Blood capillaries are under some pressure from within and from without. As a result, some fluid, whose composition will be discussed later, leaks out. If this fluid were to accumulate in tissues and organs, there would a swelling of the body called oedema. In order to counter this problem, the body has developed another system of vessels, called lymphatic system. These vessels are thin-walled and resemble veins. The fluid in them flows in one direction. This fund is called lymph.
Basically, lymph has all the elements of blood except that it does not have RBC and blood platelets. Therefore, lymph is colourless or slightly yellowish. Lymphatic system consists of lymphatic vessels, lymph capillaries and lymph nodes. Lymphatic system maintains the balance between the tissue fluids and blood. Lymph nodes produce lymphocytes, along with spleen, thymus, tonsils and payer’s patches. Proteins which cannot re-enter the blood capillaries because of their size return to circulation by the lymphatic fluid. Lymph flows in one direction i.e., from the tissues to the heart but never into the tissues.
A large number of lymph vessels and lymph capillaries are found in almost all the body organs. The lymph vessels resemble veins due to their thin walls and presence of valves. The lymph vessels from two networks in the organs, one superficial and the other deep-seated. Due to the contraction of the walls of the vessels and the body muscles, the lymph moves in the vessels. The smaller lymph vessels drain into larger vessels which are common to many organs. There are two very large vessels namely thoracic duct and lymphatic duct in the human body.
The lymph nodes are arranged in groups. They are small globular masses of lymphatic tissue. These nodes arte also called regional nodes. The lymph flows into definite lymph nodes from each region or organ of the body.
The lymphatic system runs parallel to the veins. It collects the lymph (tissue fluid) in the blind-ending, thin a walled channel of lymphatic’s which unite to form two main lymph ducts. Bothe ducts then open into a pair of veins (right and left sub-clavian veins) entering the right atrium or auricle.