Anatomy of the Circulatory and Lymphatic Systems

The circulatory and lymphatic systems are networks of vessels and a pump that transport blood and lymph, respectively, throughout the body. When these systems are infected with a microorganism, the network of vessels can facilitate the rapid dissemination of the microorganism to other regions of the body, sometimes with serious results. In this section, we will examine some of the key anatomical features of the circulatory and lymphatic systems, as well as general signs and symptoms of infection.

The Circulatory System

The circulatory (or cardiovascular) system is a closed network of organs and vessels that moves blood around the body. The primary purposes of the circulatory system are to deliver nutrients, immune factors, and oxygen to tissues and to carry away waste products for elimination. The heart is a four-chambered pump that propels the blood throughout the body. Deoxygenated blood enters the right atrium through the superior vena cava and the inferior vena cava after returning from the body. The blood next passes through the tricuspid valve to enter the right ventricle. When the heart contracts, the blood from the right ventricle is pumped through the pulmonary arteries to the lungs. There, the blood is oxygenated at the alveoli and returns to the heart through the pulmonary veins. The oxygenated blood is received at the left atrium and proceeds through the mitral valve to the left ventricle. When the heart contracts, the oxygenated blood is pumped throughout the body via a series of thick-walled vessels called arteries. The first and largest artery is called the aorta. The arteries sequentially branch and decrease in size (and are called arterioles) until they end in a network of smaller vessels called capillaries. The capillary beds are located in the interstitial spaces within tissues and release nutrients, immune factors, and oxygen to those tissues. The capillaries connect to a series of vessels called venules, which increase in size to form the veins. The veins join together into larger vessels as they transfer blood back to the heart. The largest veins, the superior and inferior vena cava, return the blood to the right atrium.

Extended description

(a) Deoxygenated blood returns to the heart from the lower body through the inferior vena cava and from the upper body through the superior vena cava. The pulmonary artery carries blood from the heart to the lungs, and the pulmonary vein carries it back from the lungs to the heart. (b) Inside the heart, blood enters the right atrium from the superior and inferior venae cavae, passes through the tricuspid valve into the right ventricle, and leaves through the pulmonary artery; oxygenated blood returns through the pulmonary vein to the left atrium, passes through the mitral valve into the left ventricle, and leaves through the aorta. (c) The heart wall is drawn in three layers: the pericardium (outer), the myocardium (thick, muscular middle layer), and the endocardium (thin inner lining).

Other organs play important roles in the circulatory system as well. The kidneys filter the blood, removing waste products and eliminating them in the urine. The liver also filters the blood and removes damaged or defective red blood cells. The spleen filters and stores blood, removes damaged red blood cells, and is a reservoir for immune factors. All of these filtering structures serve as sites for entrapment of microorganisms and help maintain an environment free of microorganisms in the blood.

The Lymphatic System

The lymphatic system is also a network of vessels that run throughout the body. However, these vessels do not form a full circulating system and are not pressurized by the heart. Rather, the lymphatic system is an open system with the fluid moving in one direction from the extremities toward two drainage points into veins just above the heart. Lymphatic fluids move more slowly than blood because they are not pressurized. Small lymph capillaries interact with blood capillaries in the interstitial spaces in tissues. Fluids from the tissues enter the lymph capillaries and are drained away. These fluids, termed lymph, also contain large numbers of white blood cells.

Extended description

The whole-body view labels the tonsil (a swelling on a lymph vessel in the mouth), the thymus (a lumpy structure near the heart), lymph nodes (swellings along the lymph vessels throughout the body), and the right lymphatic duct, which empties into a vein in the neck. The lymph-node close-up shows a rounded structure with several vessels attached and a central region labeled masses of lymphocytes and macrophages. The tissue close-up shows tissue cells surrounded by a red blood capillary network and a green lymphatic capillary network; arrows show interstitial fluid from around the cells entering the lymphatic capillaries and moving into the larger lymphatic vessels.

The lymphatic system contains two types of lymphoid tissues. The primary lymphoid tissue includes bone marrow and the thymus. Bone marrow contains the hematopoietic stem cells (HSC) that differentiate and mature into the various types of blood cells and lymphocytes (see the hematopoiesis diagram in Cellular Defenses). The secondary lymphoid tissues include the spleen, lymph nodes, and several areas of diffuse lymphoid tissues underlying epithelial membranes. The spleen, an encapsulated structure, filters blood and captures pathogens and antigens that pass into it. The spleen contains specialized macrophages and dendritic cells that are crucial for antigen presentation, a mechanism critical for activation of T lymphocytes and B lymphocytes (see Major Histocompatibility Complexes and Antigen-Presenting Cells). Lymph nodes are bean-shaped organs situated throughout the body. These structures contain areas called germinal centers that are rich in B and T lymphocytes. The lymph nodes also contain macrophages and dendritic cells for antigen presentation. Lymph from nearby tissues enters the lymph node through afferent lymphatic vessels and encounters these lymphocytes as it passes through; the lymph exits the lymph node through the efferent lymphatic vessels.

Extended description

(a) The spleen sits at the end of the pancreas just under the diaphragm; the hilum, where the spleen is closest to the pancreas, is where the splenic artery and vein enter and leave. (b) In the close-up cross-section of a lymph node, afferent lymphatic vessels bring lymph into the node and efferent lymphatic vessels carry it out; a connective-tissue capsule surrounds the node, with the subcapsular sinus just beneath it; the cortex forms the outer layer, trabeculae are cortex regions that fold inward toward the center, and germinal centers appear as red structures within the cortex.

Check Your Understanding

Infections of the Circulatory System

Under normal circumstances, the circulatory system and the blood should be sterile; the circulatory system has no normal microbiota. Because the system is closed, there are no easy portals of entry into the circulatory system for microbes. Those that are able to breach the body’s physical barriers and enter the bloodstream encounter a host of circulating immune defenses, such as antibodies, complement proteins, phagocytes, and other immune cells. Microbes often gain access to the circulatory system through a break in the skin (e.g., wounds, needles, intravenous catheters, insect bites) or spread to the circulatory system from infections in other body sites. For example, microorganisms causing pneumonia or renal infection may enter the local circulation of the lung or kidney and spread from there throughout the circulatory network.

If microbes in the bloodstream are not quickly eliminated, they can spread rapidly throughout the body, leading to serious, even life-threatening infections. Various terms are used to describe conditions involving microbes in the circulatory system. The term bacteremia refers to bacteria in the blood. If bacteria are reproducing in the blood as they spread, this condition is called septicemia. The presence of viruses in the blood is called viremia. Microbial toxins can also be spread through the circulatory system, causing a condition termed toxemia.

Microbes and microbial toxins in the blood can trigger an inflammatory response so severe that the inflammation damages host tissues and organs more than the infection itself. This counterproductive immune response is called systemic inflammatory response syndrome (SIRS), and it can lead to the life-threatening condition known as sepsis. Sepsis is characterized by the production of excess cytokines that leads to classic signs of inflammation such as fever, vasodilation, and edema (see Inflammation and Fever). In a patient with sepsis, the inflammatory response becomes dysregulated and disproportionate to the threat of infection. Critical organs such as the heart, lungs, liver, and kidneys become dysfunctional, resulting in increased heart and respiratory rates, and disorientation. If not treated promptly and effectively, patients with sepsis can go into shock and die.

Certain infections can cause inflammation in the heart and blood vessels. Inflammation of the endocardium, the inner lining of the heart, is called endocarditis and can result in damage to the heart valves severe enough to require surgical replacement. Inflammation of the pericardium, the sac surrounding the heart, is called pericarditis. The term myocarditis refers to the inflammation of the heart’s muscle tissue. Pericarditis and myocarditis can cause fluid to accumulate around the heart, resulting in congestive heart failure. Inflammation of blood vessels is called vasculitis. Although somewhat rare, vasculitis can cause blood vessels to become damaged and rupture; as blood is released, small red or purple spots called petechiae appear on the skin. If the damage of tissues or blood vessels is severe, it can result in reduced blood flow to the surrounding tissues. This condition is called ischemia, and it can be very serious. In severe cases, the affected tissues can die and become necrotic; these situations may require surgical debridement or amputation.

Check Your Understanding

Infections of the Lymphatic System

Like the circulatory system, the lymphatic system does not have a normal microbiota, and the large numbers of immune cells typically eliminate transient microbes before they can establish an infection. Only microbes with an array of virulence factors are able to overcome these defenses and establish infection in the lymphatic system. However, when a localized infection begins to spread, the lymphatic system is often the first place the invading microbes can be detected.

