Microbial Diseases of the Cardiovascular and Lymphatic Systems

Functions of the Lymphatic System

The lymphatic system plays a prominent role in immune function, fatty acid absorption, and removal of interstitial fluid from tissues.

Learning Objectives

Describe the roles of the lymphatic system

Key Takeaways

Key Points

  • The lymphatic system is a linear network of lymphatic vessels and secondary lymphoid organs. It is the site of many immune system functions as well as its own functions.
  • It is responsible for the removal of interstitial fluid from tissues into lymph fluid, which is filtered and brought back into the bloodstream through the subclavian veins near the heart.
  • Edema accumulates in tissues during inflammation or when lymph drainage is impaired.
  • It absorbs and transports fatty acids and fats as chylomicrons from the digestive system.
  • It transports white blood cells and dendritic cells to lymph nodes where adaptive immune responses are often triggered.
  • Tumors can spread through lymphatic transport.

Key Terms

  • lacteal: A lymphatic capillary that absorbs dietary fats in the villi of the small intestine.
  • interstitial fluid: Also called tissue fluid, a solution that bathes and surrounds the cells of multicellular animals.
  • white blood cell: A type of blood cell involved with an immune response. Many white blood cells (primarily lymphocytes) are transported by the lymphatic system.

The lymphatic system is the site of many key immune system functions. It is important to distinguish that immune system functions can happen almost anywhere in the body, while the lymphatic system is its own system where many immune system functions take place. Besides immune system function, the lymphatic system has many functions of its own. It is responsible for the removal and filtration of interstitial fluid from tissues, absorbs and transports fatty acids and fats as chyle from the digestive system, and transports many of the cells involved in immune system function via lymph.

Removal of Fluid

Interstitial fluid accumulates in the tissues, generally as a result of the pressure exerted from capillaries (hydrostatic and osmotic pressure) or from protein leakage into the tissues (which occurs during inflammation). These conditions force fluid from the capillaries into the tissues. One of the main functions of the lymphatic system is to drain the excess interstitial fluid that accumulates.

The lymphatic system is a blunt-ended linear flow system, in which tissue fluids, cells, and large extracellular molecules, collectively called lymph, are drained into the initial lymphatic capillary vessels that begin at the interstitial spaces of tissues and organs. They are then transported to thicker collecting lymphatics, which are embedded with multiple lymph nodes, and are eventually returned to the blood circulation through the left and right subclavian veins and into the vena cava. They drain into venous circulation because there is lower blood pressure in veins, which minimizes the impact of lymph cycling on blood pressure. Lymph nodes located at junctions between the lymph vessels also filter the lymph fluid to remove pathogens and other abnormalities.

Fluid removal from tissues prevents the development of edema. Edema is any type of tissue swelling from increased flow of interstitial fluid into tissues relative to fluid drainage. While edema is a normal component of the inflammation process, in some cases it can be very harmful. Cerebral and pulmonary edema are especially problematic, which is why lymph drainage is so important. Abnormal edema can still occur if the drainage components of the lymph vessels are obstructed.

This diagram of the lymphatic system indicates the artery containing high pressure oxygenated blood, capillary, vein containing low pressure deoxygenated blood, large lymph vessel with valves, lymph node, lymph, lymph capillary, and cells. The process includes tissue fluid with oxygen and food passing out of capillaries, tissue fluid with cell waste products entering capillaries, and tissue fluid with cell waste products enters lymph vessels.

The lymphatic system: A diagram of fluid movement in the lymphatic system.

Fatty Acid Transport

The lymphatic system also facilitates fatty acid absorption from the digestive system. During fat digestion, fatty acids are digested, emulsified, and converted within intestinal cells into a lipoprotein called chylomicrons. Lymph drainage vessels that line the intestine, called lacteals, absorb the chylomicrons into lymph fluid. The lymph vessels then take the chylomicrons into blood circulation, where they react with HDL cholesterols and are then broken down in the liver.

