Lymphatic system
The body cells are bathed in interstitial fluid, which leaks constantly out of the bloodstream through the permeable walls of the blood capillaries. It is therefore very similar in composition to blood plasma. Lymph passes through vessels of increasing size and a varying number of lymph nodes before returning to the blood.
The lymphatic system consist of the following terms-
- Lymph
- Lymph vessels
- Lymph nodes
- Lymph organs e.g.
- Diffuse lymphoid tissue e.g. Tonsils
- Bone marrow
Functions of the lymphatic system
Tissue drainage
Every day, around 21 litres of fluid from plasma, carrying dissolved substances and some plasma protein, escape from the arterial end of the capillaries and into tissues.
Absorption in the small intestine
Fat and fat-soluble materialise.g.the fat-soluble vitamins, are absorbed into the central lacteals of the villi. Immunity The lymphatic organs are concerned with the production and maturation of lymphocytes.
Lymph and lymph vessels
Lymph
Lymph is a clear watery fluid, similar in composition to plasma, with the important exception of plasma proteins, and identical in composition to interstitial fluid. Lymph transports the plasma proteins that seep out of the capillary beds back to the bloodstream. It also carries away larger particles, e.g. bacteria and cell debris from damage tissues, which can then be filtered out and destroyed by the lymph nodes.
Lymph cappilaries
These originated as blind-end tubes in the interstitial spaces. They have the same structure as blood capillaries, I.e. a single layer of endothelial cells but their walls are more permeable to all interstitial fluid constituents, including proteins and cell debris. The tiny capillaries join up to form larger lymph vessels. Nearly all tissues have a network of lymphatic vessels, important exceptions being the central nervous system, the cornea of the eye, the bones and the most superficial layers of the skin.
Larger lymph vessels
Lymph vessels are often found running alongside the arteries and veins serving the area. Their walls are about the same thickness as those of small veins and have the same layers of tissue, I.e. a fibrous covering, a middle layer of smooth muscle and elastic tissue and an inner lining of endothelium. Like veins, lymph vessels have numerous cup-shaped valve to ensure that lymph flows in a one way system towards the thorax. There is no pump like a heart, involved in the onward movement of lymph, but the muscle layer in the wall of the large lymph vessels has an intrinsic ability to contact rhythmically . Lymph vessels become larger as they join together, eventually forming two large duct and right lymphatic duct which empty lymph into the subclavian vein.
Thoracic duct
It is situated in front of the bodies of the first two lumber vertebrae. The duct about 40 cm long and opens into the left subclavian vein in the root of the neck. It drains lymph from both legs, the pelvic and abdominal cavities, the left half of the thorax, head and neck and the left arm.
Right lymphatic duct
This is dilated lymph vessels about 1 cm long. It lies in the root of the neck and opens into the right half of the thorax, head and the right arm.
Lymphatic organs and tissues
Lymph nodes
Lymph nodes are oval or bean -shaped organ that lie, often in groups, along the length of lymph vessels. The lymph drains through a number of nodes, usually 8-10, before returning to the venous circulation. These nodes vary considerably in size: some are small as a pin head and the largest are about size of an almond.
Structure
Lymph nodes have an outer capsule of fibrous tissue the dips down into the node substance forming partitions. The main substance of the node consists of reticular and lymphatic tissue containing many lymphocytes and macrophages. Reticular cells produce the network of fibres that provide internal structure within the lymph node. The lymphatic tissue contains immune and defence cells, including lymphocytes and macrophages.
Functions
Filtering and phagocytes
Lymph is filtered by the reticular and lymphatic tissue as it passes through lymph nodes. Particulate matter may include bacteria, dead and live phagocytes containing ingested microbes, cell form malignant tumours, worn-out and damaged tissue cells and inhaled particles.
Proliferation of lymphocytes
Activated T- and B lymphocytes multiply in lymph nodes. Antibiotics produced by sensitised B-lymphocytes enter lymph and blood draining the node.
Spleen
The spleen contains reticular and lymphatic tissue and is the largest lymph nodes. The spleen lies in the left lympochondriac region of the abdominal cavity between the fun-dus of the stomach and the diaphragm. It is purplish in colour and varies in size in different individuals, but is usually about 12 cm long, 7 cm wide and 2.5 cm thick. It weighs about 200 g.
Functions
Phagocytosis
old and abnormal erythrocytes are mainly destroyed in the spleen, and the break down products, bilirubin and iron.
Storage of food
The spleen contains up to 350 ml of blood, and in response to sympathetic stimulation can rapidly return most of this volume to the circulation.
Immune response
The spleen contain T- lymphocytes and B- lymphocytes, which are activated by the presence of antigens.
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 weigh about 10 to 15 g at birth and grows until puberty, when it begins to atrophy.. its maximum weighs at puberty , is between 30 and 40 g and by middle age it has returned to approximately its weight at birth.
Functions
Lymphocyte originate from stem cells in red bone marrow. Those that enter the thymus develop into activated T-lymphocytes. Thymus processing produces mature T-lymphocytes that can distinguish self tissue from foreign tissue and also provides each T-lymphocyte with the ability to react to only one specific antigen from the millions it will encounter.
Mucosa-associated lymphoid tissue (MALT)
They contain B-and T-lymphocytes, which have migrated from bone marrow and the thymus, and are important in the early detection of invaders. However, as they have no afferent lymphatic vessels, they do not filter lymph, and are therefore not exposed to disease spread by lymph.
Tonsils -these are located in the mouth and throat, and will therefore destroy swallow and inhaled antigens. Aggregated lymphoid follicles - these large collections of lymphoid tissue are found in the small intestine, and intercept swallowed antigen.
0 Comments