2. OUTLINE
What is lymphatic system
The function of lymphatic system
How the lymph formed
What are the organs composed in the lymphatic system
The difference between fluid and lymph
How the lymph move back and rejoin into blood circulation
3. Lymphatic System
The lymphatic system is part of the circulatory system, comprising a network of
inter-connected tubes known as lymphatic vessels that carry a clear fluid called
lymph towards the heart.
The lymphatic organs play an important part in the immune system. The lymphatic
system transports the white blood cells which are important in the immune
response against pathogens.
4. 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. From the lymph
capillaries the lymph flows into larger tubes called lymphatic vessels. These carry the
lymph back to join the blood circulation.
6. 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.
8. 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).
9. Diagram 10.3 - The circulation of lymph with major lymph nodes
11. 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.
12. 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.
13. 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. 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. 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.
14. A comparison of the cardiovascular and lymphatic systems
Cardiovascular System Lymphatic System
Blood is responsible for collecting and
distributing oxygen, nutrients and
hormones to the tissues of entire body.
Lymph is responsible for collecting and
removing waste products left behind in
the tissues.
Blood flows in the arteries, capillaries, and
veins.
Lymph flows in an open circuit from the
tissues into lymphatic vessels.
Blood flows towards the heart and away
from the heart.
Lymph flows in one direction only
(towards the heart).
Blood is pumped by the heart to all parts
of the body.
Lymph is not pumped. It passively flows
from the tissues into the lymph capillaries.
Blood consists of the liquid plasma that
transports the red and white blood cells
and platelets.
Lymph that has been filtered and is ready
to return to the cardiovascular system is a
clear or milky white fluid.
Blood is visible and damage to blood
vessels causes obvious signs such as
bleeding or bruising.
Lymph is colourless or translucent and
damage to the lymphatic system is
difficult to detect until swelling occurs.
Blood is filtered by the kidneys. Lymph is filtered by lymph nodes located
15.
16. 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.