Infections in the lymphatic system also trigger an inflammatory response. Inflammation of lymphatic vessels, called lymphangitis, can produce visible red streaks under the skin. Inflammation in the lymph nodes can cause them to swell. A swollen lymph node is referred to as a bubo, and the condition is referred to as lymphadenitis.

Summary

• The circulatory system moves blood throughout the body and has no normal microbiota.
• The lymphatic system moves fluids from the interstitial spaces of tissues toward the circulatory system and filters the lymph. It also has no normal microbiota.
• The circulatory and lymphatic systems are home to many components of the host immune defenses.
• Infections of the circulatory system may occur after a break in the skin barrier or they may enter the bloodstream at the site of a localized infection. Pathogens or toxins in the bloodstream can spread rapidly throughout the body and can provoke systemic and sometimes fatal inflammatory responses such as SIRS, sepsis, and endocarditis.
• Infections of the lymphatic system can cause lymphangitis and lymphadenitis.

Key terms

• artery — large, thick-walled vessel that carries blood from the heart to the body tissues.
• capillary — small blood vessel found in the interstitial space of tissue; delivers nutrients and oxygen, and removes waste products.
• vein — blood vessel that returns blood from the tissues to the heart for recirculation.
• primary lymphoid tissue — one of two types of lymphatic tissue; comprises bone marrow and the thymus.
• secondary lymphoid tissue — one of two types of lymphatic tissue; comprises the spleen, lymph nodes, Peyer’s patches, and mucosa associated lymphoid tissue (MALT).
• spleen — abdominal organ consisting of secondary lymphoid tissue that filters blood and captures pathogens and antigens that pass into it; also contains specialized macrophages and dendritic cells that are crucial for antigen presentation.
• lymph nodes — bean-shaped organs situated throughout the body that contain areas called germinal centers, which are rich in B and T lymphocytes; also contain macrophages and dendritic cells for antigen presentation.
• bacteremia — condition marked by the presence of bacteria in the blood.
• septicemia — condition in which pathogens are multiplying in blood.
• viremia — presence of virus in blood.
• toxemia — presence of toxins in the blood.
• systemic inflammatory response syndrome (SIRS) — severe inflammatory response to the presence of microbes in the blood; can lead to sepsis.
• sepsis — systemic inflammatory response to an infection that results in high fever and edema, causing organ damage and possibly leading to shock and death.
• endocarditis — inflammation of the endocardium, especially the heart valves.
• pericarditis — inflammation of the sac that surrounds the heart.
• myocarditis — inflammation of the heart muscle tissues.
• vasculitis — inflammation affecting blood vessels (either arteries or veins).
• petechiae — small red or purple spots on the skin that result from blood leaking out of damaged vessels.
• ischemia — condition marked by the inadequate flow of blood to the tissues.
• lymphangitis — inflammation of the lymphatic vessels.
• bubo — swollen, inflamed lymph node that forms as a result of a microbial infection.
• lymphadenitis — inflammation of the lymph nodes.

Practice

Describe the major anatomical features of the circulatory and lymphatic systems

Explain why the circulatory and lymphatic systems lack normal microbiota

Explain how microorganisms overcome defenses of the circulatory and lymphatic systems to cause infection

Describe general signs and symptoms of disease associated with infections of the circulatory and lymphatic systems

