Lymph Circulation
One of the primary functions of the lymphatic system is the movement of interstitial fluid from the tissues to the circulatory system. Like the veins of the circulatory system, lymphatic capillaries and vessels move lymph with very little pressure to help with circulation. To help move lymph towards the lymphatic ducts, there is a series of many one-way check valves found throughout the lymphatic vessels. These check valves allow lymph to move toward the lymphatic ducts and close when lymph attempts to flow away from the ducts. In the limbs, skeletal muscle contraction squeezes the walls of lymphatic vessels to push lymph through the valves and towards the thorax. In the trunk, the diaphragm pushes down into the abdomen during inhalation. This increased abdominal pressure pushes lymph into the less pressurized thorax. The pressure gradient reverses during exhalation, but the check valves prevent lymph from being pushed backwards.
Transport of Fatty Acids
Another major function of the lymphatic system is the transportation of fatty acids from the digestive system. The digestive system breaks large macromolecules of carbohydrates, proteins, and lipids into smaller nutrients that can be absorbed through the villi of the intestinal wall. Most of these nutrients are absorbed directly into the bloodstream, but most fatty acids, the building blocks of fats, are absorbed through the lymphatic system.
In the villi of the small intestine are lymphatic capillaries called lacteals. Lacteals are able to absorb fatty acids from the intestinal epithelium and transport them along with lymph. The fatty acids turn the lymph into a white, milky substance called chyle. Chyle is transported through lymphatic vessels to the thoracic duct where it enters the bloodstream and travels to the liver to be metabolized.
Types of Immunity
The body employs many different types of immunity to protect itself from infection from a seemingly endless supply of pathogens. These defenses may be external and prevent pathogens from entering the body. Conversely, internal defenses fight pathogens that have already entered the body. Among the internal defenses, some are specific to only one pathogen or may be innate and defend against many pathogens. Some of these specific defenses can be acquired to preemptively prevent an infection before a pathogen enters the body.
Innate Immunity
The body has many innate ways to defend itself against a broad spectrum of pathogens. These defenses may be external or internal defenses. The internal defenses include fever, inflammation, natural killer cells, and phagocytes.
External Defenses
The coverings and linings of the body constantly prevent infections before they begin by barring pathogens from entering the body. Epidermal cells are constantly growing, dying, and shedding to provide a renewed physical barrier to pathogens. Secretions like sebum, cerumen, mucus, tears, and saliva are used to trap, move, and sometimes even kill bacteria that settle on or in the body.
Stomach acid acts as a chemical barrier to kill microbes found on food entering the body. Urine and acidic vaginal secretions also help to kill and remove pathogens that attempt to enter the body. Finally, the flora of naturally occurring beneficial bacteria that live on and in our bodies provide a layer of protection from harmful microbes that would seek to colonize our bodies for themselves.