Lymphatic System

Blood circulates through the blood in a closed but leaky series of tubes. Leaving the heart these are the arteries which branch into smaller arterioles which again branch into tiny capillaries. The capillaries then come together to form larger tubes called venuoles and then finally return to the heart as veins. It is as capillaries that these vessels exchange gases, minerals, sugars and other substances with the interstitial fluid. Cells are bathed in this interstitial fluid and get the materials they require and release their wastes into this fluid via diffusion, osmosis and active transport. Cells and chemicals involved in the body’s immune response also reach the body's cells in a similar manner.

Also involved with our circulatory system is another system called the Lymphatic System. This system comprising Lymph Vessels and Lymph Nodes has three main roles, two of which I’ll discuss in some detail. One thing the lymphatic system does is aid in the absorption of lipids from the intestine. That’s all I’m saying about that role. The other two jobs the lymphatic system does is to clean ‘plasma’ and return it to the circulatory system via the vena cava near the heart, and, the most important for this topic, the lymphatic system helps fight infection. Lymph vessels are thin walled and located throughout the body. They carry a clear liquid called lymph which is comprised of water, ions, proteins, fats and leukocytes (white blood cells). Lymphatic circulation transports material from tissues towards the heart. Fluid is forced from capillaries due to pressure and accumulates in the interstitial tissue, which is then reabsorbed by the lymphatic capillaries. This is due to pressure in the interstitial fluid and the thin walled (highly permeable) nature of lymph vessels. This fluid contains cellular wastes as well as pathogens. At intervals along the lymph vessels are lymph nodes sometimes incorrectly referred to as glands. These nodes are small masses of lymph tissue, up to 25mm long. Lymph nodes house phagocytes and T and B lymphocytes and, as lymph travels through the nodes, phagocytes filter out foreign matter. T & B cells respond to foreign antigens and multiply rapidly initiating antibody mediated responses previously mentioned.

When fighting a pathogen, lymph nodes swell (particularly in the neck). We often refer to these as swollen glands. Remember that these are not glands, but lymphatic tissue. Tonsils are masses of lymph tissue and sometimes the pathogens they are fighting manage to infect the cells of lymph nodes themselves. This is what tonsillitis is.

There are other masses of lymph tissue in the body including the thymus and spleen. The thymus is the site of maturing T-lymphocytes and produces a hormone which stimulates T-cell synthesis and maturation. We discussed this briefly in the Specific Immunity essay. The spleen is the largest mass of lymph tissue in the body. It is an important reservoir of lymphocytes  and also regulates the removal of old red blood cells and platelets from circulation and their replacement with newly synthesised cells.

There is no pump to move the lymph through the lymph system. The circulatory system has the heart to pump blood around but movement of lymph through these vessels relies on muscle movement. The more active you are the more the faster the lymph moves through the vessels. Snakes inject their venom into the skin and muscles where it moves from the cells and tissues into the interstitial fluid, ending in the lymphatic vessels where it is transported. This is why if someone gets bitten by a snake you tightly bandage the limb and limit their movement. Unless you don’t like them.