I'm kicking off this second video about the blood vessels to bring you some exciting anatomy news, and that is that there may be a new organ that was described fairly recently! This was in 2018, so it's an exception to the rule that new discoveries aren't typically being made in anatomy. The study discovered this, this structure inside of the body that they call the interstitium. The interstitium would be a continuous fluid filled space inside of the body, it's filled with interstitial fluid, comprised of water and solutes, and would drain into the lymph system. The interstitial compartment is composed of connective and supporting tissues throughout the body. This network of interstitial spaces could act as a shock absorber for other parts of the body. Researchers said it seems to be a conduit for fluids to enter into the lymphatic system, which means that it could also be a route for spreading cancers throughout the body through metastasis. The interstitium was described by these researchers as a fluid-filled 3D latticework of connective tissue that can be found all over the body, in or near the lungs, skin, digestive tracts, and arteries. It's a hard thing to describe but they said it was a series of spaces, a highway of moving fluid, and a previously unknown feature of human anatomy. The authors referred to this system as an organ in its own right, but not all researchers agree with that characterization. The interstitial spaces were not previously described because they collapsed after death, and that's when anatomy is typically studied. The interstitium would collapse in on itself after death, and so it would not be easy to view on on a person except with modern imaging technology, that helped researchers to see the distribution of these collagen and elastic fibers that maintain that interstitial space. Because it has to do with the fluid compartment of the body, it's intimately connected to the lymphatic system, which is a system that will only talk about briefly in this class. The lymphatic system is a system that transports fluid and helps to fight infections. There's an excess interstitial fluid that leaked out of blood capillaries at the tissues, it gets picked up by the lymph vessels. So the lymphatic system is an organ system that's part of the cardiovascular system and part of the immune system. It's made up of a large network of lymphatic vessels, lymphatic organs, and lymphoid tissues. The vessels carry a clear fluid called lymph toward the heart. Lymph vessels return the fluid to venous circulation. If it were not removed, it would accumulate and cause edema. That's accumulation of fluid under the skin, swelling of body tissues, and that would cause a drop in blood pressure, because it would be a drop in blood volume. The..at the capillaries, some blood plasma leaks out of the capillaries, and it's then considered interstitial fluid. Once it enters into lymph capillaries, that same fluid becomes lymph, and then when it's put back into the bloodstream, it's plasma again. Let's take a look at some of these. Here's the structures of the lymphatic system, which is those lymph vessels, lymph cells, and lymph organs that help to transport interstitial fluid back into the blood and to mount an immune response when needed. Lymphatic vessels and structures are always illustrated as this sort of weird chartreuse, like a sick green or a neon green, or a green yellow. It's just an arbitrarily chosen color, and it's just been a tradition for many years to show arteries in red, veins in blue, nerves in yellow, and lymphatic vessels and nodes in green. These are distinctive colors that show up well on a page and are easy to tell apart from one another. And here you can see the distribution of these lymph vessels and nodes throughout the body. Fluid that is that has leaked out from the circulating blood into tissues, carries the nutrients to the cells, and that fluid is now the interstitial fluid, and it's bathing the cells, collecting waste products, collecting bacteria, damaged cells, and then that fluid drains into the lymphatic capillaries, and the lymphatic vessels. The vessels carry the lymph throughout the body, passing through numerous lymph nodes which filter out unwanted materials like bacteria and damaged cells. Lymph then passes into much larger vessels known as the lymph ducts, and these drain into the subclavian veins. Lymph is moved through the system by muscle contraction, similar to how blood flows through the veins by muscle contraction. Here we'll look at these lymphatic capillaries and their close association with a capillary bed. Over here on the left is a lower power image, showing a capillary bed and associated lymphatic capillaries, and then on the right we're looking at the end of just one lymphatic capillary. Lymphatic capillaries arise as these blind end vessels, meaning they only have they have one blind end, they just start somewhere, and they're among most blood capillary networks, shown in red and blue here, and the capillary, the lymph capillaries, would be in green. Lymphatic capillaries are closed-ended tubes that are interspersed among most blood capillary beds. They resemble blood capillaries, but they have overlapping endothelial cells that act as one-way entry flaps, and these flaps are attached by anchoring filaments to nearby structures. They're slightly larger than blood capillaries and they have closed ends. Their unique structure allows interstitial fluid to flow into them but not out. The ends of the endothelial cells that make up the wall of a lymphatic capillary overlap a bit. When pressure is greater in the interstitial fluid than in the lymph, then the cells would separate slightly, like the opening of a one-way swinging door, and interstitial fluid enters into the lymphatic capillary. When pressure is greater inside of the lymphatic capillary, then the cells adhere more tightly, and lymph can't escape back into the interstitial fluid. And so in this image, the flow of blood is illustrated with these black arrows passing through the capillary bed, and there some plasma would leak out and become the interstitial fluid. It would then travel into the lymphatic capillary, and the flow of lymph is shown with those green arrows.