Immune Cell Transport

In addition to tissue fluid homeostasis, the lymphatic system serves as a conduit for transport of cells involved in immune system function. Most notably, highly-specialized white blood cells called lymphocytes and antigen -presenting cells are transported to regional lymph nodes, where the immune system encounters pathogens, microbes, and other immune elicitors that are filtered from the lymph fluid. Much of the adaptive immune system response, which is mediated by dendritic cells, takes place in the lymph nodes. Lymphatic vessels, which uptake various antigens from peripheral tissues, are positively regulated by chemokines/cytokines secreted by various immune cells during inflammation. This allows antigens to enter lymph nodes, where dendritic cells can present them to lymphocytes to trigger an adaptive immune response.

While the lymphatic system is important for transporting immune cells, its transport capabilities can also provide a pathway for the spread of cancer. Lymph circulation is one of the main ways that tumors can spread to distant parts of the body, which is difficult to prevent.

The Cardiovascular System

Both the cardiovascular system and the lymphatic system are susceptible to diseases caused by microorganisms.

Learning Objectives

Compare and contrast the causes associated with: endocarditis, myocarditis, bacteremia, vasculitis and lymphatic disease

Key Takeaways

Key Points

  • Two common cardiovascular diseases caused by infection with microorganisms are endocarditis and myocarditis.
  • Endocarditis is inflammation of the inner tissue of the heart such as its valves caused by infectious agents.
  • Myocarditis is inflammation of heart muscle and it is most often due to infection by common viruses.
  • Bacteremia is the presence of bacteria in the blood.
  • Vasculitis is inflammation of the vessel wall due to an infection or autoimmune disease.
  • Lymphadenopathy is a disease of the lymph nodes due to infection, auto- immune disease, or malignancy.

Key Terms

  • Endocarditis: An inflammation of the endocardium and possibly the heart valves.
  • myocarditis: Inflammation of the myocardium.
  • lymphadenopathy: An abnormal enlargement of the lymph nodes
  • Vasculitis: Inflammation of the vessel wall due to an infection or autoimmune disease.
  • bacteremia: The presence of bacteria in the blood.

Both the cardiovascular system and the lymphatic system are susceptible to diseases caused by microorganisms.

In the cardiovascular system, the heart, the blood vessels (arteries, capillaries, and veins), and the blood are targets of pathogens. Two common cardiovascular diseases caused by infection with microorganisms are endocarditis and myocarditis.

  1. Endocarditis is inflammation of the inner tissue of the heart such as its valves caused by infectious agents. The agents are usually bacterial, but other organisms can also be responsible. Since, the valves of the heart do not receive any dedicated blood supply, the defensive immune mechanisms (such as white blood cells) cannot directly reach the valves via the bloodstream. The lack of blood supply to the valves also has implications for treatment, since drugs also have difficulty reaching the infected valve.
  2. Myocarditis or inflammatory cardiomyopathy is inflammation of heart muscle (myocardium) and it is most often due to infection by common viruses, such as parvovirus B19. It is often caused by an autoimmune reaction. Streptococcal M protein and coxsackievirus B have regions (epitopes) that are immunologically similar to cardiac myosin. During and after the viral infection, the immune system may attack cardiac myosin. Because a definitive diagnosis requires a heart biopsy, which doctors are reluctant to do because they are invasive, statistics on the incidence of myocarditis vary widely. The consequences of myocarditis thus also vary widely. It can cause a mild disease without any symptoms that resolves itself, or it may cause chest pain, heart failure, or sudden death. As most viral infections cannot be treated with directed therapy, symptomatic treatment is the only form of therapy for those forms of myocarditis. In the acute phase, supportive therapy, including bed rest, is indicated. For symptomatic patients, digoxin and diuretics provide clinical improvement.

Bacteremia is the presence of bacteria in the blood. Bacteria can enter the bloodstream as a severe complication of infections (like pneumonia or meningitis), during surgery (especially when involving mucous membranes such as the gastrointestinal tract), or due to catheters and other foreign bodies entering the arteries or veins (including intravenous drug abuse). Bacteremia can have several consequences. The immune response to the bacteria can cause sepsis and septic shock, which has a relatively high mortality rate. Bacteria can also use the blood to spread to other parts of the body (which is called hematogenous spread), causing infections away from the original site of infection. Examples include endocarditis or osteomyelitis. Treatment is with antibiotics, and prevention with antibiotic prophylaxis can be given in situations where problems are to be expected.

