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Anatomy of the Lymphatic System
The human lymphatic system ( Figure 5.1 ) consists of lymphatic vessels and the lymphoid organs. The lymphatic system is closely associated with the cardiovascular system. The lymphatic vessels take up excess tissue fluid and return it to the bloodstream. Lymphatic capillaries absorb fats and transport them to the bloodstream. Finally, the lymphatic system helps with immunity to defend the body against disease.
Lymphatic Capillaries
The lymph flows one way, from the lymphatic capillary system to the subclavian veins, where it joins the venous circulation to return to the heart. Fluid begins in the interstitial fluid between the cells. Most, but not all, of the fluid is returned to the heart via the veins of the cardiovascular system. Fluid that is not returned through the veins of the cardiovascular system enters lymphatic capillaries and flows into a lymphatic vessel. Lymphatic capillaries are closely connected to the capillaries of the cardiovascular system. The lymph capillaries take up plasma fluid, which, under great pressure, has been forced out of the capillaries of the circulatory system and has not been reabsorbed. This fluid bathes the cells assisting the capillaries in delivering glucose, oxygen, salts, amino acids, and other nutrients. Excess tissue fluid entering the lymphatic capillaries is now called lymph. Lymph flows from the lymphatic capillaries into larger lymphatic vessels until it eventually empties into venous blood of the cardiovascular system.
Lymphatic Vessels
Lymphatic (lymph) vessels extend throughout most sections of the body. Lymph vessels have one-way flow valves similar in structure to the large veins of the cardiovascular system. The valves prevent the backward flow of lymph. The return of the lymph fluid into circulation is solely dependent on the squeezing action of skeletal muscles, squeezing the fluid one way through the lymphatic vessels.
All the lymphatic vessels merge before entering venous circulation at either the thoracic duct (also called the left lymphatic duct ) or the right lymphatic duct , both
located in the shoulder region of the human body ( Figure 5.2 ). Lymph then flows from the thoracic duct into the left subclavian vein (of the cardiovascular system). The right lymphatic duct drains into the right subclavian vein (of the cardiovascular system).
The larger thoracic duct drains fluid from both lower extremities, the abdomen, the left arm, and the left side of both the head and the neck. The right lymphatic duct is smaller and drains fluid from the right arm, the right side of both the head and the neck, and the right thoracic area. See Figure 5.3 to see the areas drained by the thoracic duct and the right lymphatic duct.
Lymphoid Organs
The lymphoid organs including the lymph nodes, the spleen, the thymus gland, and the tonsils. The body has hundreds of lymph nodes which are found at junctions of lymphatic vessels. A lymph node is encapsulated in a fibrous connective tissue with many incoming and fewer outgoing lymphatic vessels ( Figure 5.4 ). Incoming vessels ( afferent vessels ) flow through a network of sinuses that contain cells. The lymph then flows out of the lymph node through the efferent vessel. The interior of the lymph node is divided into open spaces called nodules , containing lymphocytes and macrophages ( Figure 5.4 ). The lymph nodes act as a filtering center, which rid the flowing lymph of infectious organisms and other debris as it passes through a series of sinuses (cortical, subcapsular, and medullary). Lymph nodes tend to be grouped together in regions of the body, particularly the groin and axilla (armpit) regions.
The spleen is in the upper-left abdomen ( Figure 5.5 ). The spleen functions to extract old or defective blood cells and platelets. The spleen also removes debris, foreign matter, bacteria, viruses, and toxins from the blood that flows through it. The spleen is encapsulated in a thin, fragile fibrous connective tissue with an incoming splenic artery and an outgoing splenic vein. The interior of the spleen is divided into open spaces called lobules , containing lymphocytes and macrophages that carry out the functions of the spleen.
Blood enters the spleen via the splenic artery ( Figure 5.6 ). Blood leaves the spleen via the splenic vein , which flows to the hepatic portal vein (also called the hepatic portal system). The hepatic portal system carries blood drained from the veins of the spleen, intestines, stomach, and pancreas to the liver ( Figure 5.7 ). The hepatic portal vein transports blood into the liver where it is detoxified before returning to general circulation.
