Lymphatic system

Introduction
As blood circulates, some of its fluid components push out of the capillary bed into the surrounding tissue. This material forms lymph, a special protein-containing tissue fluid that bathes the cells. Lymphatic vessels reabsorb part of this lymph to return it to the circulation, thereby maintaining tissue fluid balance. The lymphatics also engage in absorption of fats and other substances from the digestive tract. Lymph node structures along the route of the lymphatics filter out foreign materials and disease-causing agents from the general circulation. Other lymphatic system structures include the tonsils, spleen, and thymus.

Structure of lymphatic system
Capillary hydrostatic pressure: fluid diffusion and reabsorption
Capillary hydrostatic pressure (filtration pressure) forces fluid out of the blood capillaries. Hydrostatic pressure results from the heart forcing blood through the narrow arterial part of capillaries. The fluid contains oxygen and nutrients that move into the surrounding tissue where they are less concentrated. Similarly, the tissue contains carbon dioxide and waste products that move into the capillaries where they are less concentrated. This process of substances moving from areas of higher concentration to areas of lower concentration is diffusion.
Fluid reabsorption begins in the lymph capillaries that are throughout the body near blood capillaries. Lymph capillaries are small microscopic tubes that collect extracellular fluid. The walls of lymph capillaries comprise loosely joined cells. The overlapping edges of the cells form mini-valves that allow extracellular fluid to pass into the capillary and prevent fluid from flowing back into the tissue. Unlike blood capillaries, lymph capillaries are blind-end tubes that lead away from the tissue.
Lymph vessels
Lymph travels through the lymph capillaries to small lymph vessels. Like veins, the walls of lymph vessels have smooth muscle that contracts and propels lymph away from the tissues. Lymph vessels contain valves that prevent lymph from flowing backward.
The lymph vessels converge into two main collecting ducts: the shorter right lymphatic duct and the longer thoracic duct. The right lymphatic duct drains lymph from the right side of the head, neck, thorax, and right upper extremity into the right subclavian vein. Lymph from the rest of the body flows into the thoracic duct that empties into the left subclavian vein. The thoracic duct begins in the abdomen as an expanded sac called the cisterna chyli. When lymph empties into the veins, it forms plasma (the liquid part of blood).
Lymph organs: nodes, nodules, spleen, thymus gland, tonsils
The lymphoid organs are the lymph nodes, spleen, thymus, and groups of lymph nodules in both the oral cavity (tonsils) and small intestine, and appendix (Peyer's patches)। A connective tissue capsule surrounds the lymph nodes. The nodes have an outer cortex and inner medulla. Within the medulla is the germinal center that produces lymphocytes. These infection-fighting white blood cells produce antibodies that identify and destroy antigens.

UNCTIONS of the LYMPHATIC SYSTEM
The main functions of the lymphatic system are as follows:
v the main function of the lymphatic system is to collect and transport tissue fluids from the intercellular spaces in all the tissues of the body, back to the veins in the blood system;
v it plays an important role in returning plasma proteins to the bloodstream;
v digested fats are absorbed and then transported from the villi in the small intestine to the bloodstream via the lacteals and lymph vessels.
v new lymphocytes are manufactured in the lymph nodes;
v antibodies and anti (manufactures in the lymph nodes) assist the body to build up an effective immunity to infectious diseases;
v lymph nodes play an important role in the defence mechanism of the body. They filter out micro-organisms (such as bacteria) and foreign substances such as toxins, etc.
v it transports large molecular compounds (such as enzymes and hormones) from their manufactured sites to the bloodstream.
Function
Lymphocytes originate from haemocytoblasts (stem cells) in red bone marrow. Those that enter the thymus mature and develop into activated T-lymphocytes i.e. able to respond to antigens encountered elsewhere in the body. They then divide into two groups
1. those that enter the blood, some of which remain in circulation and some lodge in other lymphoid tissue .
2. those that remain in the thymus gland and are the source of future generations of T-lymphocytes.
The maturation of the thymus and other lymphoid tissue is stimulated by thymosin, a hormone secreted by the epithelial cells that form the framework of the thymus gland. Involution of the gland begins in adolescence and, with increasing age the effectiveness of T- lymphocyte response to antigens declines.

