The lymphatic system. Lymph vessels are found in all tissues except the central nervous system, the bone marrow, and tissues without blood vessels such as cartilage. The lymph system vessels are extensive as the vessels of the circulatory system. The lymphatic system serves several functions. It controls fluid balance by draining and cleansing the fluids that lead the circulatory system to deliver nutrients and gases to the tissues. It interacts with the villi and the digestive system to absorb and deliver fats to the circulatory system. It also has an immunological protection from viruses, bacteria, fungi, and cellular debris that could damage the cells of the body. From your understanding of the circulatory system, you know that the blood passes through the arteries, arterios, and then the capillaries. The capillary walls allow the fluid portion of the blood to exit the capillaries into the surrounding tissues. Once the fluid leaves the capillaries, it is called interstitial fluid. About 90% of this fluid will diffuse back into the capillaries because of the difference in concentrations of the fluid. However, about 10% of the fluid will enter the open-ended lymph vessels. Once the fluid has entered the lymph vessels, it's now called lymph. These vessels eventually deliver the lymph to locations where the lymph can be cleansed of debris and checked for the presence of pathogenic organisms. How it gets the lymph there is pretty amazing. There is no heart for the system of vessels to pump the lymph around. So how does a lymph get to the locations it needs to be delivered to? The lymph moves through your body when you move your skeletal muscles. The contraction of skeletal muscles squeezes the nearby lymph vessels, pumping them. This pushes lymph through the vessels. In addition to the contraction of skeletal muscles, there are two other means by which lymph travels through the lymphatic system. There are smooth muscles at the larger lymph vessels. The contraction of these smooth muscles adds to the force provided by the skeletal muscles. Also, when we breathe, pressure changes occur in the thoracic region. When the thoracic pressure drops, that tends to pull lymph into the thoracic duct. One way valves prevent the lymph from flowing backwards. The function of fluid balance is seen best, perhaps, when it goes awry. When the lymphatic system is prevented from doing its job, the fluids build up in the tissues. Adema is the term given to this medical condition. Mile the dima can occur during pregnancy when the weight of the baby slows the ability of the vessels to move the lymph up the body. More serious levels of adema can occur in a tropical disease called elephantitis, in which a parasite blocks the vessels, and the adema that is produced looks a lot like having legs of an elephant. Some lymph tissue is very diffuse with no clear boundaries. You can actually feel some when you rubbed your lower inner lip with your tongue. Others are more organized into groups, and these are called lymph nodes. Lymph nodes have three functions. First, they are testing stations. They monitor the blood by receiving samples of the blood plasma. Second, if the sample is rife of foreign invaders, they produce lymphocytes and send them into the bloodstream to try to destroy those invaders. In addition, the lymph nodes filter the lymph that they have so they can only return clean fluid back to the blood. Eventually, the lymph is returned to the circulatory system via the right and the left subclavian veins in the shoulders just above the heart level. Lymph nodules can be found as single structures in the body, or they can be grouped together in small clumps. That's what the tonsils are. They are groups of lymph nodules under the mucus membrane in the throat. These lymph nodules form a protective ring around the throat strategically located to protect the body from foreign invaders. If the tonsils get infected, they can become inflamed and abnormally enlarged as you see here. This condition is called tonsilitis. If the condition is chronic, the tonsils can be removed in a tonsilectomy. Tonsils tend to get smaller as a person matures, and they can actually disappear altogether in an adult. Pires patches are very similar to tonsils. They are groups of lymphocytes and lymph nodules that are in the small intestines. Typically, they are found in the last third of the small intestine. Once again, they are strategically located to deal with foreign invaders. The lymphatic system's second function takes place here in the small intestine as well. The absorption of fats. We will discuss this more in depth in the topic of digestion, but for now, know that there are specialized lymph vessels called lacteals in the intestinal villi. These pick up fats that are released from digested food and absorb it into the villas tissue. The liquid in the vessels take on a milky color, instead of being called limp, this fluid is called chile. The chile eventually gets dumped in the subclavian vein just like limp. That is how the fats enter the circulatory system. The spleen is a significant lymphatic structure, and it has a lot in common with the