The lymphatic system Unit 11 – Assignment 4.
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Transcript of The lymphatic system Unit 11 – Assignment 4.
The lymphatic system Unit 11 Assignment 4 The lymphatic system
The lymphatic systemconsists of two parts, each of which performs
important functions. A network of lymphatic vessels that collect
tissue fluid and return it to the bloodstream. Lymphoid organs and
tissues that house phagocytes and lymphocytes. Lymphatic vessels
These form a one-way system and the lymph fluid inside them flows
towards the heart. There are three types of lymph vessel:
Capillaries Trunks Ducts Lymphatic capillaries
The smallest lymph vessels are called lymph capillaries. These
thin-walled, blind-ending vessels in between cells of body tissue
collect tissue fluid that has been forced out of blood capillaries
but not returned to them. They are drainage vessels. Their walls
are made of a single layer of overlapping endothelial cells and
this makes them very permeable. Fine filaments anchor the
endothelial cells to surrounding structures so, when tissues swell
with fluid, the endothelial cells of the lymph capillary wall are
pulled apart slightly. As a result tissue fluid between cells, and
many any proteins in it, can enter the lymph capillaries The cells
making up the wall of these capillaries act like one-way swing
doors. When the lymph capillaries are full of fluid they push on
the endothelial cells of the walls and shut any gaps between them
so fluid cannot leak out again. The lacteals in villi of the ileum
are specialised lymph capillaries. The fluid in them is milky white
as it also contains absorbed lipids. This fluid is called chyle.
Lymphatic ducts The lymph capillaries join to make larger and
thicker-walled collecting ducts, which join to form lymphatic
trunks and then lymphatic ducts. Two large ducts in the thoracic
region deliver their lymph fluid into the subclavian veins. By
collecting all the tissue fluid that didnt return to blood
capillaries and delivering it back to the bloodstream, the lymph
system enables the cardiovascular system to keep working. The lymph
system also collects digested lipids from the ileum and delivers
them to the blood, How does lymph fluid flow?
There is no pumping mechanism within the lymphatic system and the
fluid in them is at low pressure. The flow of lymph fluid is very
slow but this means that the rate of return of lymph fluid to the
blood is equal to the rate of loss of tissue fluid from the
bloodstream. If you are physically active, the lymph fluid flow
rate speeds up, as does production of tissue fluid from
capillaries. The contraction of skeletal muscles near the vessels
helps propel the lymph fluid in them.
There are valves in lymph vessels that prevent the backflow of
lymphatic fluid. When you breathe in, the pressure in the thoracic
cavity reduces and this helps lymph fluid move along the thoracic
ducts. Lymph trunks and ducts have smooth muscle in their walls and
when this contracts it helps propel the fluid. These vessels are
wrapped in connective tissue near to arteries so the pulsating
action of arteries also helps lymph fluid to move. Lymph nodes
There are hundreds of small lymph nodes clustered along the
lymphatic vessels. They are bean shaped and less than 2.5cm long.
Each is surrounded by a fibrous capsule. Strands of connective
tissue from the capsule extend inwards and divide the node into
compartments. Lymph nodes Lymph nodes contain many B lymphocytes.
They also contain T lymphocytes that are in transit as these cells
circulate between the blood, lymph nodes and lymphatic vessels. As
lymphatic fluid flows through these nodes macrophages engulf
bacteria, cancer cells and other particles, such as bits of debris
from dead cells. This cleanses the lymph fluid before it enters the
blood. The lymphocytes, which began life in the bone marrow but
have migrated to the lymph nodes and other lymph organs, can mount
an immune response against any pathogens in the lymphatic fluid.
The spleen The spleen extracts old erythrocytes, thrombocytes
(platelets), bacteria and viruses from the blood. It stores iron
from old erythrocytes and stores thrombocytes. Thymus gland The
thymus gland educates T lymphocytes. T lymphocytes are made in the
bone marrow and go to the thymus gland where they are sorted. Any
that have receptors that would make them mount an immune response
against the bodys own tissues are destroyed. Any that are not
capable of mounting an immune response are also destroyed. Thus,
only immunocompetent T cells remain. Tonsils The tonsils form a
ring of lymphatic tissue around the back of the throat and around
the base of the tubes that connect the pharynx to the middle ear.
The tonsils filter out pathogens that are inhaled or taken in with
food. Peyers patches Peyers patches are similar in structure to the
tonsils but are in the wall of the ileum. They contain macrophages
that can ingest bacteria and prevent them from crossing the
intestinal wall. Macrophages Macrophages are attracted to invading
pathogens by chemicals. They ingest the pathogen and break it down
into smaller pieces. They then put pieces of the pathogens outer
membrane that contains the pathogens antigens onto its surface
membrane. It is now called an antigen presenting cell T lymphocytes
Each antigen has a specific shape. Somewhere in your lymphatic
system there is one T lymphocyte that has receptors that fit this
antigen. The macrophage searches for the T lymphocyte. When it is
found, the T lymphocyte docks with the antigen on the surface of
the macrophage. This stimulates that T lymphocyte to multiply and
produce helper T cells and killer T cells. Helper T cells stimulate
B cells to divide. Killer T cells destroy our cells that are
infected with viruses and kill cancer cells. B lymphocytes There is
also a B lymphocyte with receptors to fit these antigens. When that
has been found it multiplies in the lymph node and produces lots of
identical B lymphocytes. Some become plasma cells. These produce
antibodies which enter the bloodstream and combat the pathogens.
Antibodies may clump viruses together to stop them entering cells
or they may coat bacteria so macrophages and neutrophils can ingest
them. Some form memory cells which stay in the body, ready for a
quicker and greater response of that pathogen invades again.