The Lymphatic System
The lymphatic system and the immune system are terms that are used interchangeably to refer to the body's ability to defend against pathogens. The lymphatic system is comprised of three interrelated functions: (1) Removal of excess fluids, lymph, from body tissues, (2) Absorption of fatty acids and subsequent transport of fat, chyle, to the circulatory system and (3) Formation of white blood cells (WBCs), and initiation of immunity through the formation of antibodies, lending specific resistance to pathogens.
The lymphatic system acts as a secondary circulatory system, except it collaborates with white blood cells in lymph nodes to protect the body from being infected by cancer cells, fungi, viruses or bacteria. Unlike the circulatory system, the lymphatic system is not closed and has no central pump; the lymph moves slowly and under low pressure due to peristalsis, the operation of semilunar valves in the lymph veins, and the milking action of skeletal muscles.
Lymph originates as blood plasma that leaks from the capillaries of the circulatory system, becoming interstitial fluid, filling the space between individual cells of tissue. Plasma is forced out of the capillaries by hydrostatic pressure, and as it mixes with the interstitial fluid, the volume of fluid accumulates slowly. Most of the fluid is returned to the capillaries by osmosis. The proportion of interstitial fluid that is returned to the circulatory system by osmosis is about 90% of the former plasma, with about 10% accumulating as overfill.
Edema is the swelling that forms when too much tissue fluid forms or not enough taken away. It can be caused by a variety of conditions such as allergic responses (too much vasodilation), starvation (lack of albumin in blood lowers osmotic pressure and decreases amount of fluid returning to capillaries), and lymphatic disorders.
Lymphatic Vessels and Ducts
The lymphatic vessels are similar in structure to the cardiovascular veins, meaning they also have valves. They are dependent upon the contraction of skeletal muscle, respiratory movements and valves that do not allow backward flow. The vessels merge before entering one of two ducts. 1. Thoracic Duct: This duct is much larger than the lymphatic duct. It serves the abdomen, lower extremities and the left side of the upper body (head, neck, and arm) 2. Right Lymphatic Duct: This duct serves all of the right side of the upper body and thoracic area (head, neck).