smaller nodes throughout the body. But unlike the limp nodes, the spleen does not filter limp. It is part of the lymphatic system, however, because it filters the blood. As the blood passes through the white pulp of the spleen, foreign invaders stimulate a response from the diffused lymphatic tissue or the lymph nodules. The spleen also works to clean the blood of worn out urethrocytes. Remember, red blood cells have a short lifespan, as a result, roughly 2 million urethrocytes die every second. They must be removed from the blood, and that is another job of the spleen. Before the blood leaves the spleen through the veins, it passes through the red pulp. Macrophages and the red pulp engage in phagocytosis to remove both foreign invaders and worn out red blood cells. The third function of the spleen is to act as a reservoir for oxygen-rich blood. The spleen actually holds more blood than is necessary for its own metabolism. Therefore, it is an extra blood supply full of oxygen and nutrients. This serves as a backup supply of blood in case of blood loss. If the body detects blood loss due to homage, the sympathetic division of the ANS stimulates the smooth muscles in the capsule of the spleen to contract. This pushes the backup supply of blood into the bloodstream, compensating for the blood loss. Although the backup supply of blood in the human spleen is rather minor, it is a major factor in the physiology of some other mammals. Seals use the spleen as a built in oxygen tank. When the seal dies, it can serve its oxygen as much as possible. However, when it is running low and cannot get to the surface, the smooth muscles of the spleen contract, send in the oxygen-rich blood stored there into the bloodstream. This gives the seal more time before it must surface to breathe. Although the spleen is part of the lymphatic system, you can live without it. If your spleen is ruptured due to an injury, it can be removed in a spleenectomy. This is often necessary in order to stop internal bleeding because the spleen is so vascular. Once your spleen is removed, tissues in the liver, as well as other lymphatic tissues in the body, take over the first two tasks of the spleen. Of course, the overall function is not as good as when the spleen was present in the body. As a result, people who have their spines removed are more susceptible to infections and more sensitive to hemorrhage. The spleen is roughly the size of a clenched fist. Unlike lymph nodes, however, the capsule, or outer cover of the spleen, contains smooth muscle tissue, extensions of this capsule, called Tropiculae, make up the skeleton of the node. The lymph nodes are fed by several afferent lymph vessels, however lymph exits through just one afferent lymph vessel. Reticular fibers extend from the Tropiculae, forming a net of connective tissue throughout the lymph node. Inside the spleen, there are two types of tissue, red pulp and white pulp. The white pulp is composed of diffuse lymphatic tissue and lymph nodules, much like the lymph node. This white pulp surrounds the arteries which enter the spleen. The red pulp is made of twisted veins and particular fibers, which are full of blood cells which were in the capillaries of the spleen. Lymph nodules contain germinal centers, where rapid mitosis of lymphocytes can take place in response to foreign invaders found in the lymph. Lymphocytes produced in the germinal centers are released into the lymph and eventually reach the bloodstream, where they can be transported to the tissues. Another lymphatic system structure is the thymus gland. Like the tonsils, the thymus gland changes as a person matures. When a person is young, the thymus gland is large and apportioned to the body size. During this stage of life, it is mostly lymphatic tissue. After puberty, it decreases in size and becomes mostly fibrous and fatty tissue. What does the thymus gland do? Like many things in the human body, the scientific community is still rather puzzled by the thymus gland. We know that while a person is young, immature lymphocytes known as teal lymphocytes leave the bone marrow. Remember, blood cells are made in the bone marrow, and they travel to the thymus. The remarkable maturation process sometimes referred to as thymic education. Teal lymphocytes that are beneficial to the immune system are spared, while teal lymphocytes that might evoke a detrimental immunological response are eliminated. For example, if you have type A blood, teal lymphocytes which attack the A antigen are destroyed. However, teal lymphocytes which attack the B antigen are allowed to mature and enter the bloodstream. Notice that this one is called a gland. That means that one of its functions is to secrete hormones, making it also a part of the undercune system as well as the lymphatic system. It produces hormones, principal among them is the hormone thymusin. What does thymusin in the body do? Well, we're not really sure. We know that it affects the immunological response of the body, however, the way that it's done remains unclear. One prevalent thought is that thymusin stimulates the activity of lymphocytes to migrate to other lymphatic tissues. You