microorganisms and disease. Circulating lymph fluid can carry microorganisms into filtering lymph nodes before the lymph fluid is returned to the circulatory system. Lymph nodes are masses of B and T-lymphocytes (lymphatic cells) and macrophages (big eaters of foreign objects) clustered together and encapsulated into a kidney bean shape structure. These lymph nodes are found in chains of nodules that are concentrated in number around the neck, armpits, groin, and abdomen. Faced with an infection, the lymph nodes swell due its increased numbers of T-cell, B-cells, and plasma cells accumulating within the structure. Thus, when a person is sick with a sore-throat, or a cold sore, a person may feel small hard, nodules under their chin or around their neck, since the lymph nodes in that region swell as the body mounts an attack against the invading microbe. The primary function of the lymph nodes is to cleanse and filter lymph fluid of foreign microorganisms and cellular debris before it is returned to the blood. Larger organs like the spleen, tonsils, and thymus also accumulate lymphocytes in response to foreign microorganisms. ²⁷