VAN 121: Veterinary Gross Anatomy-II (2+2)

Lymphatic circulation

LYMPHATIC SYSTEM

• This system consists of the lymph vessels and lymph glands connected with the venous part of the blood vascular system, containing the lymph.
• The lymph is a colorless tissue fluid drained by lymphatics.
• The vessels are other than the blood vessels, which transport tissue fluid to the blood stream.
• The walls of the vessels are very much thinner than those of the veins but are provided with numerous valves, which allow the lymph to flow at one direction. When full, they present characteristic beaded appearance.
• The flow of lymph is very slow. The vessels pass through lymph nodes.
• All the lymph vessels in the body ultimately unite together into two large trunks, the thoracic duct and right lymphatic duct which open into the venous system.
• The lymph glands or nodes are oval or bean shaped structures of variable sizes ranging from very minute bodies to the size of the lemon.
• They are intercalated in the course of the lymph vessels. The hilum is in a depressed area where the blood vessels and efferent lymph vessels enter or leave the gland. Lymph passes through at least one lymph gland before it empties into the venous system.
• Lymph vessels, which carry lymph towards the lymph gland are called as afferents and those, which carry the lymph away from the lymph gland, are known as efferents.
• The afferent vessel found at different points along the convex surface of the node Although variable in number the lymph nodes are constantly found in the certain definite areas such as inguinal region, mesentery, prescapular area and supramammary, etc.
• The haemal lymph glands are dark red in colour and these have no afferents or efferents but have the blood directly passing through their channels.
• Lymph capillaries drain into lymph vesels, lymph ducts and lymph trunks. They carry the tissue fluid to blood stream. Ultimately lymph empties into the venous angle or jugular confluence or to cranial vena cava. Wall of lymph vessel presents individual muscular segments developed between adjacent valves. Such a segment is known as lymphangion. It is the basic unit of lymph transportation. The vessels pass through lymph nodes. All the lymph vessels in the body ultimately unite together into two large trunks, the thoracic duct and right lymphatic duct which open into the venous system.

Lymph vessels

• It has already been pointed out that the lymph capillary is followed by the lymph vessel (240 A, B) which has a wider lumen. It is the function of this vessel to carry the lymph from the lymph capillary to the lymph node and finally to the blood stream. Since the first lymph .valves are situated in this region, the direction of flow is centripetal and we therefore call these vessels "conducting vessels". Like the capillary netWork, these conducting vessels have numerous branches which are interconnected into a eticulation. They do not differ from the capillaries in the structure of their walls but they are of greater calibre and bear a large number of cusped valves. The forces which drive thelymph cenuipetally along these vessels are exerted from outside. In the conducting vessels of the skin such forces can result from the tension of the vessels during movement. In the conducting vessels of the locomotor apparatus the forces are due to the contraction of neighbouring muscle bundles. Co';;'duccing vessels are situated along the intestines running from the mucous membrane to the peritoneum and passing between the bundles of intestinal muscles and they are therefore expressed during peristaltic movement.
• On the other hand the more distally situated sections of lymph vessels have muscle fibres in their own walls and are themselves capable of movement. For this reason we refer to them as the "transport vessels". They also have numerous, regularly-distribUted valves and these divide the transport vessel into a chain of valved segments. Each valve segment, also known as a lymphangion, therefore consists of a pair of valves and the following length of vessel, which has a lining endothelium and a atff of muscle tissue. Depending on the size of this vessel, the cuff consists of one or more layers of muscle fibres arranged in a complex pattern. It is interesting to note that. the walls of small transport vessels are free from muscle fibres at the point of insertion of the valves. Thus the muscular cuffs of the individual lymphangion are separated from one another and can function independendy. If lymph flows from the periphery towards a valve segment then it distends betWeen the valves, because the muscular cuff relaxes and the vessel can be stretched. When the segment is filled, muscle contraction follows with the result that the lymph in the valve segment is put under pressure. The entry valve now closes and the lymph is released centripetally through the passively opened exit valve into the next lymphangion. In this latter lymphangion the muscular cuff is again stretched and the process repeated.