The lymphatic system Unit 11 – Assignment 4

The lymphatic system The lymphatic system consists 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 didn’t 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 body’s 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.

Peyer’s patches Peyer’s 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 pathogen’s outer membrane that contains the pathogen’s 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.