Saladin Outline Ch.21 Page 22
Saladin 5e Extended Outline
Chapter 21
The Lymphatic and Immune Systems
I. The Lymphatic System (pp. 816–829)
A. The lymphatic system is composed of a network of vessels that penetrate nearly every tissue, and a collection of tissues and organs that produce immune cells. (p. 816) (Fig. 21.1)
B. The lymphatic system has three functions. (p. 816)
1. Fluid recovery. Fluid continually filters from blood capillaries into tissue spaces, and the lymphatic system reabsorbs any excess and return it to the blood.
a. The blood capillaries reabsorb about 85% of fluid, leaving 15% extra.
b. This amounts to 2 to 4 L of water and ¼ to ½ of the plasma protein; a person would die within hours if this were not returned via the lymph system.
c. Even partial interference with lymphatic drainage can lead to severe edema. (Fig. 21.2)
2. Immunity. The lymphatic system also picks up foreign cells and chemicals from tissues.
a. The fluid passes through lymph nodes where immune cells stand guard.
b. These cells activate a protective immune response.
3. Lipid absorption. In the small intestine, special lymphatic vessels called lacteals absorb dietary lipids not absorbed by blood capillaries.
C. The components of the lymphatic system are lymph, lymphatic vessels, lymphatic tissue, and lymphatic organs. (p. 816)
D. Lymph is a clear, colorless fluid, similar to blood plasma but low in protein, that originates as tissue fluid taken up by the lymphatic vessels. (pp. 818–820)
1. The composition of lymph varies from place to place and at different times.
2. Lymph leaving the lymph nodes contains a large number of lymphocytes and may also contain macrophages, hormones, bacteria, viruses, cellular debris, or even cancer cells.
3. Lymphatic vessels are similar to blood vessels.
a. The vessels begin with microscopic lymphatic capillaries (terminal lymphatics); these are present almost everywhere but are absent from the CNS, cartilage, cornea, bone, and bone marrow.
b. They are closely associated with blood capillaries, but are closed at one end. (Fig. 21.3)
c. A lymphatic capillary consists of a sac of thin endothelial cells that overlap like shingles of a roof.
i. They are tethered to surrounding tissue by protein filaments that prevent the sac from collapsing.
d. Lymphatic endothelial cells are not joined by tight junctions and do not have a continuous basal lamina; thus cells and particles can enter with the fluid.
e. The overlapping edges of the endothelial cells act as valvelike flaps that can open and close in response to tissue pressure.
f. Lymphatic vessels form in the embryo by budding from veins, and the larger ones have a similar histology.
i. A tunica interna has an endothelium and valves. (Fig. 21.4)
ii. A tunica media has elastic fibers and smooth muscle.
iii. A tunica externa is the thin outermost layer.
g. The lymphatic vessels converge and become larger vessels with changing names: lymphatic capillaries à collecting vessels à six lymphatic trunks à two collecting ducts à subclavian veins. (Fig. 21.5)
h. The lymphatic capillaries converge to form collecting vessels.
i. These vessels often travel alongside veins and arteries and share a common connective tissue sheath with them.
i. Collecting vessels empty into lymph nodes at irregular intervals; lymph leaves the nodes through another collecting vessel and may continue through several nodes.
j. Eventually the collecting vessels converge to form larger lymphatic trunks, each of which drains a major portion of the body.
i. The six lymphatic trunks are named by their locations and the parts of the body they drain: jugular, subclavian, bronchomediastinal, intercostal, intestinal, and lumbar.
k. The lymphatic trunks converge to form the two collecting ducts, the largest of the lymphatic vessels. (Fig. 21.6)
i. The right lymphatic duct is formed by the convergence of the right jugular, subclavian, and bronchomediastinal trunks in the right thoracic cavity; it drains the right arm, right side of the thorax, and head, and it empties into the right subclavian vein.
ii. the thoracic duct, on the left, is larger and longer, beginning just below the diaphragm anterior to the vertebral column near L2.
iii. The two lumbar trunks and the intestinal trunk join to form a prominent sac called the cisterna chyli, names for the large amount of chyle (fatty lymph) that it collects after a meal.
iv. The thoracic duct than passes upward through the diaphragm with the aorta and ascend the mediastinum; as it passes through the thorax, it connects with the left bronchomediastinal, left subclavian, and left jugular trunks, then empties into the left subclavian vein.
