week 5 The Lymphatic System Overview

 

The Lymphatic System Overview

The Lymphatic System

The lymphatic system is a network of vessels that transports fluids, the cells and chemicals that travel within the vessels, and the organs and glands that produce them. The three major functions of the lymphatic system are maintaining fluid balance, lipid absorption, and body defense, as summarized in the table below.

3 Main Functions of the Lymphatic System
Fluid Balance	Fat Absorption	Defense
Absorbs excess tissue fluid to return it to the bloodstream	Absorbs lipids and fat-soluble vitamins from the digestive tract	Removes harmful substances from lymph and blood

Fluid Balance

As a subset of the body’s circulatory system, the lymphatic system carries away excess fluid from tissues and returns it to the bloodstream, which helps maintain fluid balance.  Roughly 30 liters (L) of fluid passes from blood capillaries into the area between cells, known as the interstitial spaces.  However, only 27 L is reabsorbed by the blood capillaries. If the extra 3 L of fluid remained in the interstitial space, edema (swelling) would result, causing tissue damage and eventually death. Instead, the 3 L of fluid enters lymphatic capillaries and passes through various lymphatic vessels to return to the bloodstream.

Fat Absorption

The second function of the lymphatic system is the absorption of fats and fat-soluble vitamins (vitamins A, D, E, K) from the digestive tract. The mucosa that lines the small intestine is covered with fingerlike projections called villi. In the center of each villus are blood capillaries and special lymphatic capillaries called lacteals. The blood capillaries absorb most nutrients, but fats and fat-soluble vitamins are absorbed by the lacteals. The lymph passing through lacteals to enter venous circulation appears white due to its high fat content and is called chyle.

Defense

The third and most familiar function of the lymphatic system is defense against disease. Pathogens (disease-causing organisms) are filtered from lymph by the lymph nodes and from blood by the spleen. Lymphatic organs, including the thymus and tonsils contain lymphocytes and other immune cells capable of inactivating and destroying pathogens.  This immunity function of the lymphatic system also attacks foreign substances such as toxins and cancer cells, and when this function becomes abnormal, can cause cancer, autoimmune disorders, and allergies.

Lymph

Lymph, also called lymphatic fluid, is a clear to faintly yellow fluid similar in composition to blood plasma. Lymph is derived from blood plasma as fluids pass through blood capillary walls at the arterial end. Although 90% of the fluid is reabsorbed at the venous end of the capillaries, 10% remains in the interstitial spaces. To prevent edema, the excess interstitial fluid is collected by tiny lymphatic capillaries and is then called lymph. Lymph is roughly 96% water.  The remaining 4% is composed of white blood cells known as lymphocytes and dissolved solutes from 2 main sources. Substances in blood plasma, such as ions, nutrients, dissolved gases, and small proteins, pass from blood capillaries into the interstitial spaces to become part of the reabsorbed lymph. Substances such as hormones, enzymes, and waste products are produced by living cells within the tissues and also become part of the reabsorbed lymph.

Lymphatic Pathways

The lymphatic system, unlike the circulatory system, does not circulate fluid to and from tissues.  Instead, the lymphatic system only carries lymph in one direction, from body tissues to the circulatory system.  The lymphatic pathways are a network composed of all tubes that carry lymph.  The lymphatic pathways begin as lymphatic capillaries. These tiny tubes merge to form larger lymphatic vessels. These, in turn, merge to form larger lymphatic trunks.  Finally, several lymphatic trunks merge to form to the largest tubes called lymphatic ducts, which combine with veins in the thorax.

Lymphatic Pathway
Interstitial fluid	Lymphatic capillary	Lymphatic vessel	Lymph node	Lymphatic vessel	Lymphatic trunk	Lymphatic duct	Subclavian vein	Blood

Lymphatic Capillaries

Lymphatic capillaries are the smallest lymphatic vessels. They are microscopic, close-ended tubes that extend into interstitial spaces to form a network of vessels that parallels blood capillaries.  Like blood capillaries, the walls of lymphatic capillaries are formed from a single layer of simple squamous epithelium. However, lymphatic capillaries are more permeable since they lack a basement membrane, allowing interstitial fluid to enter easily. In addition, the cells of the epithelium partially overlap, which allows the capillary walls to act as simple valves that prevent the backflow of fluid.    Lymphatic capillaries are present in most tissues of the body, with the exception of the central nervous system, bone marrow, and tissues lacking blood vessels, such as the epidermis and cartilage.

