8B - Physiology - The Lymphatic System
Functions of the Lymphatic system:
The lymphatic system has three main functions in
the body. Firstly, the lymphatic system is
responsible for fluid balance. Fluid is found
between tissues spaces and cavities, these
spaces are enclosed by interstitial spaces.
Therefore, it regulates the fluid balance. Any
excess fluid and protein will be returned but not
via the blood vessels. The majority of blood
plasma is returned in the tissues by the venous
capillaries and along veins. However, the
remaining amount of blood vessels is drained by
the lymphatic system. Approximately 3 litres of plasma is returned each day. The loss of the
lymphatic system is a life threatening consequence, it is fatal within 24 hours. As the
lymphatic system drains fluid, if it was missing, the tissues would swell, blood volume would
decrease and the body's pressure would increase.
Another function is absorption, most of the body's fats get absorbed in the gastrointestinal
tract. In the tract, the gut membrane is uniquely adapted by the lymphatic system. Therefore,
all fats will be taken up there. Within the small intestine there are small hair-like structures
known as villi, there are small lymphatic vessels where the villi is formed. The villi are
specifically seen in the small fold of the gut surface. Therefore, these small lymphatic
vessels will absorb the fat solubles similarly to vitamins. Once it has been absorbed, a milky
fluid is made known as chyle. Uniquely, chyle contains fatty acids and lymph. When chyle
reaches the venous blood circulation, it will deliver nutrients indirectly, whilst the blood
capillaries will directly deliver the other nutrients.
Another function is the defence mechanism. On a daily basis, the body is exposed to various
deadly pathogens, however, the lymphatic system acts as the defence against them. The
first line of defence in the body is the acidic environment in the stomach, and good bodily
bacteria. However, these initial lines of defence can fail, so the lymphatic system creates the
response to release white blood cells, which will rid of the deadly pathogen. Within the body,
there are various types of immune cells and molecules to fight against infections, like the
lymphocytes and phagocytes. Without the lymphatic system, the body wouldn't be able to
fight these pathogens, which will cause a great deal of harm to the body, and will be fatal.
Hydrostatic Pressure Maintenance:
Hydrostatic pressure occurs when a fluid in a confined space exerts pressure. As a result,
the lymphatic capillaries take lymph fluid from tissues to maintain interstitial fluid pressure.
When fluid accumulates, and pressure exceeds its maximum in the lymph, a valve will open
so the excess can enter a lymphatic capillary. To prevent backflow, the capillaries will stick
together due to the high amounts of pressure. So, when the tissue swells, the anchoring
filaments will begin to pull apart.
,In doing so it signals the capillaries to expand even further, which increases capacity for fluid
but decreases its internal pressure. So, the hydrostatic pressure has increased in the lymph
capillaries as there is a higher concentration of protein than blood plasma in the lymph.
Additionally, it must be considered that the lymphatic capillaries are much larger than
cardiovascular, meaning it can take up more fluid proteins into lymph than plasma, which is
the main contribution of hydrostatic pressure. All proteins will exert hydrostatic pressure, and
fluid follows them, therefore, lymph can flow easily into the capillaries.
Anatomy of the Lymphatic System:
Spleen:
The spleen is one of the lymphatic organs, located between the stomach and diaphragm in
the left abdominal region. Internal spleen feels spongy and has small blood vessels. The
spleen sinuses are hollow as they store blood. In the event of an injury, the spleen will
release the stored blood into the circulatory system to replace what was lost. Additionally,
within the spleen are platelets, which blood clots from to prevent blood loss.
The spleen is enclosed in vessels and sinuses, and is split with both white and red pulp
between the marginal zone. Red pulp has sieve-like fibres that filter out any old red blood
cells when blood flows through the spleen. The spleen will capture the red blood cells and
recycle it to irons and proteins, which are components of haemoglobin. Red pulp is the filter
of the spleen, it removes the toxins, waste and damaged cells from the bloodstream.
