Lymphatic trunks merge until the lymph enters the two lymphatic ducts. The right lymphatic duct drains lymph from the upper right quadrant of the body. The thoracic duct drains all the rest. 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.
Lymphatic organs are characterized by clusters of lymphocytes and other cells, such as macrophages, enmeshed in a framework of short, branching connective tissue fibers. The valves is to prevent backflow of fluid, so that lymph eventually flows forward instead of falling backwards. When the pressure of lymph fluid increase to a certain point due to filling with more lymph fluid or from smooth muscle contraction, the fluid will be pushed through the valve opening it into the next chamber of the vessel called a lymphangion.
As the pressure falls, the open valve then closes so that the lymph fluid cannot flow backwards. Lymph Vessel : Diagram representing propulsion of lymph through a lymph vessel.
A lymphangion is the term for the space between two semilunar valves in a lymphatic vessel, functional unit of the lymphatic system. Lymph fluid can only flow forward through lymphangions due to the closing of valves after fluid is pushed through by fluid accumulation, smooth muscle contraction, or skeletal muscle contraction. Without valves, the lymphatic system would be unable to function without a central pump.
Smooth muscle contractions only cause small changes in pressure and volume within the lumen of the lymph vessels, so the fluid would just move backwards when the pressure dropped. Blood vessels also have valves, but only in low pressure venous circulation. They function similarly to lymphatic valves, though are comparatively more dependent on skeletal muscle contractions. Learning Objectives Describe lymphatic circulation and the structure of lymphatic vessels.
Key Points Lymph or lymphatic vessels are thin-walled valved structures that carry lymph. Lymph vessels are lined by endothelial cells and have a thin layer of smooth muscles and adventitia that bind the lymph vessels to the surrounding tissue. Lymph capillaries have a greater oncotic pressure a pulling pressure exerted by proteins in solution than blood plasma due to the greater concentration of plasma proteins in lymph.
Additionally, the greater size of lymphatic capillaries compared to cardiovascular capillaries allows them to take more fluid proteins into lymph compared to plasma, which is the other reason for their greater levels of oncotic pressure. This also explains why lymph flows into the lymph capillaries easily, since fluid follows proteins that exert oncotic pressure. Under normal conditions, lymph capillaries prevent the accumulation of edema abnormal swelling in the tissues.
However, edema will still occur during acute inflammation or diseases in which lymph vessels are obstructed. During inflammation, fluid leaks into the tissues at a rate faster than it can be removed by the lymph capillaries due to the increased permeability of cardiovascular capillaries. During lymph vessel obstruction such as through elephantiasis infection , lymph will be unable to progress normally through the lymphatic system, and pressure within the blocked off lymph capillaries increases to the point where backflow into tissues may occur, while the pressure of interstitial fluid gradually rises.
Note: how the tissue fluid is entering the blind ends of lymph capillaries indicated by deep green arrows. The lymphatic capillaries bring lymph further into the lymphatic vessels.
The capillaries have external valves but no internal valves or smooth muscle, so the pressure of lymph accumulation itself must propel the fluid forward into the larger vessels.
Because lymphatic capillaries have a closed end and minivalves normally prevent backflow into tissues, the pressure of lymph becomes higher as more lymph is collected from the tissues, which sends the lymph fluid forward. Multiple capillaries converge in collecting vessels, where the internal valves and smooth muscle start to appear.
This moves lymph further along the system despite the fall in pressure that occurs when moving from the higher-pressure capillaries to the lower-pressure collecting vessels. Learning Objectives Describe the location, structure, and role of lymphatic capillaries in maintaining the pressure of the interstitial fluid.
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