Lymphatic Vessels

The lymphatic capillaries merge to form larger lymphatic vessels. Structurally, lymphatic vessels resemble small veins. Like veins, their walls are composed of 3 layers. The tunica intima is the innermost layer composed of smooth endothelium. The tunica media is the middle layer composed of smooth muscle and elastic fibers. The tunica externa (or adventitia) is the strong outer layer composed of connective tissue and collagen. Like veins, lymphatic vessels also have many one-way valves, giving them a beaded appearance. When lymphatic vessels are compressed, the valves prevent the backflow of lymph.  Like veins, there is no pump; the forward movement of lymph is due to compression of the lymphatic vessels by 3 methods: (1) contraction of surrounding skeletal muscles during activity; (2) periodic contraction of smooth muscle in the tunica media, and (3) pressure changes in the thorax due to breathing movements.  Larger lymphatic vessels feature specialized organs called lymph nodes along their length, which filter lymph as it flows through.

Lymphatic Trunks and Ducts

After leaving the lymph nodes, lymphatic vessels merge to form larger lymphatic trunks, which drain wider regions of the body.  Lymphatic trunks then converge to drain into the largest lymphatic vessels, the lymphatic ducts. There are 2 lymphatic ducts in the body. The right lymphatic duct is the smaller and shorter duct. It receives lymph from the right arm and the right side of the head, neck and thorax and empties into the right subclavian vein, located under the clavicle. The thoracic duct is the much larger and longer lymphatic duct.  It receives lymph from the rest of the body and empties into the left subclavian vein.  After leaving the lymphatic ducts, lymph enters the venous system just before the blood returns to the heart to become part of regular circulation.

LYMPHATIC SYSTEM ORGANS

The lymphatic system is a network of vessels that transports a fluid called lymph, the cells and chemicals that travel within the vessels, and the organs and glands that produce them. The 3 major functions of the lymphatic system are maintaining fluid balance, lipid absorption, and body defense.

Primary vs. Secondary Lymphoid Organs

The organs of the immune and lymphatic systems are collectively called lymphoid organs. There are two types of lymphatic organs: primary and secondary.  Primary lymphoid organs are the site of production of special immune cells called lymphocytes from stem cells. These organs include the bone marrow and the thymus. Secondary lymphoid organs are the sites where lymphocytes mature and do their actual job of fighting pathogens and foreign substances. These organs include lymph nodes, tonsils, Peyer’s patches, appendix, and spleen.

2 Types of Lymphoid Organs
Type	Primary Lymphoid Organs		Secondary Lymphoid Organs
Role	Site of production of new lymphocytes from stem cells and their maturation		Sites of interaction of lymphocytes with antigens in order to activate them
Examples	Bone marrow, thymus		Lymph nodes, tonsils, Peyer’s patches, appendix, spleen

Primary Lymphoid Organs

 

Bone Marrow

Bone marrow is a spongy tissue found inside many bones, including the vertebrae, sternum, and the ends of long bones such as the humerus and femur. Bone marrow is the site of blood cell formation, a process called hemopoiesis.  Hemopoiesis is responsible for the formation of all types of blood cells from hemopoietic stem cells, including red blood cells, called erythrocytes, and white blood cells, called lymphocytes, which include B-cells and T-cells.  Once these cells are produced in the bone marrow they move to other organs and tissues through the bloodstream.

Thymus

The thymus is a soft, bi-lobed gland located in the chest area between the lungs, called the mediastinum. The primary function of the thymus is the creation and maturation of lymphocytes called T-cells (where T stands for thymus). To do this, the thymus secretes thymosin, a hormone that stimulates bone marrow to produce new T-cells. The T-cells then migrate to the thymus to mature. Once fully mature, the cells enter the blood and travel to other lymphatic organs. The thymus is relatively large in children but begins to atrophy after puberty so that in older adults it is quite small.

Secondary Lymphoid Organs

Secondary lymphoid organs are characterized by clusters of lymphocytes and other immune cells, such as macrophages, surrounded by a connective tissue framework that hold the cells in place. The lymphocytes originate from the bone marrow and the thymus and are carried by the blood to the secondary lymphoid organs. The immune cells monitor body fluids, including lymph and blood, for foreign pathogens and other harmful substances. When they detect an invader, the lymphocytes proliferate within the lymphatic organs and then travel via the blood to the invasion site. The secondary lymphoid organs include the lymph nodes, tonsils, Peyer’s patches, appendix, and spleen.

Secondary Lymphoid Organs
lymph nodes	tonsils	Peyer’s patches	appendix	spleen

Lymph Nodes

Lymph nodes are small oval structures that are widely distributed throughout the body along lymphatic vessels. Lymph nodes have 2 primary functions: (1) filtering pathogens and harmful substances from lymph; and (2) activating the immune system when pathogens are detected by acting as a site for lymphocyte proliferation. Lymph nodes are the most common lymphoid organ; there are approximately 500-600 in an average adult. Although lymph nodes are found throughout the body, there are 3 larger clusters of lymph nodes on each side of the body: the cervical nodes in the neck, the axillary nodes in the armpit, and the inguinal nodes in the groin.