In addition, white pulp is the part of the spleen consisting of lymph tissues. Within the lymph
tissues is where T and B lymphocytes are found. These are forms of white blood cells that
will destroy the pathogens and encourage the production of antibodies.
Thymus gland:
The thymus gland plays an important role in the endocrine system by releasing hormones
into the bloodstream. It is one of the lymphatic organs, located in the chest, between their
lungs and behind the sternum.The thymus gland, in the lymphatic system, trains immature T
cells which are made in bone marrow. T cells will be trained and adapted to fight infections
and foreign materials in the body. However, only mature T cells can fight these infections.
The thymus consists of two identical lobes, enclosed in a tough capsule. Inside the lobes is
an exterior region of tissues (the cortex), and within this is a tissue layer (the medulla). Both
lobes of the thymus consist of epithelial and lymphatic tissue, and contain dendritic cells and
macrophages. Once they have reached maturity, they enter the bloodstream, travelling to
the lymph nodes. All of the body's T cells are produced in the Thymus gland before the
person reaches puberty. Once puberty is completed, it becomes less active and is replaced
by fat tissue.
T cells first are found in the cortex of the thymus, and meet epithelial cells that contain
various antigens. The immature T cells will respond to the antigens, then the foreign cells will
, be chosen to survive, mature or move to the medulla. From this, the remaining will die
through apoptosis, and are removed from the body by the macrophages.
The remaining T cells will reach the medulla, continuing to mature, and will slowly be
introduced to the antigens in the body. Here, the T cells will bind to the antigens tested for
signs of autoimmunity. These cells attack their own bodies rather than pathogens. The
autoimmune T cells will be eliminated through apoptosis, meaning only 2% of T cells will be
matured.
Most thymus hormones are made in the thymus, as they support T cells being matured
before being released into the bloodstream. Then, the mature cells will patrol the body to
recognise and eliminate pathogens using B lymphocytes. Here the B cells will be activated
and create an antibody response, the B cells will recognise any reappearing pathogens.
Many organs in the body will grow until the body has fully developed, however the thymus
grows during childhood, but begins to shrink at puberty. Once it has completed this, it is
replaced with fats.
Tonsils:
Tonsils are located in the back of the throat, when the mouth is opened, the tonsils are
visible. Tonsils are round fleshy masses, acting like lymph nodes. They are oval shaped and
are made from dense tissues. Tonsils are made from mucous membranes, veins and nerves.
They have pockets lined with epithelium known as crypts. Primarily, tonsils protect the
respiratory and digestive systems from infections. In addition to this, they help trap bacteria,
to allow for more time to create antibodies and destroy the pathogens. They filter out germs
which enter the nose and mouth, to protect the body from infection. Tonsils contain large
amounts of white blood cells, which help kill germs. Lastly, tonsils help with lymph fluid
circulation as they work alongside other lymphoid tissues.
Lymph Glands / Nodes:
Lymph nodes circular shaped organs, (2.6cm in length), and are found in the neck, groin and
armpits. Lymph glands play two crucial roles in the body, filtering the lymph and building a
defence mechanism against pathogens. Filtration occurs in the lymph nodes sinuses, and
immunological responses are produced by the lymphatic tissues in the cortex and medulla.
When an immune response occurs, the dendritic cells will link lymphocytes and lymph
nodes. The cells are delivered by lymph to the nodes. They begin to interact with the
lymphocytes and trigger an immune response. For lymph filtration to begin, the tissues and
other lymph nodes in the body downstream in the lymphatic circulatory system, and the
afferent veins deliver lymph to the nodes. Within the capsule, lymph will pass through the
immune cells and reticular fibres, and a sieve-like mass of lymph is made, before it reaches
the lymph sinuses. Through the lymph veins, filtered lymph leaves the nodes after being
captured by the reticular fibres. Then, any large particles (tumour cells) will be digested by
the macrophages.
Lymph vessels:
