The Lymphatic System

The Lymphatic System Chapter 20

Introduction • The lymphatic system supports the function of the cardiovascular and immune systems of the body • The lymphatic system consists of two semi-independent parts • A network of lymphatic vessels • Lymphoid organs scattered throughout the body

Introduction • The lymphatic vessels transport fluids that have escaped from the cardio-vascular system • The main components of the immune system (lymphocytes, lymphoid tissue, and lymphoid organs) fight infections and confer immunity to disease

The Lymphatic System • An elaborate system of lymphatic vessels runs throughout the body • These vessels collect a fluid called lymph from the loose connective tissue around blood capillaries and carry this fluid to the great veins at the root of the neck

The Lymphatic System • Because lymph flows only toward the heart, the lymphatic vessels form a one-way system rather than a full circuit

The Lymphatic System • There are several orders of vessels • Lymph capillaries • Lymphatic collecting vessels • Lymph nodes • Lymph trunks • Lymph ducts

The Lymphatic System • Recall that all blood capillaries are surrounded by a loose connective tissue that contains tissue fluid or interstitial fluid • The fluid arises from blood filtered through the capillary walls

The Lymphatic System • Tissue fluid consists of small molecules of blood plasma, water, various ions, nutrient molecules, and respiratory gases

The Lymphatic System • Tissue fluid is continuously leaving and re-entering the blood capillaries • For complex reasons slightly more fluid arises from the arteriole end than re-enters the venule end • This amounts to about 3 liters a day

The Lymphatic System • The lymphatic vessels function to collect this excess fluid and return it to the bloodstream • Any blockage of the lymphatic vessels causes the affected body region to swell with excess tissue fluid resulting in edema

The Lymphatic System • The lymphatic vessels also perform another related function • Blood proteins leak slowly from blood capillaries into the surrounding tissue fluid • Lymph vessels return leaked proteins to the bloodstream • This is important because proteins in blood generate osmotic forces that are essential for keeping water in the bloodstream

Lymph Capillaries • Lymph capillaries are permeable vessels that receive the tissue fluid • They are located near blood vessels in the loose connective tissue • Like blood capillaries their walls consists of a single layer of endothelial cells

Lymph Capillaries • Lymph capillaries are very permeable • The permeability results from the structure and arrangement of the endothelial cells • They have few intercellular junctions

Lymph Capillaries • In lymph capillaries the adjacent edges of cells overlap to form easily opened minivalves • These valves open to allow tissue fluid to enter the lymphatic capillary • Collagen filaments anchor the cells to the connective tissue

Lymph Capillaries • The minivalves work because the collagen filaments anchor portions of the cell wall to the surrounding connective tissue • Any fluid pressure in the volume of the tissue fluid separates the minivalves • This opens gaps in the wall of the capillary allowing fluid to enter • Once in the lymphatic capillary, it cannot leak out, because backflow pressure forces the minivalve flaps together

Distribution of Lymphatic Vessels • Lymph capillaries are widespread, occurring almost everywhere blood capillaries occur • Lymph capillaries are absent from bone and teeth, bone marrow, and the entire central nervous system

Lymph Capillaries • Once interstitial fluid enters the lymphatic ducts it is called lymph • Although the high permeability of lymph capillaries allows the uptake of large quantities of tissue fluid and protein molecules it also allows bacteria, viruses, or cancer cells in the loose connective tissue to enter these capillaries with ease

Lymph Capillaries • These pathogenic agents can then travel throughout the body via the lymphatic vessels • However, most pathogenic agents are destroyed in the lymph nodes by various antibodies before reaching the general circulation • Cancer cells can be the most problematic and can actually use lymph nodes as a site to metastasize further

Lymph Capillaries • Lymph capillaries are widespread, occurring almost everywhere blood capillaries occur

Lymph Capillaries • Lymph capillaries are absent from bone and teeth, from bone marrow, and from the entire CNS • In the CNS excess fluid drains into the cerebrospinal fluid and then returns it to the blood at the superior sagittal sinus

Lymph Collecting Vessels • From the lymph capillaries, lymph enters lymphatic collecting vessels which accompany blood vessels

Lymph Collecting Vessels • In general, the superficial lymphatic collecting vessels in the skin travel with superficial veins • Deep lymphatic collecting vessels of the trunk and digestive viscera travel with the deep arteries

Lymph Collecting Vessels • Lymphatic collecting vessels are narrow and delicate and are usually not seen in a laboratory dissection • The vessels have the same tunics as blood vessels but their walls are always thinner • The thinness reflects the fact that lymph flows under very low pressure and have no pump to move the lymph along

Lymph Collecting Vessels • To direct the flow of lymph, lymphatic collecting vessels contain more valves than do veins • At the base of each valve, the vessel bulges, forming a pocket in which lymph collects and forces the valve shut

Lymph Collecting Vessels • Because of these bulges, each collecting vessel resembles a string of beads • This distinctive appearance, which characterizes the larger lymph ducts as well allow MD’s to recognize lymph vessels in X-rays

Lymph Collecting Vessels • Unaided by pressure from a pump, lymph is propelled through lymph vessels by a series of weaker mechanisms • The action of contracting skeletal muscle and the pulsation of nearby arteries push on lymph vessel, squeezing lymph through them

Lymph Collecting Vessels • The muscular tunica media of the lymph vessels also contacts to help propel the lymph • Additionally, the normal movements of the limbs and trunk keep the lymph flowing • Despite these mechanisms, the transport of lymph is slow • People who are inactive or who stand for long times often develop edema

Lymph Nodes • Lymph nodes, which cleanse the lymph of pathogens are bean shaped organs situated along lymphatic collecting vessels • The term lymph gland is not correct because they are not glandular in their function • There are about 500 lymph gland in the body

Lymph Nodes • Large clusters of superficial lymph nodes in the cervical, axillary, and inguinal regions • The superficial cervical nodes along the jugular and carotid arteries receive lymph from the head and neck

Lymph Nodes • Axillary nodes in the armpit filter lymph from the upper limbs

Lymph Nodes • The inguinal nodes in the superior thigh filter lymph from the lower limb

Lymph Nodes • Nodes in the mediastinum such as the deep tracheobronchial nodes receive lymph from the thoracic viscera

Lymph Nodes • Deep nodes along the abdominal aorta, called aortic nodes, filter lymph from the posterior abdominal wall

Lymph Nodes • Finally, deep nodes along the iliac arteries, called iliac nodes, filter lymph from the pelvic organs and the lower limbs

Lymph Nodes • The microscopic anatomy of a lymph node suggests its role as a body filter • The node is surrounded by a fibrous capsule of dense connective tissue

Lymph Nodes • Fibrous strands of connective tissue called trabecule extend inward to divide the node into compartments

Lymph Nodes • Lymph enters the convex aspect of the node through several afferent lymphatic vessels and exits from the indented region on the other side, the hilus, through efferent lymphatic vessels

Lymph Nodes • Between the afferent and efferent vessels, lymph percolates through lymph sinuses • These large lymph capillaries are spanned internally by a crisscrossing network of reticular fibers

Lymph Nodes • The reticular fibers are covered by star-shaped endothelial cells • Many macrophages live on this fiber network, phagocytizing pathogens and foreign particles in the lymph that flows through the sinuses

Lymph Nodes • Most lymph passes through several nodes • It is usually free of pathogens by the time it leaves its last node and enters the lymph trunks on its way to the great veins in the neck

Lymph Nodes • Along with its lymph sinuses, a lymph node also contains tadpole shaped masses of lymphoid tissue • This tissue is divided into outer (cortex) and inner (medulla) regions and are part of the immune system

Lymph Trunks • After leaving the lymph nodes, the largest lymphatic collecting vessels converge to form lymph trunks • These trunks drain large areas of the body

Lumbar Trunks • The paired lumbar trunks lie along either side of the aorta in the inferior abdomen • They receive all lymph draining from the lower limbs, pelvic organs and some of the anterior abdominal wall

Intestinal Trunk • The unpaired intestinal trunk lies near midline on the posterior abdominal wall • It receives fatty lymph (chyme) from the stomach, intestines, and other digestive organs

Brachiomediastinal Trunks • These paired trunks ascend near the sides of the trachea • They collect lymph from the thoracic viscera and thoracic wall

Subclavian Trunks • These are paired trunks located near the sides of the trachea • These trunks receive lymph from the upper limbs • They also drain the inferior neck and the superior thoracic wall

Jugular Trunks • These trunks are located in the neck at the base of each internal jugular vein • These trunks drain lymph from the head and neck

Lymph Ducts • The lymph trunks drain into the largest vessels the lymph ducts • The number of ducts in an individual may vary from two to only only duct

Thoracic Duct • The thoracic duct is present in all individuals • Its most inferior part, located at the union of the lumbar and intestinal trucks is the cisterna chyli which lies on the bodies of vertebrae L1 + L2

Thoracic Duct • The thoracic duct ascends along the vertebral bodies • In the superior thorax, it turns left and empties into the venous circulation at the junction of the internal jugular and left subclavian veins

The Lymphatic System