The typical lymph node is kidney shaped. Lymphatic vessels enter the node on the convex side; these vessels are called afferent lymphatic vessels. Lymph vessels exit the lymph node from an indentation on the concave side called the hilum; these vessels are called efferent lymphatic vessels. Each node is surrounded by a fibrous connective tissue capsule featuring many inward extensions called trabeculae that subdivide the node into smaller compartments called lymph nodules. Lymph nodules are the structural units of the lymph node and are composed of dense masses of lymphocytes, particularly B-cells and T-cells. Spaces within the lymph nodules called lymph sinuses form a complex network of channels through which the lymph travels. Mobile immune cells called macrophages are most concentrated in the lymph sinuses where they actively patrol the lymph.

Lymph enters the lymph node through the afferent vessels, moves through the lymphatic sinuses, and exits through the efferent vessels. As lymph circulates through the lymph node, the presence of pathogens and other harmful substances stimulates naïve lymphocytes in the lymph nodules to divide.  Lymphatic nodules that feature rapidly dividing lymphocytes are called germinal centers. The newly produced lymphocytes are then released into the lymph where they circulate through the body to enter other lymphatic tissues and mount a larger and more widespread immune response. 

MALT

Small clusters of lymphatic tissue exist in various mucosal membranes of the body including the digestive, respiratory tracts, and reproductive tracts. Collectively, these are called mucosa-associated lymphatic tissue or MALT, which makes up 50% of all lymphatic tissue. The main job of MALT is to protect the body from pathogens or other harmful substances that may enter through mucous membranes. MALT is often subdivided into specific categories based on location, such as gut-associated lymphatic tissue (GALT) and bronchus-associated lymphatic tissue (BALT).

Subdivisions of MALT
GALT	BALT	NALT	SALT
Gut-associated lymphatic tissue (intestine)	Bronchus-associated lymphatic tissue (lungs)	Nasal-associated lymphatic tissue (nose)	Skin-associated lymphatic tissue (epidermis)

Tonsils

Tonsils are clusters of lymphatic tissue located under the mucous membranes lining the nose, mouth, and throat (pharynx), forming a protective ring. As a component of MALT, the job of the tonsils is to protect the body from harmful foreign substances that may enter the body through the oral cavity. There are 3 groups of tonsils. The pharyngeal tonsils are located where the back of the nasal cavity meets the pharynx. When these tonsils become enlarged, they are commonly called adenoids. The palatine tonsils are located where the back of the oral cavity meets the pharynx and are commonly referred to as “the tonsils”. The lingual tonsils are located on both sides of the base of the tongue.

Peyer’s Patches

Peyer’s patches are small egg-shaped nodules of lymphatic tissue found under the mucous membranes of the small intestine, particularly in the lower region called the ileum. As a component of MALT, their job is to protect the body from pathogens or harmful substances that may enter through the small intestine.

Appendix

The appendix is a small, finger-shaped pouch of intestinal tissue located at the junction of the small and large intestine. As a component of MALT, the job of the appendix is to protect the body from pathogens or harmful substances that may enter through the large intestine. Current research suggests that the organ also serves as a reservoir for beneficial gut bacteria.

Spleen

The spleen is the body’s largest lymphatic organ. It is a soft, purple organ located in the upper left side of the abdomen, next to the stomach. The spleen has several jobs: it filters the blood by removing cell debris and damaged red blood cells, it monitors the blood for pathogens and other harmful substances, it produces lymphocytes (especially B-cells) in response to infection, and it stores blood and releases it during blood-loss emergencies such as hemorrhages.

Structurally, the spleen resembles a large lymph node. The concave region of the spleen where the splenic artery enters and the splenic vein exits is the hilus. The spleen in surrounded by a connective tissue capsule with inward extensions called trabeculae that divide it into smaller compartments called lobules. Each lobule features 2 types of tissue. White pulp is lymphatic tissue that consists of densely packed lymphocytes. Red pulp fills the remaining spaces of the lobule and consists of spaces called sinuses, capillaries, and macrophages.

Blood enters the spleen via the splenic artery and circulates through the capillaries and sinuses.  There, lymphocytes in the white pulp monitor the blood for pathogens. If a pathogen is detected, the lymphocytes are stimulated to divide and circulate throughout the body in the same manner as in lymph nodes.  Macrophages in the red pulp remove foreign substances and worn-out red blood cells through phagocytosis. The filtered blood then exits the spleen through the splenic vein to return to general circulation.

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