The thymus gland is located on the anterior surface of the heart. The thymus secretes
membranes and then brought up to the throat to be spit out or swallowed and killed in the stomach acid.
Other non-specific immune responses include fevers and the inflammatory response. The inflammatory response is a localized response in the tissue. Inflammation increases blood flow to the infected region causing swelling. Tissue swelling dilates blood vessels in the affected area to help increase the number of immune cells responding to the infection. The increase of blood also causes redness and pain, helping to bring a conscious awareness of the infection ( Figure 5.8 ).
Within the blood circulate many cytokines , which are not cells but rather are secreted proteins ( Figure 5.8 ). Cytokines help to regulate and signal both the specific and nonspecific immune systems. The two main groups of cytokines include interferons and interleukins. Both types of cytokines are produced by a variety of immune cells, including macrophages, T-cells, B-cells, and fibroblasts.
Interferons inhibit viral replication and assist in activating natural killer cells. Interleukins function as chemical activators, sending signals throughout the body to increase the immune response. There are many subtypes of interleukins that function to activate different types of immune cells. During an infection, certain subtypes of interleukins called pyrogens reset the body’s thermostat in the hypothalamus. The temperature set-point during homeostasis (normal body temperature) is raised to create a fever. Fevers help the body fight infection by interfering with the growth and replication of pathogens. Fevers also cause lysosomes, an organelle inside cells, to break down. The lysosomes release digestive enzymes that lyse (destroys) a cell infected by a virus ( Figure 5.8 ). In addition, fevers can promote the activity of white blood cells. Mild fevers of short duration can assist in recovery.
White Blood Cells
White blood cells ( leukocytes ) are divided into two major categories: granulocytes and agranulocytes. Granulocytes have granules in the cytoplasm while agranulocytes do not contain granules. Granulocytes include neutrophils, eosinophils, and basophils. All the granulocytes are capable of phagocytosis.
Neutrophils are the most abundant white blood cell ( Figure 5.8 , Figure 5.9 ). Neutrophils are responsible for fighting infections, especially those that involve bacteria. Neutrophils use phagocytosis or the ingestion of foreign materials ( Figure 5.10 ). A cell, such as a neutrophil, recognizes a pathogen by its cell surface receptors. The neutrophil then binds to the pathogen and brings it inside the cell forming a vacuole
( Figure 5.10 ). The vacuole fuses with cell lysosomes, which release digestive enzymes to destroy the pathogen. Once the pathogen is destroyed, the contents are released from the cell. Phagocytosis protects the body by engulfing and destroying pathogens. Neutrophils have a short life span and die quickly after the pathogen is ingested. After cell death, neutrophils exit the body as pus (white fluid).
Neutrophils ( Figure 5.11 ) have a multi-lobed nucleus and (when stained) light pink granules in their cytoplasm. In the photos that follow, there is an intense purple color in the nucleus of each cell. To distinguish the cells, look in the cytoplasm for the color of the granules.
Eosinophils ( Figure 5.12 ) respond to allergic reactions and parasitic infections. They are similar in appearance to neutrophils, except that their granules stain a darker pink to red and are less commonly seen. As a granulocyte, eosinophils are also capable of phagocytosis.
Basophils ( Figure 5.13 ) are the rarest of the granulocytes and are also capable of phagocytosis. They are involved in the release of histamines and heparin. Histamines are a vasodilator ,which increase blood flow through the dilation of vessels and capillaries. Heparin is a blood anticoagulant, which helps to prevent the formation of blood clots. Basophils have similar morphology (shape) to neutrophils and eosinophils, but the granules stain dark blue/purple.
There are two types of agranulocytes: lymphocytes and monocytes. Monocytes ( Figure 5.14 ) are large white blood cells with a “U” or kidney bean shaped nucleus. Monocytes can move into the tissue where they are then called macrophages. Monocytes and macrophages are the greatest phagocytes of all the blood cells. These cells are more effective than neutrophils because they live longer and have greater phagocytic ability.
