FUNCTIONS OF THE LYMPHATIC SYSTEM
One of the basic tenets of life is that many organisms consume or use other organisms in order to survive. Some microorganisms, such as certain bacteria or viruses, use humans as a source of nutrients and as an environment where they can survive and reproduce. As a result, some of these microorganisms can damage the body, caus-ing disease or even death. Any substance or microorganism that causes disease or damage to the tissues of the body is considered a pathogen. Not surprisingly, the body has ways to resist or destroypathogens.
The lymphatic (lim-fat′ ik) system functions are
1. Fluid balance. About 30 liters (L) of fluid pass from theblood capillaries into the interstitial spaces each day, whereas only 27 L pass from the interstitial spaces back into the blood capillaries . If the extra 3 L of interstitial fluid remained in the interstitial spaces, edema would result, causing tissue damage and eventually death. Instead, the 3 L of fluid enters the lymphatic capillaries,where it is called lymph (limf), and it passes through the lymphatic vessels to return to the blood. In addition to water, lymph contains solutes derived from two sources:(a) Substances in plasma, such as ions, nutrients, gases, and some proteins, pass from blood capillaries into the interstitial spaces and become part of the lymph; (b) substances such as hormones, enzymes, and waste products, derived from cells within the tissues, are also part of the lymph.
2. Lipid absorption. The lymphatic system absorbs lipids andother substances from the digestive tract (see figure 16.14) through lymphatic vessels called lacteals (lak′ t̄e -̆a lz) located in the lining of the small intestine. Lipids enter the lacteals and pass through the lymphatic vessels to the venous circulation.The lymph passing through these lymphatic vessels appears white because of its lipid content and is called chyle (k̄ı l).
3. Defense. Pathogens, such as microorganisms and otherforeign substances, are filtered from lymph by lymph nodes and from blood by the spleen. In addition, lymphocytes and other cells are capable of destroying pathogens. Because the lymphatic system is involved with fighting infections, as well as filtering blood and lymph to remove pathogens, many infectious diseases produce symptoms associated with the lymphatic system (see the Diseases and Disorders table).