Lymphatic system is essentially a drainage system which is accessory to the venous system. Most of the tissue fluid formed at the arterial end of capillaries is absorbed back into the blood by the venous ends of the capillaries and the postcapillary venules. The rest of the tissue fluid (10-20%) is absorbed by the lymphatics which begin blindly in the tissue spaces.
It is important to know that the larger particles (proteins and particulate matter) can be removed from the tissue fluid only by the lymphatics. Therefore, the lymphatic system may be regarded as ‘drainage system of coarse type’ and the venous system as ‘drainage system of fine type’.
Certain parts of the lymphatic system (lympho-reticular organs), however, arc chiefly involved in phagocytosis, raising immune responses. and contributing to cell populations of the blood and lymph. The tissue fluid flowing in the lymphatics is called lymph. It passes through filters (lymph nodes) placed in the course of lymphatics, and finally drains into the venous blood. Lymph from most of the tissues is clear and colorless, but the lymph from small intestine is milky-white due to absorption of fat. The intestinal milky lymph is called chyle, and lymph vessels, the lacteals.
For information on Gross anatomy of the lymphatic system of human body, visit the page: Lymphatic System.
Components of Lymphatic System:
The lymphatic system comprises:
- Lymph vessels
- Central lymphoid tissues
- Peripheral lymphoid organ
- Circulating lymphocytes.
1. Lymph Vessels:
The lymph capillaries begin blindly in the tissue spaces and form intricate networks. Their calibre is greater and less regular than that of blood capillaries, and their endothelial wall is permeable to substances of much greater molecular size.
Lymph capillaries are absent from the cellular structures like brain, spinal cord, splenic pulp, and bone marrow.
The lymph capillaries join to form lymphatics, which are superficial and deep lymphatics. The superficial lymphatics accompany veins, while the deep lymphatics accompany arteries.
The lymph passes through filters or barriers of the regional lymph nodes which trap the particulate matter. The filtered lymph passes through larger lymphatics and is eventually collected into two large trunks, the thoracic duct and right lymphatic duct, which pour their lymph into the brachiocephalic veins. Thoracic duct drains both lower limbs, abdomen, left halves of thorax, head and neck and left upper limb. Right lymphatic duct drains right halves of thorax, head and neck and right upper limb.
The lymphatics anastomose freely with their neighbors of the same side as well as of the opposite side. Larger lymphatics are supplied with their vasa vasorum and are accompanied by a plexus of fine blood vessels which form red streaks seen in lymphangitis.
2. Central Lymphoid Tissues:
Central lymphoid tissues comprise bone marrow and thymus.
All pluripotent lymphoid stem cells are initially produced by bone marrow, except during early fetal life when these are produced by liver and spleen. The stem cells undergo differentiation in the central lymphoid tissues, so that the lymphocytes become competent defensive elements of the immune system.
Bone marrow helps differentiation of the (committed) B-lymphocytes which are capable of synthesizing antibodies after getting transformed into plasma cells.
In birds, B-cells arc differentiated in the wall of the bursa of Fabricius, a hindgut diverticulum. Thymus helps differentiation of immunologically competent but uncommitted T-lymphocytes (10% of thymic population) which are long-lived, join the circulating pool of lymphocytes, and populate the thymus-dependent areas of lymph nodes and other peripheral lymphoid organs.
T-cells being uncommitted can react, to a wide range of foreign antigenic stimuli. These respond by cytotoxic cell killing (killing virus-infected cells, neoplastic cells, fungi, tissue grafts, etc.), by ‘arming’ macrophages, and by triggering the large mononuclear cells (killer cells) and the ‘helper’ activity of B-lymphocytes.
3. Peripheral Lymphoid Organs:
Peripheral lymphoid organs comprise lymph nodes, spleen, and epithelio-lymphoid tissues (lymphoid nodules developed in the alimentary and respiratory tracts). Any part of this may become overactive on appropriate stimulation.
The progenies of B- and T-lymphocytes reach these organs where the cells may proliferate and mature into competent cells. The mature lymphocytes join the circulating pool of lymphocytes.
4. Circulating Pool of Lymphocytes:
The pool contains mature progenies of B- and T-lymphocytes which may be called upon during antigenic emergencies.