Lymphocytes
T-cells, B-cells, and natural killer cells are three of the major types of lymphocytes. The specific immune response depends on the activity of lymphocytes. Lymphocyte functions include making antibodies, attacking foreign cells, and destroying body cells that have lost normal function. Lymphocytes have a large dark nucleus with little cytoplasm. The main lymphocytes involved in specific immune responses are the T-cells and the B-cells ( Figure 5.15 ).
Graft Rejection
Recall that T-cells contain antigen receptors that bind to specific glycoproteins in cell membranes. Each person has a genetically unique surface cell receptor present on all the cells of the body in the cell membranes. The glycoprotein surface receptors on all the cells of the body are called the major histocompatibility complex (MHC). The MHC enables the immune system to determine what cells are the body’s cells and what cells are foreign. Cells of the body also have a way of displaying through the MHC complex if there is something abnormal within the cell (such as cancer). Killer T-cells (cytotoxic T cells) recognize and destroy invading cells through this self or not-self recognition of the MHC complexes on each cell.
Graft rejection is the rejection of a transplanted organ by an organ donor. Tissue grafts and organ transplants most often originate from another person's body. The MHC on the surface of the graft are recognized by the host body as a foreign pathogen. An identical twin can make donations to his/her sibling without complications. However, most patients need to be placed on immunosuppressant drugs to suppress the immune system. Suppressing the immune system is necessary so that the body does not reject the organ transplant or tissue graft. While taking these immunosuppressant drugs, the patient is more susceptible to disease.
Autoimmune Diseases
When lymphocytes launch an attack against a person's own body, it is called an autoimmune disease. An autoimmune disease occurs when antibodies and T-cells attack the body's own tissues. In rheumatoid arthritis (RA) , T-cells attack the synovial lining inside joints. T-cells produce interleukins, which cause inflammation inside the joints systemically (body-wide). See Figure 5.16 below to view a normal joint and a joint affected by RA.
Multiple sclerosis ( Figure 5.17 ) is another autoimmune disease (discussed in the nervous system module). Antibodies attack and/or prevent the formation of the myelin sheath around nerve cells, resulting in muscular weakness.
Allergies
Sometimes immune responses are inconvenient or too excessive. In allergic reactions , antibodies are produced against mild antigens called allergens. These allergens are typically common environmental factors, such as pollen or dust mites. Many people that display allergies have some type of genetic predisposition to this type of body reaction. Allergic reactions vary from person to person, but generally include red/watery eyes, sneezing, runny nose, and headaches.
Some people have excessive immune responses to an allergen, called anaphylaxis. Anaphylaxis is a severe, life-threatening allergic reaction against a pathogen, insect bite, or drug. In anaphylaxis, antibodies are overproduced by B-cells. These circulating antibodies increase capillary permeability throughout the body, causing symptoms within minutes. Increasing the permeability of the capillaries causes symptoms throughout the body. Some symptoms include swelling in the form of hives (raised red bumps) and narrowed respiratory pathways, causing difficulty breathing ( Figure 5.18 ). In severe cases, called anaphylactic shock , a person’s capillaries become so dilated it causes their blood pressure to drop too low, leading to collapse. A person can die from anaphylactic shock because the heart stops beating or the airway passages completely close, so the person can no longer breathe.
People who have severe allergic reactions should always carry an epinephrine auto injector (EpiPen, Figure 5.19 ). When anaphylaxis begins to occur, the EpiPen is injected into the thigh to release the hormone epinephrine into the bloodstream. Epinephrine helps to offset the symptoms of anaphylaxis, constricting blood vessels and raising blood pressure.
Other Diseases of the Lymphatic System
An accumulation of too much fluid in tissues can cause localized swelling called edema. Lymphedema ( Figure 5.20 ) is a specific form of edema where the lymphatic system is not functioning properly to return fluid back into circulation. A lymph vessel can become blocked, or the lymph nodes can be removed in an area for testing, causing difficulty with lymph drainage. The interstitial fluid slowly accumulates in the limb (arm or leg), and it becomes extremely swollen and distended. Lymphedema can become a serious condition if not treated because the swollen tissues are vulnerable to infection. If left untreated the connective tissues and vessels become permanently stretched and