4. Flow of lymph is governed by forces similar to those for venous return, except that the lymphatic system has no pump like the heart, and lymph flows at an even lower pressure and speed compared to venous blood.
a. The primary mechanism of flow is rhythmic contractions of the lymphatic vessels themselves in response to distention.
b. The valves of lymphatic vessels prevent fluid from flowing backward.
c. Lymph flow is also produced by skeletal muscles squeezing the lymphatic vessels.
d. Because lymphatic vessels are often next to an artery inside a common sheath, arterial pulsation may also contribute to lymph flow.
e. A thoracic (respiratory) pump promotes the flow of lymph from the abdominal to the thoracic cavity as one inhales.
f. At the point where the ducts empty into the subclavian veins, the rapidly flowing bloodstream draws the lymph into it.
g. Physical exercise significantly increases the rate of lymphatic return.
E. Lymphatic cells range from loosely scattered cells in different tracts to compact populations in lymphatic organs; a variety of lymphocytes and other cells have roles in immunity. (pp. 820–822)
1. Natural killer (NK) cells are large lymphocytes that attack and destroy bacteria, transplanted tissue cells, and host cells that are infected with virus or have turned cancerous.
2. T lymphocytes (T cells) mature in the thymus and later depend on thymic hormones; the T stands for thymus-dependent.
3. B lymphocytes (B cells) are lymphocytes that mature in the bone marrow and differentiate into plasma cells that secrete the antibodies of the immune system.
4. Macrophages are very large, avidly phagocytotic cells of the connective tissues; they develop from monocytes that have emigrated from the bloodstream and phagocytize tissue debris, dead neutrophils, bacteria, and other foreign matter. (Fig. 21.7)
a. They also process foreign matter and display antigenic fragments of it on their surface to alert certain T cells of a foreign presence; cells that do this are collectively termed antigen-presenting cells (APCs).
5. Dendritic cells are branched, mobile APCs found in the epidermis, mucous membranes, and lymphatic organs; in the skin they are often called Langerhans cells.
a. They help alert the immune system to pathogens that have breached body surfaces.
b. They engulf foreign matter by receptor-mediated endocytosis rather than phagocytosis and are included in the macrophage system.
6. Reticular cells are branched stationary cells that contribute to the stroma of lymphatic organs and act as APCs in the thymus; they should not be confused with reticular fibers. (Fig. 21.10)
F. Lymphatic (lymphoid) tissues are aggregations of lymphocytes in the connective tissues of mucous membranes and various organs. (p. 822)
1. Diffuse lymphatic tissue has lymphocytes scattered rather than densely clustered and is prevalent in body passages open to the exterior (respiratory, digestive, etc.) where it is called mucosa-associated lymphatic tissue (MALT).
a. In the respiratory and digestive tracts is it sometimes called bronchus-associated (BALT) and gut-associated (GALT) lymphatic tissue.
2. In some places, lymphocytes and macrophages congregate in dense masses called lymphatic nodules (follicles). (Fig. 21.8)
a. These come and go as pathogens invade tissues and are dealt with.
b. Abundant lymphatic nodules are a relatively constant feature of the lymph nodes, tonsils, and appendix.
c. In the ileum, they form clusters called Peyer patches.
G. Lymphatic (lymphoid) organs have well-defined anatomical sites and at least partial connective tissue capsules that separate lymphatic tissue from neighboring tissue. (pp. 822–828)
1. These organs include the red bone marrow, thymus, lymph nodes, tonsils, and spleen.
2. Red bone marrow and thymus are regarded as primary lymphatic organs because they are sites where B and T lymphocytes become immunocompetent.
3. Lymph nodes, tonsils, and spleen are called secondary lymphatic organs because they are populated with immunocompetent lymphocytes only after the cells have matured in the primary organs.
4. Red bone marrow is involved in hemopoiesis (blood formation) and immunity.
a. In children, red bone marrow occupies the medullary spaces of nearly the entire skeleton, whereas in adults, it is limited to parts of the axial skeleton and the proximal heads of the humerus and femur.
b. Red bone marrow is a soft, loosely organized, highly vascular material separated from osseous tissue by the endosteum.
c. It produces all classes of formed elements of the blood, and its red color is from the abundance of erythrocytes.
d. Numerous arteries penetrate the bone via nutrient foramina and empty into large sinusoids in the marrow. (Fig. 21.9)
e. The sinusoids drain into a central longitudinal vein that exits via the same foramina.
f. The sinusoids are lined by endothelial cells, like other blood vessels, and are surrounded by reticular cells and reticular fibers.
i. The reticular cells secrete colony-stimulating factors that induce formation of leukocytes.
ii. In the long bones, aging reticular cells accumulate fat and transform into adipose cells, eventually replacing red bone marrow with yellow bone marrow.
g. The spaces between the sinusoids are occupied by islands (cords) of hemopoietic cells—macrophages and blood cells in all stages of development.
i. The macrophages destroy malformed blood cells and nuclei discarded by developing erythrocytes.
ii. As blood cells mature, they push their way through the reticular and endothelial cells to enter the sinus and the bloodstream.
