Structure of Lymphatic Vessels

00:01 The lymph nodes have a lot of fluid in them so they are a reservoir for plasma. If you start to get dehydrated, the lymph nodes are capable of releasing that fluid to help hold the vascular volume up.

00:15 And there’s also a constant transport that goes on here involving the lymphatic vessels and the lymph nodes in order to prevent oedema, or swelling. And we’re going to talk about oedema and swelling at the end of this lecture. It happens when the system becomes overwhelmed and tissue fluid accumulates. And we’ll talk about it in more detail.

00:40 Just to give you a quick idea about the structure of the larger lymph vessels.

00:47 They have an endothelial lining just like the arteries, just like the veins, just like the arterioles, just like the venules. Their basement membrane beneath the endothelial layer is not as tight so there is even a little bit of leakage from the lymphatics. But basically since they’re constantly draining, any leakage from the lymphatic gets taken up by little small lymph vessels and, eventually, returned into the system. The larger lymph vessels have smooth muscle as I said to help move the lymph fluid up into the chest. And, of course, on the outer layer there’s adventitia – fibrous tissue and fatty tissue – that holds the lymph vessels in place. And, as I’ve also said before, the further from the periphery you get, the lymph vessels become more complex – more smooth muscle, more adventitia. And the very small lymphatics down in the tissues often have no muscular layer and no adventitia. So the system gets progressively complex until you arrive in the ducts in the chest where they return to the vein. These are quite substantial vessels.

01:56 And I’ll just say a few more words about the lymph ducts. They are the largest vessels in the lymphatic system. No surprise because all the fluid arrives at them. They drain the lymph fluid into one of the subclavian veins and return it to the general circulation.

02:11 The large lymphatic ducts are one on the right that drains the right side of the body and the thorax and the arm. And the thoracic duct is the largest lymph vessel. It drains the lymph from the rest of the body.

02:24 Interestingly enough, the intestines also have a number of lymph vessels. Little tiny lymph vessels in the wall and these are used for absorption following digestion of food.

02:38 And in fact, if you feed an animal or even at surgery a human being a fatty meal, you will see fatty fluid being drained by these lacteals back into the circulation where many of these fatty components are metabolized – that is chewed up – in the liver and converted into nutritional substances that can be used by the cells.

03:08 Again to emphasize that the lymphatic system is a major part of the immune system. It removes cellular debris and invading bacteria, as we talked about, in the lymph nodes. It produces lymphocytes which are a critical part of the immune system. We know that because, for example when a retrovirus such as HIV destroys the lymphocytes, terrible infections develop.

03:33 So the lymphocytes are critical to our well-being.

03:37 And, as you know, there are lymph nodes throughout the body. They’re in the groin, they’re in the cervical area, they’re in the axilla – in the armpit. And also, of course, the tonsils are part of this as well.

03:53 Interestingly enough, both the spleen and the thymus gland are part of the lymph system because the spleen is a blood filter, another filter like the lymph nodes. The spleen’s job is more destroying defective red blood cells and acting as a reservoir for platelets and red blood cells but also it has a lot of lymphocytes and helps in destroying any bacteria that get into the bloodstream.

04:21 The thymus gland up in the neck is also a lymphatic organ. And it’s more commonly involved in early life – in childhood and early adolescence – in developing and maintaining the immune system. And it partially involutes in adults although a very nasty form of cancer can develop in adult life if it doesn’t involute appropriately.

04:48 I mentioned the tonsils before. You know that they’re localized in the back of the throat and they also help by trapping and removing throat bacteria and they’re a kind of lymph node.

05:00 In the intestine, there are something called Peyer's patches, named after an anatomist, Peyer. And they’re also like small collections of lymphocytes or small lymph node type areas.

05:13 And they also prevent bacteria from penetrating the intestinal wall and getting into the system.

05:20 And there are small collections of lymphoid tissue in the skin and connective tissue throughout the body.

05:25 So you see that the lymphatic system is very complex. It isn’t just a bunch of little tubes for draining fluid. It also has a very important defence component. And it lives in intimate contact with the lymphatic system, the lymph nodes and the lymphocyte production which, as I’ve said, are critical to defence against invading bacteria.