Vasculitis is inflammation of the vessel wall due to an infection (or autoimmune disease). Blood vessel permeability is increased in inflammation. Damage, due to trauma or spontaneously, may lead to hemorrhage due to mechanical damage to the vessel endothelium.

Lymphatic disease is a class of disorders that directly affect the components of the lymphatic system. Lymphadenopathy is a term meaning disease of the lymph nodes due to infection, auto-immune disease, or malignancy. Enlarged lymph nodes are a common symptom in a number of infectious diseases, of which some are as follows:

  1. Acute infection (e.g., bacterial, or viral), or chronic infections (tuberculous lymphadenitis, cat-scratch disease).
  2. The most distinctive symptom of bubonic plague is extreme swelling of one or more lymph nodes that bulge out of the skin as “buboes. ” The buboes often become necrotic and may even rupture.
  3. Infectious mononucleosis is an acute viral infection, the hallmark of which is marked enlargement of the cervical lymph nodes.
  4. It is also a symptom of cutaneous anthrax, measles and Human African trypanosomiasis, the latter two giving lymphadenopathy in lymph nodes in the neck.
  5. Toxoplasmosis, a parasitic disease, gives a generalized lymphadenopathy.
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Endocarditis: A mitral valve vegetation caused by bacterial endocarditis.

Structure of the Lymphatic System

The lymphatic system consists of lymphatic vessels and associated lymphoid organs.

Learning Objectives

Describe the structure and function of the lymphatic system

Key Takeaways

Key Points

  • The lymphatic system is a circulatory system that drains fluid from the blood vessels.
  • Lymph vessels are the site of fluid drainage and pump lymph fluid using smooth muscle and skeletal muscle action. The larger vessels contain valves to prevent backflow and pump towards the heart to return lymph fluid to the bloodstream by the subclavian veins.
  • A lymph node is an organized collection of lymphoid tissue through which the lymph passes on its way to returning to the blood. Lymph nodes are located at intervals along the lymphatic system.
  • Lymphoid tissue contains lymphocytes and other specialized cells and tissues that have immune system functions.

Key Terms

  • lymph node: Small oval bodies of the lymphatic system, distributed along the lymphatic vessels clustered in the armpits, groin, neck, chest, and abdomen. They filter through lymph fluid.
  • lymph: A colorless, watery, bodily fluid carried by the lymphatic system, consisting mainly of white blood cells.

The lymphatic system is a collection of structures and vessels that drains lymph from blood and has several other functions. It is a circulatory system for lymph fluid and the site of many key immune system functions.

Lymphatic Vessels

The lymphatic vessels are the lymphatic system equivalent of the blood vessels of the circulatory system and drain fluid from the circulatory system. The network of lymph vessels consists of the initial collectors of lymph fluid, which are small, valveless vessels, and goes on to form the precollector vessels, which have rudimentary valves that are not fully functional. These structures then form increasingly larger lymphatic vessels which form colaterals and have lymph-angions (lymph hearts).  The larger lymph vessels contain valves that prevent the backflow of lymph.

The lymphatic system is an active pumping system driven by segments that have a function similar to peristalsis. They lack a central pump (like the heart in the cardio vascular system), so smooth muscle tissue contracts to move lymph along through the vessels. Skeletal muscle contractions also move lymph through the vessels. The lymphatic vessels make their way to the lymph nodes, and from there the vessels form into trunks. In general, the lymph vessels bring lymph fluid toward the heart and above it to the subclavian veins, which enable lymph fluid to re-enter the circulatory system through the vena cava.

Lymphatic Tissues and Organs

Lymphoid tissue is found in many organs including the lymph nodes, as well as in the lymphoid follicles in the pharynx such as the tonsils. Lymph nodes are found primarily in the armpits, groin, chest, neck, and abdomen. Lymphoid tissues contain lymphocytes (a type of highly differentiated white blood cell), but they also contain other types of cells for structural and functional support, such as the dendritic cells, which play a key role in the immune system. The system also includes all the structures dedicated to the circulation and production of lymphocytes, including the spleen, thymus, and bone marrow.

