The lymphatic vessels are thin-walled vessels structured like blood vessels, that carry lymph. As part of the lymphatic system, lymph vessels are complementary to the cardiovascular system. Lymph vessels are lined by endothelial cells, have a thin layer of smooth muscle, adventitia that bind the lymph vessels to the surrounding tissue. Lymph vessels are devoted to the propulsion of the lymph from the lymph capillaries, which are concerned with absorption of interstitial fluid from the tissues. Lymph capillaries are larger than their counterpart capillaries of the vascular system. Lymph vessels that carry lymph to a lymph node are called afferent lymph vessels, those that carry it from a lymph node are called efferent lymph vessels, from where the lymph may travel to another lymph node, may be returned to a vein, or may travel to a larger lymph duct. Lymph ducts drain the lymph into one of the subclavian veins and thus return it to general circulation. Lymph flows away from the tissues to lymph nodes and to either the right lymphatic duct or the largest lymph vessel in the body, the thoracic duct.
These vessels left subclavian veins respectively. The general structure of lymphatics is based on that of blood vessels. There is an inner lining of single flattened epithelial cells composed of a type of epithelium, called endothelium, the cells are called endothelial cells; this layer functions to mechanically transport fluid and since the basement membrane on which it rests is discontinuous. The next layer is that of smooth muscles that are arranged in a circular fashion around the endothelium, which by shortening or relaxing alter the diameter of the lumen; the outermost layer is the adventitia. The general structure described here is seen only in larger lymphatics; the smallest vessels lack both the outer adventitia. As they proceed forward and in their course are joined by other capillaries, they grow larger and first take on an adventitia, smooth muscles; the lymphatic conducting system broadly consists of two types of channels—the initial lymphatics, the prelymphatics or lymph capillaries that specialize in collection of the lymph from the ISF, the larger lymph vessels that propel the lymph forward.
Unlike the cardiovascular system, the lymphatic system has no central pump. Lymph movement occurs despite low pressure due to peristalsis and compression during contraction of adjacent skeletal muscle and arterial pulsation; the lymphatic circulation begins with blind ending permeable superficial lymph capillaries, formed by endothelial cells with button-like junctions between them that allow fluid to pass through them when the interstitial pressure is sufficiently high. These button-like junctions consist of protein filaments like platelet endothelial cell adhesion molecule-1, or PECAM-1. A valve system in place here prevents the absorbed lymph from leaking back into the ISF. There is another system of semilunar valves that prevents back-flow of lymph along the lumen of the vessel. Lymph capillaries have many interconnections between them and form a fine network. Rhythmic contraction of the vessel walls through movements may help draw fluid into the smallest lymphatic vessels, capillaries. If tissue fluid builds up the tissue will swell.
As the circular path through the body's system continues, the fluid is transported to progressively larger lymphatic vessels culminating in the right lymphatic duct and the thoracic duct. The system collaborates with white blood cells in lymph nodes to protect the body from being infected by cancer cells, viruses or bacteria; this is known as a secondary circulatory system. The lymph capillaries drain the lymph to larger contractile lymphatics, which have valves as well as smooth muscle walls; these are called the collecting lymphatics. As the collecting lymph vessel accumulates lymph from more and more lymph capillaries in its course, it becomes larger and is called the afferent lymph vessel as it enters a lymph node. Here the lymph is removed by the efferent lymph vessel. An efferent lymph vessel may directly drain into one of the lymph ducts, or may empty into another lymph node as its afferent lymph vessel. Both the lymph ducts return the lymph to the blood stream by emptying into the subclavian veins The functional unit of a lymph vessel is known as a lymphangion, the segment between two valves.
Since it is contractile, depending upon the ratio of its length to its radius, it can act either like a contractile chamber propelling the fluid ahead, or as a resistance vessel tending to stop the lymph in its place. Lymph vessels act as reservoirs for plasma and other substances including cells that have leaked from the vascular system and transport lymph fluid back from the tissues to the circulatory system. Without functioning lymph vessels, lymph cannot be drained and edema results; the afferent lymph vessels enter at all parts of the periphery of the lymph node, after branching and forming a dense plexus in the substance of the capsule, open into the lymph sinuses of the cortical part. It carries unfiltered lymph into the node. In doing this th
A varix is an anatomical feature of the shell of certain sea snails, marine gastropod molluscs. Gastropods whose shells have varices are families and species within the taxonomic groups Littorinimorpha and Neogastropoda; the varix is a thickened axial ridge, a subcylindrical protrusion, in the shell which exists in some families of marine gastropods. It is an important shell character in generic classification. A varix is located at intervals around the whorl, is formed by considerable thickening of the outer lip during a resting stage in the growth of the shell. In other words, in gastropods whose shells have varices, the shells are characterised by episodic growth - the shell grows in spurts, during the resting phase the varix forms. In many gastropod whose shells have varices, for example the Cassinae, the varix is merely a thickening and swelling of the shell at that point, but in some genera within the family Muricidae, such as Chicoreus, Hexaplex and Pterynotus, within the genus Biplex, of the family Ranellidae, the varices are characterised by elaborate ruffles, frills or lamellae.
Some other genera, for example Murex, are armed with protective spines which may be straight or curved, which are formed by the varices closing or curling around their axis