5. The thymus is a member of the endocrine, lymphatic, and immune systems.
a. It houses developing lymphocytes and secretes hormones that regulate their later activity.
b. It is a bilobed organ located between the sternum and aortic arch in the superior mediastinum; it degenerates with age as described earlier. (Fig. 17.8)
c. The fibrous capsule of the thymus give off trabeculae (septa) that divide the gland into several angular lobules.
i. Each lobule has a dense, dark-staining cortex and a lighter medulla populated by T lymphocytes. (Fig. 21.10)
ii. Reticular epithelial cells seal off the cortex from the medulla and surround blood vessels and lymphocyte clusters, forming the blood–thymus barrier that isolates developing lymphocytes from blood-borne antigens.
d. After developing in the cortex, the T cells migrate to the medulla, where they spend another 3 weeks.
i. There is no blood–thymus barrier in the medulla; mature T cells enter blood or lymphatic vessels and leave the thymus.
ii. In the medulla, the reticular epithelial cells form whorls called thymic (Hassall) corpuscles, useful for identifying the thymus histologically.
e. Reticular epithelial cells also produce signaling molecules that promote development and action of T cells, including thymosin, thymopoietin, thymulin, interleukins, and interferon.
f. If the thymus is removed from newborn mammals, they waste away and never develop immunity.
g. Other lymphatic organs depend on thymosins or T cells and develop poorly in thymectomized animals.
6. Lymph nodes are the most numerous lymphatic organs, numbering about 450 in a typical young adult. (Fig. 21.11)
a. They serve to functions: cleansing the lymph, and acting as a site of T and B cell activation.
b. A lymph node is an elongated or bean-shaped structure, usually less than 3 cm long, often with an indentation called the hilum on one side. (Fig. 21.12)
i. It is enclosed in a fibrous capsule with trabeculae that partially divide the interior into compartments.
ii. Between the capsule and parenchyma is a narrow, relatively clear space called the subcapsular sinus, which contains reticular fibers, macrophages, and dendritic cells.
iii. Deep to this sinus the gland consists mainly of a stroma of reticular connective tissue and a parenchyma of lymphocytes and APCs.
c. The parenchyma is divided into an outer C-shaped cortex that encircles about 4/5 of the organ and an inner medulla that extends to the surface at the hilum.
i. The cortex consists of lymphatic nodules that acquire germinal centers for B cell multiplication when the lymph node is fighting a pathogen.
ii. The medulla consists of a branching network of medullary cords composed of lymphocytes, plasma cells, macrophages, reticular cells, and reticular fibers.
iii. Both regions also contain lymph-filled sinuses continuous with the subcapsular sinus.
d. Afferent lymphatic vessels lead into a node along its convex surface.
i. Lymph flows from these vessels into the subcapsular sinus and percolates slowly through the sinuses of the cortex and medulla.
e. The lymph leaves the node through one to three efferent lymphatic vessels that emerge from the hilum.
f. No other lymphatic organs have afferent lymphatic vessels, and lymph nodes are the only organs that filter lymph.
i. Macrophages and reticular cells of the sinuses remove about 99% of the impurities before lymph leaves the node.
ii. Sequential passage through a number of nodes thoroughly cleanses the lymph before it returns to the blood.
g. Blood vessels also penetrate the hilum of a lymph node.
i. Arteries follow the medullary cords and give rise to capillary beds in the medulla and cortex.
ii. In the deep cortex, lymphocytes can emigrate from the bloodstream into the parenchyma of the node; most are T cells.
h. Lymph nodes are widespread, but especially concentrated in seven locations. (Fig. 21.1)
i. Cervical lymph nodes occur in deep and superficial groups in the neck, monitoring lymph coming from the head and neck.
ii. Axillary lymph nodes are concentrated in the armpit and receive lymph from the upper limb and the female breast. (Fig. 21.6b)
iii. Thoracic lymph nodes occur in the thoracic cavity, especially embedded in the mediastium.
iv. Abdominal lymph nodes occur in the posterior abdominopelvic wall and monitor lymph from the urinary and reproductive system.
v. Intestinal and mesenteric lymph nodes are found in the mesenteries and adjacent to the appendix and intestines. (Fig. 21.11a)