Cardiovascular and Lymphatic System Defenses

The circulatory system has a defence against microbial invaders in the form of the lymphatic system.

Learning Objectives

Summarize the role of the lymphatic system during an infection

Key Takeaways

Key Points

  • The lymphatic system works in close cooperation with other body systems to destroy pathogens and filter waste.
  • The lymphatic system contains immune cells called lymphocytes, which protect the body against antigens (viruses, bacteria, etc. ) that invade the body. Lymph nodes are sites of both microbial destruction and the production of antibodies against other foreign invaders.
  • While the lymph system acts as a defence against microbial invaders it can also cause problems. Notably over-swelling, and acting as a home for bacterial invaders, even spreading bacteria or cancerous cells.
  • Lymph is the fluid that is formed when interstitial fluid enters the initial lymphatic vessels of the lymphatic system and transports antigen-presenting cells (APCs), such as dendritic cells, to the lymph nodes where an immune response is stimulated.

Key Terms

  • lymphatic system: In mammals, including humans, a network of lymph vessels and lymph nodes that transport fluid, fats, proteins, and lymphocytes to the bloodstream as lymph, and remove microorganisms and other debris from tissues.
  • lymphocytes: type of white blood cells in the vertebrate immune system

The cardiovascular and lymphatic are both integral parts of the circulatory system. The cardiovascular system basically moves blood throughout the body. While the lymphatic system is part of the circulatory system, comprising a network of conduits called lymphatic vessels. Rather than blood the lymph systems carries a clear fluid called lymph (from Latin lympha, meaning “water goddess”) unidirectionally towards the heart. The lymph system is not a closed system. The circulatory system processes an average of 20 liters of blood per day through capillary filtration which removes plasma while leaving the blood cells. While the circulatory system is essential for survival, it also is the source of a major problem when dealing with microbial infections. Many microbes take advantage of the circulatory system to spread throughout the body. Not surprising then the lymphatic system is critical for the bodies immune response to microbial infections. Lymphatic organs play an important part in the immune system, having a considerable overlap with the lymphoid system. Lymphoid tissue is found in many organs, particularly the lymph nodes, and in the lymphoid follicles associated with the digestive system such as the tonsils. Lymphoid tissues contain lymphocytes, but they also contain other types of cells for support. The system also includes all the structures dedicated to the circulation and production of lymphocytes (the primary cellular component of lymph), which includes the spleen, thymus, bone marrow, and the lymphoid tissue associated with the digestive system.

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The lymphatic system: This diagram shows the network of lymph nodes and connecting lymphatic vessels in the human body.

As well as filtering the lymph, lymph nodes produce the white cells known as lymphocytes. Lymphocytes are also produced by the thymus, spleen and bone marrow. There are two kinds of lymphocyte. The first attach invading micro organisms directly while others produce antibodies that circulate in the blood and attack them. When micro-organisms invade the body, or the body encounters antigens (such as pollen), antigens are transported to the lymph. Lymph is carried through the lymph vessels to regional lymph nodes. In the lymph nodes, the macrophages and dendritic cells phagocytose the antigens, process them, and present the antigens to lymphocytes, which can then start producing antibodies or serve as memory cells. The function of memory cells is to recognize specific antigens in the future. The function of the lymphatic system can therefore be summarized as transport and defense. It is important for returning the fluid and proteins that have escaped from the blood capillaries to the blood system and is also responsible for picking up the products of fat digestion in the small intestine. Its other essential function is as part of the immune system, defending the body against infection.

While the lymph nodes do battle infections, there are problems with lymph nodes and the lymphatic system. During infection of the body the lymph nodes often become swollen and tender because of their increased activity. This is what causes the swollen ‘glands’ in your neck during throat infections, mumps and tonsillitis. Sometimes the bacteria multiply in the lymph node and cause inflammation. Cancer cells may also be carried to the lymph nodes and then transported to other parts of the body where they may multiply to form a secondary growth or metastasis. The lymphatic system may therefore contribute to the spread of cancer. Inactivity of the muscles surrounding the lymphatic vessels or blockage of these vessels causes tissue fluid to ‘back up’ in the tissues resulting in swelling or edema.