The Lymphatic System
The lymphatic system is often considered part of the cardiovascular system (see chapter 1). Excess fluid that leaks out of capillaries to bathe the body's cells is collected by the vessels of the lymphatic system and returned to the blood. By doing so, the lymphatic system maintains the fluid balance in the body. The lymphatic system further assists the cardiovascular system in absorbing nutrients from the small intestine. These necessary actions, however, are only part of the system's vitally important overall function. It is the body's main line of defense against foreign invaders such as bacteria and viruses. The lymphatic system is responsible for body immunity, filtering harmful substances out of tissue fluid (which fills the spaces between the cells) before that fluid is returned to the blood and the rest of the body. For this reason, it is sometimes referred to as the immune system.