LYMPHOID TISSUE AND ORGANS
I. DIFFUSE LYMPHOID TISSUE
Lymphocytes, especially T-cells, wander throughout the body even crossing basement membranes to wander among epithelial cells. They are classed as connective tissue cells and wherever they congregate, except in the thymus, they associate with reticular fibers forming reticular connective tissue. Lymphoid tissues, sites with large numbers of lymphocytes, vary in their degree of organization from diffuse concentrations of lymphocytes to highly structured organs.
I. DIFFUSE LYMPHOID TISSUE
A. Mucosa-associated lymphoid tissue (MALT) consists of large numbers of lymphocytes, plasma cells and macrophages in the lamina propria of G.I., respiratory and genitourinary tracts, especially near glands. These are sites of potential invasion. Look for areas of diffuse lymphoid tissue in esophagus and oviduct isthmus.
B. Epithelial lymphocytes - penetrate epithelial basement membrane and migrate among epithelial cells of wet epithelium.
C. After antigen exposure activated lymphocytes and antigen-presenting cells migrate to regional lymph nodes where immunologically active cells are produced. These effector T and B cells, plasma cell precursors and memory cells are returned via the circulation to the mucosa that is threatened.
Lymphatic tissues begin to develop by the end of the fifth week of embryonic life. Lymphatic vessels develop from lymph sacs that arise from developing veins, which are derived from mesoderm.
The first lymph sacs to appear are the paired jugular lymph sacs at the junction of the internal jugular and subclavian veins. From the jugular lymph sacs, lymphatic capillary plexuses spread to the thorax, upper limbs, neck and head. Some of the plexuses enlarge and form lymphatic vessels in their respective regions. Each jugular lymph sac retains at least one connection with its jugular vein, the left one developing into the superior portion of the thoracic duct.
The next lymph sac to appear is the unpaired retroperitoneal lymph sac at the root of the mesentery of the intestine. It develops from the primitive vena cava and mesonephric veins. Capillary plexuses and lymphatic vessels spread form the retroperitoneal lymph sac to the abdominal viscera and diaphragm. The sac establishes connections with the cisterna chyli but loses its connections with neighboring veins.
The last of the lymph sacs, the paired posterior lymph sacs, develop from the iliac veins. The posterior lymph sacs produce capillary plexuses and lymphatic vessels of the abdominal wall, pelvic region, and lower limbs. The posterior lymph sacs join the cisterna chyli and lose their connections with adjacent veins.
With the exception of the anterior part of the sac from which the cisterna chyli develops, all lymph sacs become invaded by mesenchymal cells and are converted into groups of lymph nodes. The spleen develops from mesenchymal cells between layers of the dorsal mesentery of the stomach. The thymus arises as an outgrowth of the third pharyngeal pouch. Lymphatic tissue is also scattered throughout the body in different major organs and in and around the gastrointestinal tract.

Spleen
The spleen is another important lymphatic organ. It processes lymphocytes from incoming blood. The tonsils and adenoids are secondary lymphatic organs.
The spleen is the "largest single mass of lymphoid tissue in the body. The spleen is a soft, purple, highly vascular organ located in the upper left region of your abdomen. It lies just behind and partly lateral to the stomach and just under the diaphragm. It is shaped like a very small smooth rounded catcher's mitt with notches at its upper anterior edge."
The spleen helps control the amount of blood and blood cells that circulate through the body and helps destroy damaged cells. Spleen - large, flattened oval organ covered by peritoneum (mesothelium), with blood vessels entering and leaving at the hilus. Learn to recognize the following features and the distribution of macrophages and reticular fibers.
1. Capsule (CT and muscle)
2. Trabeculae (CT and Muscle)
3. Trabecula containing blood vessel
4. White pulp (lymphoid tissue)
5. "Central" artery in white pulp
6. Red pulp (surrounds trabeculae and white pulp)
The spleen is the largest lymphoid organ. It has two types of tissue: the red pulp, which contains many red blood cells (erythrocytes) and macrophages; and the white pulp, which stores lymphocytes. The macrophages in the red pulp remove foreign substances and damaged or dead erythrocytes and platelets from the blood. And, the red pulp stores platelets, which are important for blood clotting. The lymphocytes within the white pulp are used for the body immune system.
In the thymus gland lymphocytes become specialized. The thymus plays an important role in lymphocyte specialization and immunity.
Thymus gland
The thymus gland lies in the upper part of the mediastinum behind the sternum and extends upwards into the root of the neck. It weighs about 10 to 15 g.(about half an ounce) at birth and begins to grow until the individual reaches puberty when it begins to atrophy. It’s maximum weight is around 30 - 40g (around 1 to 1.5 ounces) by the age of 40 it has returned to it’s weight at birth. The thymus consists of two lobes connected by areolar tissue. The lobes are enclosed in a fibrous capsule which dips into their substance dividing them into lobules that consist of an irregular branching framework of epithelial cells and lymphocytes.

