lymphatic system:
A system of blind vessels i.e. lymphatics that drains lymph from all over the body into the blood stream is called lymphatic system. In addition to lymphatics and lymph it consists of lymph nodes, splan, thymus, tonsils and some of the patches of tissues in vermiform appendix and small intestine.
Lymph vascular system starts at capillary bed, where tissue fluid (intestinal fluid surrounding the cells of the tissues) enters the lymph capillaries which are closed towards the tissue sinuses. These are thin walled anastomosing microscopic vessels which form a network in every organ except the nervous system. The lymph capillaries merge into lymph vessels which have large diameter. These vessels contain smooth muscles in their walls as well as internal valves to prevent backward flow of lymph. The lymph circulates through the lymph vessels by the contraction of surrounding skeletal muscles in one direction, towards the heart. These vessels converge into collecting ducts that drain into veins in the lower neck.
All body tissues are bathed in a watery fluid derived from the blood stream. This inter cellular or tissue fluid is formed when blood passes through the capillaries. The capillary walls are permeable to all components of blood except RBC and blood protein. The fluid passes from the capillary into the intercellular spaces as the intercellular or tissue fluid. About 85% fluid returns into the blood at the venous end of the capillary. The rest 15% of the tissue fluid drains into blindly ending lymphatic capillaries as lymph along with WBC cell debris and microorganisms like bacteria are transported back to the heart through lymphatic system. Thus lymph can be defined as a colourless body fluid that contains lymphocytes (agranular WBC), small proteins and fats. Lymph takes fluid substances from cells of tissues and intercellular spaces which cannot penetrate the blood capillaries. It is a medium of exchange between blood and body cells.
Throughout the course of lymphatics lie lymph nodes through which lymph flows. Lymph nodes vary considerably in size from microscopic to about one inch in diameter. Each node consists of thin, fibrous outer capsule and an inner mass of lymphoid tissue. Penetrating the capsule are several small, lymphatic vessels which carry lymph into lymph node, while single large vessel carries it out. The lymphoid tissue contains anti bodies, lymphocytes and macrophages.
These nodes act as filters that trap microorganisms and other foreign bodies in the lymph. The lymphocytes and macrophages present here, neutralize and engulf the microorganisms.
Function of lymphatic system:
Drainage System: Lymphatic vessels act as drainage channels for water and plasma proteins that have leaked away from blood at capillary bed and that must be delivered back to blood circulation, without which death can occur in 24 hours.
Defense of the body: Microorganisms, foreign cells, cellular debris in the lymph are removed by macrophages residing in the lymph nodes. These are also sight for differentiation of B cells into antibody secreting cells.
Absorption and delivery of fats: Lymph capillaries called lacteals penetrate the villi of small intestine where fats are absorbed and delivered to blood circulatory system.
Edema: Whenever the tissue fluid accumulates rather than being drained into blood by lymphatic system, tissues and body cavities become swollen. This condition is known as edema.
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