Immune system module 1: anatomy of the lymphatic and immune systems

The immune system    is the complex collection of cells and organs that destroy pathogens . Pathogens are substances that can cause illness or death. The lymphatic system works together with the immune system so closely that they are usually studied together. The lymphatic system    is the system of vessels, cells, and organs that carries excess fluids from between the cells to the bloodstream and filters pathogens from the blood. The swelling of lymph nodes during an infection and the transport of lymphocytes via the lymphatic vessels are but two examples of the many connections between these critical organ systems.

Functions of the lymphatic system

A major function of the lymphatic system is to drain body fluids and return them to the bloodstream. Blood pressure causes leakage of fluid from the capillaries, resulting in the accumulation of fluid in the spaces between individual cells in the tissues. In humans, 20 liters of plasma is released into the spaces between the cells of the tissues each day. Once this filtrate is out of the bloodstream and in the tissue spaces, it is referred to as interstitial fluid . Of this, 17 liters is reabsorbed directly by the blood vessels. But what happens to the remaining three liters? This is where the lymphatic system comes into play. It drains the excess fluid and empties it back into the bloodstream via a series of vessels, trunks, and ducts. Lymph is the term used to describe interstitial fluid once it has entered the lymphatic system. When the lymphatic system is damaged in some way, too much interstitial fluid accumulates in the tissue spaces. When too much fluid accumulates it creates a condition called lymphedema which may lead to serious medical consequences.

Cells of the immune system not only use lymphatic vessels to make their way from interstitial spaces back into the circulation, but they also use lymph nodes as areas for the development of critical immune responses. A lymph node    is one of the small, bean-shaped organs located throughout the lymphatic system.

Structure of the lymphatic system

The lymphatic vessels begin as open-ended capillaries, which feed into larger and larger lymphatic vessels, and eventually empty into the bloodstream by a series of ducts. Along the way, the lymph travels through the lymph nodes, which are commonly found near the groin, armpits, neck, chest, and abdomen. A major distinction between the lymphatic and cardiovascular systems in humans is that lymph is not actively pumped by the heart, but is forced through the vessels by the movements of the body, the contraction of skeletal muscles during body movements, and breathing. One-way valves (semi-lunar valves) in lymphatic vessels keep the lymph moving toward the heart. Lymph flows from the lymphatic capillaries, through lymphatic vessels, and then is dumped into the circulatory system via the lymphatic ducts located on the neck.