1 2 3 4 5 6 7 In animals with open circulatory systems, there is no distinction between blood and lymph and therefore there is no separate lymphatic system. There are undoubtedly many possible ways in which organisms could organize their microbe-fighting tissues. Some invertebrates possess “lymphoid tissues”, “white bodies”, or “branchial spleens” which produce cells which function in immune responses as do those cells of vertebrate lymphatic tissues but are not homologous with vertebrate lymphatic tissues (Hoar, 1983). Higher vertebrates do possess a separate lymphatic system which returns fluid to the circulatory system, transports lipids from the digestive tract, and is essential in combating disease.
8 9 Lymph is a fluid similar in composition to blood plasma. It is derived from blood plasma as fluids pass through capillary walls at the arterial end. As the interstitial fluid begins to accumulate, it is picked up and removed by tiny lymphatic vessels and returned to the blood. As soon as the interstitial fluid enters the lymph capillaries, it is called lymph. Returning the fluid to the blood prevents edema and helps to maintain normal blood volume and pressure.
10 The wall of the lymph capillary is composed of endothelium in which the simple squamous cells overlap to form a simple one-way valve. This arrangement permits fluid to enter the capillary but prevents lymph from leaving the vessel. Like veins, the lymphatic tributaries have thin walls and have valves to prevent backflow of blood. There is no pump in the lymphatic system like the heart in the cardiovascular system. The pressure gradients to move lymph through the vessels come from the skeletal muscle action, respiratory movement, and contraction of smooth muscle in vessel walls.
11 The wall of the lymph capillary is composed of endothelium in which the simple squamous cells overlap to form a simple one-way valve. This arrangement permits fluid to enter the capillary but prevents lymph from leaving the vessel. Like veins, the lymphatic tributaries have thin walls and have valves to prevent backflow of blood. There is no pump in the lymphatic system like the heart in the cardiovascular system. The pressure gradients to move lymph through the vessels come from the skeletal muscle action, respiratory movement, and contraction of smooth muscle in vessel walls.
12 The wall of the lymph capillary is composed of endothelium in which the simple squamous cells overlap to form a simple one-way valve. This arrangement permits fluid to enter the capillary but prevents lymph from leaving the vessel. Like veins, the lymphatic tributaries have thin walls and have valves to prevent backflow of blood. There is no pump in the lymphatic system like the heart in the cardiovascular system. The pressure gradients to move lymph through the vessels come from the skeletal muscle action, respiratory movement, and contraction of smooth muscle in vessel walls.
13 14 Lymph nodes are small bean-shaped structures that are usually less than 2.5 cm in length. They are widely distributed throughout the body along the lymphatic pathways where they filter the lymph before it is returned to the blood. Lymph nodes are not present in the central nervous system. There are three superficial regions on each side of the body where lymph nodes tend to cluster. These areas are the inguinal nodes in the groin, the axillary nodes in the armpit, and the cervical nodes in the neck.
15 The typical lymph node is surrounded by a connective tissue capsule and divided into compartments called lymph nodules. The lymph nodules are dense masses of lymphocytes and macrophages and are separated by spaces called lymph sinuses. The afferent lymphatics enter the node at different parts of its periphery, which carry lymph into the node; entering the node on the convex side. The lymph moves through the lymph sinuses and enters an efferent lymphatic vessel, which, located at an indented region called the hilum, carries the lymph away from the node.
16 Lymphocytes and macrophages in the tonsils provide protection against harmful substances and pathogens that may enter the body through the nose or mouth.
17 18 Like other lymphatic tissue, it produces lymphocytes, especially in response to invading pathogens. The sinuses in the spleen are a reservoir for blood. In emergencies such as haemorrhage, smooth muscle in the vessel walls and in the capsule of the spleen contracts. This squeezes the blood out of the spleen into the general circulation.
19 The spleen is surrounded by a connective tissue capsule, which extends inward to divide the organ into lobules, the spleen consists of two types of tissue called white pulp and red pulp. The white pulp is lymphatic tissue consisting mainly of lymphocytes around arteries. The red pulp consists of venous sinuses filled with blood and cords of lymphatic cells, such as lymphocytes and macrophages. Blood enters the spleen through the splenic artery, moves through the sinuses where it is filtered, then leaves through the splenic vein.
20 21 After the lymphocytes have matured, they enter the blood and go to other lymphatic organs where they help provide defence against disease. The thymus also produces a hormone, thymosin, which stimulates the maturation of lymphocytes in other lymphatic organs.
22 After the lymphocytes have matured, they enter the blood and go to other lymphatic organs where they help provide defence against disease. The thymus also produces a hormone, thymosin, which stimulates the maturation of lymphocytes in other lymphatic organs.
23 24 Already in 1919 Mayer tried to explain the contradictory findings about the piscine lymphatics by the existence of a secondary vascular system. However, only in 1981 could Vogel and Claviez experimentally prove this existence. The secondary vascular system constitutes a separate, parallel circulatory system and includes the vessels earlier assumed to be lymphatics (Hoyer 1934). It starts from the systemic arteries, forms its own capillary networks, which supply mainly the oral mucous membranes and the skin, and returns to the systemic venous system. It functions presumably in skin respiration, osmoregulation and immune defence. The only lymphatic tissue identified in lampreys to date is gut-associated lymphatic tissue (GALT) which is also present in higher vertebrates (Flajnik, 2007; Varner, 1991). In jawless fish, the spleen is not a separate organ, but rather diffuse tissue associated with the digestive tract (Torrey, 1979). In hagfish, the liver seems to sequester red blood cells, which may indicate that hagfish livers perform a function observed in the spleen of higher vertebrates (Forster, 2001). The gnathostome spleen is a single structure caudal to the stomach.
25 Based on its anatomical and functional characteristics it has been hypothesized that the secondary circulation might be an evolutionary predecessor of the lymphatic system (Steffensen and Lomholt 1992). There is evidence for a "true" lymphatic system in lungfish and it is thus reasonable to speculate that the first occurrence of a lymphatic system was associated with the transition from aquatic to terrestrial life (Laurent et al. 1978). Growth factors and receptors that regulate lymphatic growth and development in higher vertebrates are present in fish, but their relevance for the secondary vascular system has not been analyzed (Stainier et al. 1995).
26 Lymph can be propelled by skeletal muscle contractions and even by contraction of regions referred to as “lymph hearts”. These lymph hearts consist of smooth muscle surrounding the regions where lymphatic vessels enter veins and are known in every group of vertebrates except mammals and cartilaginous fish (Webster, 1974).
27 Lymph hearts range in number from four to six in frogs to over two hundred in caecilians. Unlike their name suggests, the main function of lymph hearts is probably not the propulsion of the lymph, but rather maintaining the directionality of lymphatic flow and regulating the entry of lymph fluid into the circulation.
28 29 Vertebrates differ significantly in the number of lymph nodes: in humans there are between 400 and 500 lymph nodes while ducks have only four of them (Weidenreich et al. 1934).
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