Structure of the Thymus

Begin with slide 117, a thymus from a young pig. A low-power view is shown above. It's covered by a thin connective tissue capsule, and lobulated by septa originating from this. The thymic lobules are clearly divided into a dark staining cortex and a lighter medulla by the differential density of the cells in each region. The cell types in these areas are the same, but their distribution within the lobules isn't uniform, and there are important physiological differences between the two populations.
This shows the distinct lobulation of the organ. You won't find germinal centers in the thymus: it's a sort of "training academy" for the T-lymphocytes, so what you have here are T-lymphocytes that have not yet become immune competent (in the cortex) or have just "graduated" and have started out as rookie T-cells. While the thymus is an important immune organ, it's not a site of antibody production. Hence there can be no germinal centers.


This somewhat higher magnification shows two lobules. The darkly stained cortex and lighter medulla are obvious. The staining difference is attributable to the much higher density of T-cells in the cortex.
The thymic lobules are well demarcated by CT, but they are actually all interconnected. If you were to enter the cortex at the point you see here, you could move from any place in it to any other place in the cortex of this organ freely. The lobulation, in other words, is incomplete. So the septation of the thymus isn't complete, as it is in some other lobulated organs. Serial sectioning would easily show that the different lobes and lobules are continuous with each other through bridges of parenchymal material. Blood vessels enter the thymus through the capsule, and travel along septa to the corticomedullary border, at which point they enter the parenchyma. Blood vessels are pretty obvious in this image. On close examination you can see them not only between lobules, but as small capillary beds and venules in the parenchyma. Arterioles entering the thymus send capillaries to the cortex, which branch at the periphery and return. At the corticomedullary junction, postcapillary venules are found, which represent specialized sites of transit of matured lymphocytes into the blood.
The thymus is relatively largest (in relation to body weight) very early in life, and actually continues to grow in absolute size for a while after birth. Nevertheless, in the normal course of aging, the thymus undergoes involution, by which the total volume of active tissue is reduced. It's replaced by connective tissue and adipose tissue for the most part, but some functional tissue remains throughout life. Many stimuli (such as overexposure to radiation and some chemicals) can cause accidental involution, and in these cases the organ is usually capable of regeneration if the offending stimulus is removed.