This section is adapted from Microbiology, Section 25.1: Anatomy of the Circulatory and Lymphatic Systems by Nina Parker, Mark Schneegurt, Anh-Hue Thi Tu, Philip Lister, Brian M. Forster, and OpenStax, © OpenStax, licensed under CC BY-NC-SA 4.0. Access the original for free at openstax.org. Changes: the source Fill in the Blank keyed “the subclavian veins” is rendered as a multiplechoice over vessel and organ names the module prints, because the module’s own prose never states the key (it says only “veins just above the heart”) and a blind solver cannot recover a word the page never prints; all five vendored source figures re-encoded as WebP and rendered as mediafigures after image inspection, all kind="diagram" except the Lymph_img clinical photograph (kind="photo"); the CircSyst figure carries eager="true" as the page’s first figure; the CircSyst, LymphSyst, and spleenLnode alts are rewritten under the 600-character cap with a longdesc walk-through added for each (the source alts ran 630–860 characters); the spleenLnode figure’s panel (b) alt and longdesc are further corrected to describe a lymph node rather than “the spleen” — the source CNXML’s own alt for this panel misnames it as a spleen close-up even though the source’s own caption correctly reads “Lymph nodes,” and the artwork itself (afferent/efferent lymphatic vessels, germinal centers) is lymph-node anatomy, not splenic; this source alt/caption mismatch is reported as a source defect; the CapBed alt is rewritten from the image rather than reused, since the source alt duplicates its own caption’s first two sentences almost verbatim; the Clinical Focus box’s “Barbara’s was also running a fever” is corrected to “Barbara was also running a fever” (a one-word typo, reported as a source defect) and its closing “Jump to the next Clinical Focus box” link is replaced with a sentence naming where the case continues, Bacterial Infections of the Circulatory and Lymphatic Systems; its closing questions stay inside the callout as unanswered plain bullets; the Link to Learning box’s “this website” anchor text is replaced with a descriptive phrase naming the destination, its own URL kept; the three cross-references to other modules ([Cellular Defenses], [Major Histocompatibility Complexes and Antigen-Presenting Cells], [Inflammation and Fever]) are rendered as links to their authored pages; the source Multiple Choice item “Which of the following is where are most microbes filtered out of the fluids that accumulate in the body tissues?” is corrected to “Which of the following is where most microbes are filtered out…” (a one-word-order typo, reported as a source defect), its options, order, and key otherwise unchanged; the other three source Multiple Choice items and both Fill in the Blank items keep their source options, order, and keys unchanged, with “the subclavian veins” reworded so the stem carries the article and the textin key is “subclavian veins”; the section’s one unkeyed Short Answer question and one of its two unkeyed Critical Thinking questions (bacteremia vs. septicemia) are each a selfcheck whose model answer is assembled from this module’s own sentences, since the module does not state a single fixing sentence for either and, for the bacteremia/septicemia question, states no comparative severity judgment at all; the exercise image OSC_Microbio_25_01_Lymph_img (the other unkeyed Critical Thinking item, which carries a real <figure> with a credit-only caption and no source key) is rendered as a mediafigure with an author-written descriptive caption immediately followed by one selfcheck carrying the source’s two-part question verbatim, its model answer keying the term “lymphangitis” from one module sentence and describing the likely cause only as far as the module’s own general statement that lymphatic infections trigger an inflammatory response, naming no organism; a separate textin recall for “lymphangitis,” built from the Key terms definition rather than the clinical sentence the selfcheck’s model answer uses, is added under the fourth objective; of this section’s three body Check Your Understanding bullets (two boxes), one — “What is the main function of the lymphatic system?” — is graded as a multiplechoice, keyed verbatim by the Link to Learning box’s single fixing sentence (“filters fluids that have accumulated in tissues before they are returned to the blood,” not the earlier draft’s “drains,” which was a composite with the LymphSyst caption), with distractors drawn from the module’s own sibling facts about pulmonary circulation, the heart, and the kidneys — the pulmonary-vein distractor replaces an earlier draft’s spleen-filtration distractor, since the module classifies the spleen as lymphatic tissue and a learner could otherwise defend it as a lymphatic-system function too; the other two — on why the circulatory system lacks normal microbiota, and why microbes in it can have serious consequences — remain self-checks, since each needs more than one module sentence assembled; no source exercise or Check Your Understanding bullet is omitted; two textin fillers (“primary lymphoid tissue,” “spleen”) and two more (“sepsis,” “toxemia”) are added under the first and third objectives respectively, each built from a single Key terms definition of this module, to bring every objective group to the book’s three-item floor with at least one auto-graded item apiece; key terms are compiled from the module’s 22 <term> elements (no repeats) and the book’s Glossary appendix, all 22 taken directly from the appendix with no sentence-derived definitions needed; “secondary lymphoid tissue” is rendered as the single bold run “secondary lymphoid tissues,” matching how the module prints it in the body; three Practice fillers (“primary lymphoid tissue,” “spleen,” and “lymphangitis”) are sequenced before, rather than after, the source-keyed multiple choice items whose printed distractor lists include that same word (“veins,” “spleen,” and “lymphangitis” respectively), so no filler’s key is printed as a wrong option in an item the learner has not yet answered.