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The human body contains two circulatory systems which work together to serve very critical functions:

• The Cardiovascular System consists of the blood, blood vessels, and heart. Blood delivers oxygen, nutrients, hormones and antibodies to all parts of the body and carries away waste materials. The heart pumps the blood through the vascular system (arteries, veins, and capillaries) so that circulation continues without any interruption.

• The Lymphatic System includes a network of lymphatic vessels, tissue and organs such as bone marrow, spleen, thymus gland, tonsils, appendix, and lymph nodes that contain specialized cells called lymphocytes that help the body fight infection. The complex network of interconnected vessels drains and filters fluid called "lymph" from tissue throughout the body and delivers it into the blood circulatory system via entry points into the subclavian vein. The walls of lymph vessels are thinner than those of veins. Some are found close to the surface of the skin while others course through deeper fatty tissue.

These two life-sustaining systems are responsible for maintaining homeostasis, the balance of fluids in the body. The lymphatic system is responsible for draining water, protein, and other by products of cell metabolism from the tissue and transporting them to the cardiovascular circulatory system.

There are several components of the lymphatic system. These include:

• Lymph - a clear or straw-colored liquid consisting of water, large-molecule proteins, salts, and waste products (e.g., dead cells)
• Lymphatic capillaries - very small vessels located under the skin which are the first point of collection for lymph
• Pre-collector vessels - these vessels are larger in diameter than capillaries and are located deeper in the tissue
• Collector vessels - these are yet larger than pre-collector vessels located quite deep in the body
• Lymph nodes - dense clusters of tissue which filter lymph
• Lymph ducts - these are the largest diameter vessels of which there are two, the thoracic duct and the right lymphatic duct that drain the lymph into the left and right subclavian veins at the level of the shoulders/neck.

While the circulatory system is based on the constant pumping action of the heart, the lymphatic system depends on more subtle factors to achieve constant drainage, including:

• Pressure changes within the tissue which propel the lymph into draining vessels
• Smooth muscle within the walls of the larger lymphatic pre-collector and collector vessels which contract and push the lymph forward with the help of valves
• Contractions of surrounding structures, such as arteries and muscles which compress lymph vessels and propels fluid forward.

Within the lymph vessels, valves and muscles help move the fluid towards the larger collector vessels. Each section of collector vessels is called a lymphangion and fluid is propelled through each lymphangion until it reaches a lymph node.

Lymph nodes range in size from 1 mm to 25 mm, about the size of an almond. There are approximately 600 lymph nodes scattered throughout the trunk with some situated very deep in the tissue and others lying more superficially. They start in the neck region (cervical nodes) continuing down to the armpits (axillary nodes) and groin area (inguinal nodes). Some stand individually while others may be grouped into clusters. The outermost layer of the lymph node consists of densely packed lymphocytes which activate the immune system if needed. Lymphocytes consist of several types of white blood cells which are crucial to protecting the body from infection and overcoming infection that has already become established.