# Anatomy and Physiology of Animals/Lymphatic System

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## 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).

![](Anatomy_and_physiology_of_animals_Capillary_bed_with_lymphatic_capilaries.jpg "Anatomy_and_physiology_of_animals_Capillary_bed_with_lymphatic_capilaries.jpg")

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).

![](Anatomy_and_physiology_of_animals_Lymphatic_system.jpg "Anatomy_and_physiology_of_animals_Lymphatic_system.jpg")

Diagram 10.2 - The lymphatic system

![](Anatomy_and_physiology_of_animals_Circulation_of_lymph_w_major_lymph_nodes.jpg "Anatomy_and_physiology_of_animals_Circulation_of_lymph_w_major_lymph_nodes.jpg")

Diagram 10.3 - The circulation of lymph with major lymph nodes

![](Anatomy_and_physiology_of_animals_Lymph_node.jpg "Anatomy_and_physiology_of_animals_Lymph_node.jpg")

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

-   <http://www.cancerhelp.org.uk/help/default.asp?page=117> Cancerhelp

A nice clear explanation here with great diagrams of the (human)
lymphatic system.

-   <http://www.jdaross.cwc.net/lymphatics2.htm> Lymphatic system

Introduction to the Lymphatic System. A good description of lymph
circulation with an animation.

-   <http://en.wikipedia.org/wiki/Lymphatic_system> Wikipedia

Good information here on the (human) lymphatic system, lymph circulation
and lymphoid organs.

## Glossary

-   Link to
    Glossary

