The lymphatic system
The lymphatic or lymph system involves an extensive network of vessels that passes through almost all our tissues to allow for the movement of a fluid called lymph. Lymph circulates through the body in a similar way to blood.
There are about 600 lymph nodes in the body. These nodes swell in response to infection, due to a build-up of lymph fluid, bacteria, or other organisms and immune system cells.
A person with a throat infection, for example, may feel that their "glands" are swollen. Swollen glands can be felt especially under the jaw, in the armpits, or in the groin area. These are, in fact, not glands but lymph nodes.
They should see a doctor if swelling does not go away, if nodes are hard or rubbery and difficult to move, if there is a fever, unexplained weight-loss, or difficulty breathing or swallowing.
Fast facts about the lymphatic system
- The lymphatic system plays a key role in the immune system, fluid balance, and absorption of fats and fat-soluble nutrients.
- As lymph vessels drain fluid from body tissues, this enables foreign material to be delivered to the lymph nodes for assessment by immune system cells.
- The lymph nodes swell in response to infection, due to a build-up of lymph fluid, bacteria, or other organisms and immune system cells.
- Lymph nodes can also become infected, in a condition known as lymphadenitis.
- If lymph nodes remain swollen, if they are hard and rubbery, and if there are other symptoms, you should see a doctor.
Definition
Lymph nodes, or "glands" may swell as the body responds to a threat.
The lymphatic system has three main functions
- It maintains the balance of fluid between the blood and tissues, known as fluid homeostasis.
- It forms part of the body's immune system and helps defend against bacteria and other intruders.
- It facilitates absorption of fats and fat-soluble nutrients in the digestive system.
The system has special small vessels called lacteals. These enable it to absorb fats and fat-soluble nutrients from the gut.
They work with the blood capillaries in the folded surface membrane of the small intestine. The blood capillaries absorb other nutrients directly into the bloodstream.
Anatomy
The lymphatic system consists of lymph vessels, ducts, nodes, and other tissues.
Around 2 liters of fluid leak from the cardiovascular system into body tissues every day. The lymphatic system is a network of vessels that collect these fluids, or lymph. Lymph is a clear fluid that is derived from blood plasma.
The lymph vessels form a network of branches that reach most of the body's tissues. They work in a similar way to the blood vessels. The lymph vessels work with the veins to return fluid from the tissues.
Unlike blood, the lymphatic fluid is not pumped but squeezed through the vessels when we use our muscles. The properties of the lymph vessel walls and the valves help control the movement of lymph. However, like veins, lymphatic vessels have valves inside them to stop fluid from flowing back in the wrong direction.Lymph is drained progressively towards larger vessels until it reaches the two main channels, the lymphatic ducts in our trunk. From there, the filtered lymph fluid returns to the blood in the veins.
The vessels branch through junctions called lymph nodes. These are often referred to as glands, but they are not true glands as they do not form part of the endocrine system.
In the lymph nodes, immune cells assess for foreign material, such as bacteria, viruses, or fungus.
The thymus gland
The thymus gland is a lymphatic organ and an endocrine gland that is found just behind the sternum. It secretes hormones and is crucial in the production, maturation, and differentiation of immune T cells.
It is active in developing the immune system from before birth and through childhood.
The bone marrow
Bone marrow is not lymphatic tissue, but it can be considered part of the lymphatic system because it is here that the B cell lymphocytes of the immune system mature.
Liver of a fetus
During gestation, the liver of a fetus is regarded as part of the lymphatic system as it plays a role in lymphocyte development.
Below is a 3-D model of the lymphatic system, which is fully interactive.
Explore the model using your mouse pad or touchscreen to understand more about the lymphatic system.
Function
The lymph system has three main functions
Fluid balance
The lymphatic system helps maintain fluid balance. It returns excess fluid and proteins from the tissues that cannot be returned through the blood vessels.
The fluid is found in tissue spaces and cavities, in the tiny spaces surrounding cells, known as the interstitial spaces. These are reached by the smallest blood and lymph capillaries.
Around 90 percent of the plasma that reaches tissues from the arterial blood capillaries is returned by the venous capillaries and back along veins. The remaining 10 percent is drained back by the lymphatics.
Each day, around 2-3 liters is returned. This fluid includes proteins that are too large to be transported via the blood vessels.
Loss of the lymphatic system would be fatal within a day. Without the lymphatic system draining excess fluid, our tissues would swell, blood volume would be lost and pressure would increase.
Absorption
Most of the fats absorbed from the gastrointestinal tract are taken up in a part of the gut membrane in the small intestine that is specially adapted by the lymphatic system.
The lymphatic system has tiny lacteals in this part of the intestine that form part of the villi. These finger-like protruding structures are produced by the tiny folds in the absorptive surface of the gut.
Lacteals absorb fats and fat-soluble vitamins to form a milky white fluid called chyle.
This fluid contains lymph and emulsified fats, or free fatty acids. It delivers nutrients indirectly when it reaches the venous blood circulation. Blood capillaries take up other nutrients directly.
The immune system
The lymphatic system produces white blood cells, or lymphocytes that are crucial in fending off infections.
The third function is to defend the body against unwanted organisms. Without it, we would die very soon from an infection.
Our bodies are constantly exposed to potentially hazardous micro-organisms, such as infections.
The body's first line of defense involves:
- physical barriers, such as the skin
- toxic barriers, such as the acidic contents of the stomach
- "friendly" bacteria in the body
However, pathogens often do succeed in entering the body despite these defenses. In this case, the lymphatic system enables our immune system to respond appropriately.
If the immune system is not able to fight off these micro-organisms, or pathogens, they can be harmful and even fatal.
A number of different immune cells and special molecules work together to fight off the unwanted pathogens
Diseases
The lymphatic system can stop working properly if nodes, ducts, vessels, or lymph tissues become blocked, infected, inflamed, or cancerous.
Lymphoma
Cancer that starts in the lymphatic system is known as lymphoma. It is the most serious lymphatic disease.
Hodgkin lymphoma affects a specific type of white blood cell known as Reed-Sternberg cells. Non-Hodgkin lymphoma refers to types that do not involve these cells.
Cancer that affects the lymphatic system is usually a secondary cancer. This means it has spread from a primary tumor, such as the breast, to nearby or regional lymph nodes.
Lymphadenitis
Sometimes, a lymph node swells because it becomes infected The nodes may fill with pus , creating an abscess. The skin over the nodes may be red or streaky.
Localized lymphadenitis affects the nodes near the infection, for example, as a result of tonsilitis.
Generalized lymphadenitis can happen when a disease spreads through the bloodstream and affects the whole body. Causes range from sepsis to an upper respiratory tract infection.
Lymphedema
If the lymphatic system does not work properly, for example, if there is an obstruction, fluid may not drain effectively. As the fluid builds up, this can lead to swelling, for example in an arm or leg. This is lymphedema .
The skin may feel tight and hard, and skin problems may occur. In some cases, fluid may leak through the skin.
Obstruction can result from surgery, radiation therapy injury, a condition known as lymphatic filariasis, or—rarely—a congenital disorder
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