Closely connected with the blood and circulatory system, the lymphatic system is an extensive drainage system that returns water and proteins from various tissues back to the blood stream.
When blood reaches the arterial end of the capillary beds in the body tissues, fluid from the plasma is forced out of the capillaries by the blood pressure in to a space between the capillaries and the tissue cells. This fluid, called tissue fluid, is plasma that has permeated through the blood capillary walls and has surrounded the cells to bring them nutrient and to remove their waste substances. Tissue fluid is an aqueous solution containing glucose, amino acids, fatty acids, inorganic ions, oxygen and hormones, which diffuse into the tissue cells to carry out the metabolic reactions.
Most of the tissue fluid, now containing fewer nutrients and less oxygen but more carbon dioxide and metabolic waste products, passes back into the venous end of the capillary network and then back to the heart. The small amount of Tissue fluid that remains is picked up by tiny vessels called the lymph capillaries. The cells forming the walls of the lymph capillaries are loosely fitted together, thus making the wall very porous. Even the large serum proteins that filtered through the capillary wall pass easily from the tissue fluid into the interior of the lymph capillary. The lymph capillaries drain into still larger vessels that make up the lymphatic system. The flow through the lymph vessels is quite slow. The lymphatics resemble the veins in their structure having;
1)An outer coat of fibrous tissue
2)A middle coat of muscular elastic tissue
3)An inner lining made up a single layer of epithelial cells
Also like the blood in the veins, contraction of skeletal muscles compresses the lymph vessels and squeezes the plasma fluid- now called lymph- along. Again like return of the blood in the veins, the lymph can flow only in one direction because of valves in the vessels.
There are also several hundred lymph nodes, (concentrated areas of lymphocytes and macrophages) scattered throughout the lymphatic system especially in the
These contain cavities called sinuses, into which the lymph flows through the afferent vessel. The walls of the sinuses are lined with phagocytic cells, which engulf foreign bodies e.g. bacteria which may be present in the lymph. Tests have demonstrated that over 99% of the bacteria carried to the node are screened out before the lymph leaves the node and returns to the blood. The filtering mechanism is one of the most important body defences against infectious disease. When combating a heavy infection, the lymph nodes enlarge because of dead white cells, to give what we know as ‘swollen Glands’.
I conclude to say that the lymphatic system is part of the circulatory system because the lymph comes from the blood and returns to the blood, and because its vessels are very similar to the veins and capillaries of the circulatory system. Throughout the body, wherever there are blood vessels, there are lymph vessels, and the two systems work together.