Lymphatic Vessels

00:01 Let’s finally look at a lymphatic vessel. Here are two images of a section through a lymph node. Lymph nodes are structures that I’ll talk about in more detail when we look at the lymphoid tissues. But when fluid passes out of the blood vessels in capillaries, into the interstitium, generally it’s returned back into the capillary ends or the venule ends of the capillaries. So there’s a balance between the amount of fluid in plasma and the amount of fluid in the interstitium. And that’s controlled mainly by the presence of the plasma protein, albumin in the blood. But often, there’s excessive fluid accumulating in tissues. And that’s returned back into the veins up in the neck region through having lymphatic channels, very small blind ended tubes in the tissues of the body that collect that fluid, and then that passes along lymphatic vessels all the way back to the veins in the neck to be returned back into the vascular system. Now, those lymph vessels, on their way back to the vein, pass through structures shown here called “lymph nodes.” And that’s a good idea because that enables lymph or interstitial fluid to be exposed to lymphocytes. So the lymphocytes can then check all that lymph tissue and see whether or not there are pathogens present anywhere in the interstitium around cells everywhere in the body. And that’s the role of these lymph nodes. But if you reflect back on blood, you will recall that lymphocytes travel through blood and then leave the blood system to survey the body tissues, to try and identify any antigens that they are being programmed to identify. Well here, in this lymph node, lymphocytes have passed out of the vascular system, gone through the lymph node, and then they leave the lymph node through lymphatic vessels. Now, those lymphatic vessels actually pass through a chain of lymph nodes. And here, you see a lymph vessel arriving at a lymph node, and it’s full of lymphocytes. They’re on their way back to the vascular system.

02:42 These lymphocytes, in this efferent or arriving vessel, will leave the lymphatic system or leave these lymphatic vessel in sinuses in the lymph node, they'll wander about through the lymph node to check out whether or not there are any antigens present, and then they’ll leave via these efferent lymph vessels to finally join up into the vascular system. So that’s why you see in lymph vessels, lots and lots of lymphocytes.