Professor Dave here, I wanna tell you about the lymphatic system. After discussing blood, the heart, and all the various blood vessels, one might suspect that the entire circulatory system should be covered, but we would be missing a crucial component, and that’s the lymphatic system. Let’s take a quick look at this system now. We know that as the blood makes its way through the body, it enters capillary beds. It is here that hydrostatic and colloid osmotic pressures force some of the fluid out of the blood, to join the interstitial fluid. This allows for oxygen and various nutrients to be absorbed by nearby tissues. Most of this fluid is reabsorbed into the capillaries at the venous end, but not all of it. Around fifteen percent remains behind, along with important plasma proteins. This would be a problem, as the bloodstream certainly can’t continuously lose so much volume. Luckily we have the lymphatic system. The lymphatic vessels that are part of this system will collect this excess interstitial fluid and return it to the bloodstream. So the lymphatic system is comprised of these lymphatic vessels, the fluid within the vessels, which is called lymph once it is absorbed, and the lymph nodes that clean the lymph as it circulates. The lymphatic vessels start out as lymphatic capillaries that weave themselves between the blood capillaries we learned about and the surrounding tissue. These are highly permeable, much more so than blood vessels, so it is easy for interstitial fluid to get inside, thanks to the fact that the endothelial cells that comprise the walls of lymphatic capillaries are connected in very loose fashion, and form flaps that encourage fluid to enter and then prevent it from leaving. It’s not just fluid that enters lymphatic vessels, however. Proteins can enter, as well as even larger particles, like bacteria and other pathogens. This is actually fortunate, as lymph must make it through the lymph nodes, where it is examined by the immune system, so certain issues can be dealt with. So we can see that interstitial fluid, along with various other components, will enter lymphatic capillaries. From there, these combine to form larger lymphatic vessels, just like veins do. These also combine to form lymphatic trunks, which drain various parts of the body, and are named after the region they drain. The interesting thing is that while blood circulates due to the capacity of the heart to act like a pump, the lymphatic system has no such pump. Instead, lymph relies on pressure changes and valves to flow in the correct direction, and lymph flow is stronger during physical activity. There are also other components in the lymphatic system. Lymphoid cells are immune system cells like lymphocytes, which we will discuss later. Lymphoid tissue is made of loose reticular connective tissue, found all over the system except the thymus, and macrophages live on the fibers of this network. This can be either a diffuse lymphoid tissue, or lymphoid follicles, which are tightly packed spherical bodies. Clustered along the lymphatic vessels are many lymph nodes, which are the main lymphoid organ. These nodes act as filters, and macrophages destroy pathogens and other things that don’t belong there, preventing them from spreading throughout the body. Lymph nodes also serve to activate the immune system. Looking a bit more closely at the structure of a lymph node, these consist of a dense fibrous capsule, and strands of connective tissue called trabeculae that divide the node into compartments. Inside, we can see the cortex, packed with follicles, and the medulla, which is comprised of medullary cords and sinuses. The sinuses are where the macrophages patrol for intruders. There are a few other lymphoid organs of note, the first of which is the spleen. The spleen is adjacent to the stomach in the upper left part of the abdomen, and this serves as a site for immune surveillance, and to cleanse blood of foreign matter. It also has the ability to salvage iron for hemoglobin production, and store other components of blood for release when necessary. Structurally, the spleen has two components, which are called white pulp and red pulp. White pulp houses the immune functions of the spleen, while red pulp is where old and damaged red blood cells are removed from circulation. The next lymphoid organ is the thymus, which is found in the thorax just below the neck. This is where certain lymphocytes develop, and it is more active in newborns than in adults. And finally, though not complete organs, there are also patches of lymphoid tissue that are distributed around strategic locations in the body within mucous membranes. The first of these are the tonsils. These are distributed around the mouth and throat, and they gather and remove many of the pathogens that would enter the body through ingestion or inhalation. Peyer’s patches are clusters of lymphoid follicles that serve a similar purpose, and they are found in the small intestine. And finally, we have the appendix, which is also rich in lymphoid follicles which serve to destroy bacteria. So that’s what we need to know about the lymphatic system, which serves to maintain blood volume, but also works closely with the immune system. The immune system is absolutely essential to our ability to stay healthy, so let’s move forward and learn about that next.