Anatomy and Physiology of Animals/Lymphatic System

Objectives
After completing this section, you should know:
 * the function of the lymphatic system
 * what the terms tissue fluid, lymph, lymphocyte and lymphatic mean
 * how lymph is formed and what is in it
 * the basic structure and function of a lymph node and the position of some important lymph nodes in the body
 * the route by which lymph circulates in the body and is returned to the blood system
 * the location and function of the spleen, thymus and lacteals

Lymphatic System
When tissue fluid enters the small blind-ended lymphatic capillaries that form a network between the cells it becomes lymph. Lymph is a clear watery fluid that is very similar to blood plasma except that it contains large numbers of white blood cells, mostly lymphocytes. It also contains protein, cellular debris, foreign particles and bacteria. Lymph that comes from the intestines also contains many fat globules following the absorption of fat from the digested food into the lymphatics (lacteals) of the villi (see chapter 11 for more on these). From the lymph capillaries the lymph flows into larger tubes called lymphatic vessels. These carry the lymph back to join the blood circulation (see diagrams 10.1 and 10.2).



Diagram 10.1 - A capillary bed with lymphatic capillaries

Lymphatic vessels
Lymphatic vessels have several similarities to veins. Both are thin walled and return fluid to the right hand side of the heart. The movement of the fluid in both is brought about by the contraction of the muscles that surround them and both have valves to prevent backflow. One important difference is that lymph passes through at least one lymph node or gland before it reaches the blood system (see diagram 10.2). These filter out used cell parts, cancer cells and bacteria and help defend the body from infection.

Lymph nodes are of various sizes and shapes and found throughout the body and the more important ones are shown in diagram 10.3. They consist of lymph tissue surrounded by a fibrous sheath. Lymph flows into them through a number of incoming vessels. It then trickles through small channels where white cells called macrophages (derived from monocytes) remove the bacteria and debris by engulfing and digesting them (see diagram 10.4). The lymph then leaves the lymph nodes through outgoing vessels to continue its journey towards the heart where it rejoins the blood circulation (see diagrams 10.2 and 10.3).



Diagram 10.2 - The lymphatic system



Diagram 10.3 - The circulation of lymph with major lymph nodes



Diagram 10.4 - A lymph node

As well as filtering the lymph, lymph nodes produce the white cells known as lymphocytes. Lymphocytes are also produced by the thymus, spleen and bone marrow. There are two kinds of lymphocyte. The first attack invading micro organisms directly while others produce antibodies that circulate in the blood and attack them.

The function of the lymphatic system can therefore be summarized as transport and defense. It is important for returning the fluid and proteins that have escaped from the blood capillaries to the blood system and is also responsible for picking up the products of fat digestion in the small intestine. Its other essential function is as part of the immune system, defending the body against infection.

Problems with lymph nodes and the lymphatic system
During infection of the body the lymph nodes often become swollen and tender because of their increased activity. This is what causes the swollen ‘glands’ in your neck during throat infections, mumps and tonsillitis. Sometimes the bacteria multiply in the lymph node and cause inflammation. Cancer cells may also be carried to the lymph nodes and then transported to other parts of the body where they may multiply to form a secondary growth or metastasis. The lymphatic system may therefore contribute to the spread of cancer. Inactivity of the muscles surrounding the lymphatic vessels or blockage of these vessels causes tissue fluid to ‘back up’ in the tissues resulting in swelling or oedema.

Other Organs Of The Lymphatic System
The spleen is an important part of the lymphatic system. It is a deep red organ situated in the abdomen caudal to the stomach (see diagram 10.3). It is composed of two different types of tissue. The first type makes and stores lymphocytes, the cells of the immune system. The second type of tissue destroys worn out red blood cells, breaking down the haemoglobin into iron, which is recycled, and waste products that are excreted. The spleen also stores red blood cells. When severe blood loss occurs, it contracts and releases them into the circulation.

The thymus is a large pink organ lying just under the sternum (breastbone) just cranial to the heart (see diagram 10.1). It has an important function processing lymphocytes so they are capable of recognising and attacking foreign invaders like bacteria.

Other lymph organs are the bone marrow of the long bones where lymphocytes are produced and lymph nodules, which are like tiny lymph nodes. Large clusters of these are found in the wall of the small intestine (called Peyer’s Patches) and in the tonsils.

Summary

 * Fluid leaks out of the thin walled capillaries as they pass through the tissues. This is called tissue fluid.
 * Much of tissue fluid passes back into the capillaries. Some enters the blind-ended lymphatic capillaries that form a network between the cells of the tissues. This fluid is called lymph.
 * Lymph flows from the lymphatic capillaries to lymph vessels, passing through lymph nodes and along the thoracic duct to join the blood system.
 * Lymph nodes filter the lymph and produce lymphocytes.
 * Other organs of the lymphatic system are the spleen, thymus, bone marrow, and lymph nodules.

Worksheets
Lymphatic System Worksheet

Test Yourself
1. What is the difference between tissue fluid and lymph?

2. By what route does lymph make its way back to join the blood of the circulatory system?

3. As the lymphatic system has no heart to push the lymph along what makes it flow?

4. What happens to the lymph as it passes through a lymph node?

5. Where is the spleen located in the body?

6. Where is the thymus located in the body?

7. What is the function of lymphocytes?

/Test Yourself Answers/

Websites
A nice clear explanation here with great diagrams of the (human) lymphatic system.
 * http://www.cancerhelp.org.uk/help/default.asp?page=117 Cancerhelp

Introduction to the Lymphatic System. A good description of lymph circulation with an animation.
 * http://www.jdaross.cwc.net/lymphatics2.htm Lymphatic system

Good information here on the (human) lymphatic system, lymph circulation and lymphoid organs.
 * http://en.wikipedia.org/wiki/Lymphatic_system Wikipedia

Glossary

 * Link to Glossary