This example of an involuted thymus is from an elderly cat. Involution is a normal process. As an animal ages, the functional parenchyma of the organ is replaced with fat and connective tissue, and the organ as a whole diminishes in size and as a proportion of body weight. It does not become totally nonfunctional, however. As more and more T-lymphocytes become "trained" there is less need for the thymus, but even late in life it may be necessary to produce immunocompetent T-cells; so the parenchyma never completely disappears and it always retains some level of function. Natural involution, as in this specimen, is an irreversible process. But accidental involution due to some exogenous agent, such as chemical or radiation insult, can usually be reversed once the offending stimulus is removed.
LYMPHOCYTIC CIRCULATION - lymph to blood and back to lymph.
Small memory cells, mostly T-cells, leave blood stream in thymic dependent areas of lymphoid tissue (post-capillary venules of lymph nodes) and reenter via the lymph. Memory and effector cells from a primary or secondary immune response also leave the lymph nodes by way of the lymph, and seed other lymphoid tissues. There is some organ specificity in this seeding. For example: medium to large lymphocytes in the thoracic duct lymph, come largely from gut associated lymphoid tissue and mesenteric lymph nodes, and are rapidly concentrated in the lamina propria of the gut, where they produce secretory immunoglobulin (IgA).
This animation of a section through a lymph capillary shows how pressure in the interstitial fluid surrounding the capillary pushes open the overlapping cells.The arrows represent the direction of flow of the lymph. Note the internal valve which allows the lymph to flow in one direction only.
Lymphatic circulation
Unlike the circulatory system, the lymphatic system is not closed and has no central pump. Lymph movement occurs with low pressure due to peristalsis, valves, and the milking action of skeletal muscles. Like veins, lymph travels through vessels in one way only, due to semilunar valves. This depends mainly on the movement of skeletal muscles to squeeze fluid through them, especially near the joints. Rhythmic contraction of the vessel walls through movements may also help draw fluid into the smallest lymphatic vessels, capillaries. Tight clothing can restrict this, thus reducing the removal of wastes and allowing them to accumulate. If tissue fluid builds up the tissue will swell; this is called edema. As the circular path through the body's system continues, the fluid is then transported to progressively larger lymphatic vessels culminating in the right lymphatic duct (for lymph from the right upper body) and the thoracic duct (for the rest of the body); both ducts drain into the circulatory system at the right and left subclavian veins. The system collaborates with white blood cells in lymph nodes to protect the body from being infected by cancer cells, fungi, viruses or bacteria. This is known as a secondary circulatory system.
The lymphatic circulation is a drainage system. Its job in maintaining fluid balance is to:
1. collect excess interstitial fluid and return it to the blood (approximately 3 litres daily).
2. return plasma proteins to the blood.
Once interstitial fluid enters a lymph capillary, it is referred to as lymph. The three main types of lymphatic vessels are lymph capillaries, lymphatics, and lymph ducts.
Lymph capillaries are microscopic tubes located between cells. Lymph capillaries resemble blood capillaries somewhat, but differ in important ways. Whereas a blood capillary has an arterial and a venous end, a lymph capillary has no arterial end. Instead, each lymph capillary originates as a closed tube. Lymph capillaries also have a larger and more irregular lumen (inner space) than blood capillaries and are more permeable.
The wall of a lymph capillary is constructed of endothelial cells that overlap one another. When fluid outside the capillary pushes against the overlapping cells, they swing slightly inward--like a swinging door that moves in only one direction. Fluid inside the capillary cannot flow out through these openings.
Lymph capillaries branch and interconnect freely and extend into almost all tissues of the body except the CNS (Central Nervous System) and the avascular tissues such as the epidermis and the cartilage. Lymph capillaries join to form larger vessels called lymphatics or lymph veins. These resemble blood-conducting veins but have thinner walls and relatively larger lumen, and they have more valves. In the skin, lymphatics are located in subcutaneous tissue and follow same paths as veins. In the viscera, lymphatics generally follow arteries and form plexuses (networks) around them.
At certain locations lymphatics enter lymph nodes. These are structures that consist of lymphatic tissue. As the lymph flows slowly through the lymph sinuses within the tissue of the lymph node, it is filtered. Macrophages remove bacteria and other foreign matter as well as debris.
Lymphocytes are added to the lymph as it flows through the sinuses of a lymph node. Thus the lymph leaving the node is both cleaner of debris and richer in lymphocytes. Lymphatics leaving lymph nodes are called efferent lymph vessels and conduct lymph toward the shoulder region. Large lymphatics that drain groups of lymph nodes are often called lymph trunks.
Lymphatics from the lower portion of the body converge to form a dilated lymph vessel, the cisterna chyli, in the lumbar region of the abdominal cavity. The cisterna chyli extends for about 6 centimetres just to the right of the abdominal aorta. At the level of the twelfth thoracic vertebra, the cisterna chyli narrows and becomes the thoracic duct.
Lymphatic vessels from all over the body, except the upper right quadrant, drain into the thoracic duct. This vessel delivers the lymph into the base of the left subcIavian vein at the junction of the left subcIavian and internal jugular veins. In this way lymph is continuously emptied into the blood where it mixes with the plasma. At the junction of the thoracic duct and the venous system, a valve prevents blood from flowing backward into the duct.
Only about 1 centimetre in length, the right lymphatic duct receives lymph from the lymphatic vessels in the upper right quadrant of the body. The right lymphatic duct empties lymph into the base of the right subclavian vein (at the point where it unites with the internal jugular vein to form the brachiocephalic)
An example of the pattern of lymph circulation is:
Lymph capillaries lymphatic lymph node lymphatic cisterna chyli thoracic duct
Lymphatic vessels and lymph nodes can be visualised by the process of lymphangiography. A radiopaque (not transparent to x-rays) contrast material is injected into the a lymphatic vessel. This will show up the vessel and it’s connections to other lymph vessels. The fluid is left in the system for 24 hours and the lymph nodes can then be observed by X-rays. This technique is quite important in the treatment of neoplasms and other disorders of the lymphatic system. The technique is also used to locate lymph nodes for radiation therapy or for surgical removal.