Chapter 22 Lymphatic System
I. INTRODUCTION
A. The ability to ward off the pathogens that produce disease is called resistance
or (immunity).
B. Lack of resistance is called susceptibility.
C. Resistance to disease can be grouped into two broad areas. – Innate and
Adaptive
1. Nonspecific resistance or innate immunity are present at birth and
includes defense mechanisms that provide general protection against
invasion by a wide range of pathogens.
2. Adaptive (specific) immunity refers to specific recognition of a
microbe once it has breached the innate defenses.
Immunity involves activation of specific lymphocytes that combat a particular pathogen
or other foreign substance.
D. The body system that carries out immune responses is the lymphatic system.
II. LYMPHATIC SYSTEM STRUCTURE AND FUNCTION
A. The lymphatic system consists of a clear fluid called lymph flowing within
lymphatic vessels, several structures and organs that contain lymphatic tissue
(specialized reticular tissue containing large numbers of lymphocytes), and
bone marrow, which is the site of lymphocyte production (Figure 22.1).
Lymphocytes – agranular WBCs T and B cells
Interstitial Fluid – fluid that bathes body cells
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1. Interstitial fluid and lymph are basically the same.
2. Their major difference is location. Interstitial fluid – between cells,
Lymph – within lymphatic vessels and tissues.
B. The lymphatic system functions to:
a.
drain interstitial fluid and return leaked plasma proteins to the blood.
b. transport dietary fats vitamins A D E and K.
c. protect against invasion by nonspecific defenses and specific immune
responses.
C. Lymphatic Vessels and Lymph Circulation
1. Lymphatic vessels begin as lymph capillaries located in tissue spaces
between cells (Figure 22.2).
a. Interstitial fluid drains into lymphatic capillaries, thus forming
lymph.
b. Lymph capillaries merge to form larger vessels, called
lymphatic vessels, which convey lymph into and out of
structures called lymph nodes (Figure 22.1).
2. Lymphatic Capillaries
a. Lymphatic capillaries are found throughout the body except in
avascular tissue, the CNS, portions of the spleen, and red bone
marrow.
b. Lymphatic capillaries have a slightly larger diameter than
blood capillaries and have overlapping endothelial cells which
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work as one-way valves for fluid to enter the lymphatic
capillary.
c. Anchoring filaments attach endothelial cells to surround tissue
(Figure 22.2).
d. A lymphatic capillary in the villus of the small intestine is the
lacteal. It functions to transport digested fats from the small
intestine into blood.
3. Lymph Trunk and Ducts
a. The principal lymph trunks, formed from the exiting vessels of
lymph nodes, are the lumbar, intestinal, bronchomediastinal,
subclavian, and jugular trunks (Figure 22.3).
b. The thoracic duct begins as a dilation called the cisterna chyli
(Figure 22.3) and is the main collecting duct of the lymphatic
system.
1) The thoracic duct receives lymph from the left side of
the head, neck, and chest, the left upper extremity, and
the entire body below the ribs.
2) It drains lymph into venous blood via the left
subclavian vein.
c. Right Lymphatic Duct (Figure 22.3)
1) The right lymphatic duct drains lymph from the upper
right side of the body.
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2) It drains lymph into venous blood via the right
subclavian vein.
4. Formation and Flow of Lymph
a. Interstitial fluid drains into lymph capillaries.
b. The passage of lymph is from arteries and blood capillaries
(blood) to interstitial spaces (interstitial fluid) to lymph
capillaries (lymph) to lymphatic vessels to lymph trunks to the
thoracic duct or right lymphatic duct to the subclavian veins
(blood) (Figure 22.4).
1) Lymph flows as a result of the milking action of
skeletal muscle contractions and respiratory
movements.
2) It is also aided by lymphatic vessel valves that prevent
backflow of lymph.
D. Lymphatic Organs and Tissues
1. Introduction
a. The primary lymphatic organs are those in which cells become
immunocompetent (capable of mounting an immune response)
the red bone marrow (B cells) and the thymus gland (T cells).
b. The secondary lymphatic organs are the lymph nodes and
spleen.
c. Included as secondary lymphatic tissues are lymphatic nodules.
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d. Most immune responses occur in secondary lymphatic organs
and tissues.
2. Thymus Gland
a. The thymus gland lies between the sternum and the heart and
functions in immunity as the site of T cell maturation (Figure
22.5).
b. The thymus gland is large in the infant and after puberty is
largely replaced by adipose and areolar connective tissue.
3. Lymph Nodes
a. Lymph nodes are encapsulated oval structures located along
lymphatic vessels (Figures 22.1 and 22.6).
b. They contain T cells, macrophages, follicular dendritic cells,
and B cells.
c. Lymph enters nodes through afferent lymphatic vessels, and
exits through efferent lymphatic vessels.
1) Foreign substances filtered by the lymph nodes are
trapped by nodal reticular fibers.
2) Macrophages then destroy some foreign substances and
cells by phagocytosis. (come from monocytes fixed or
wandering)
3) Lymphocytes bring about the destruction of other cells
by immune responses.
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d. Lymph nodes are the site of proliferation of plasma cells and T
cells.
e. Knowledge of the location of the lymph nodes and the
direction of lymph flow is important in the diagnosis and
prognosis of the spread of cancer by metastasis; many cancer
cells are spread by way of the lymphatic system, producing
clusters of tumor cells where they lodge. (Clinical Connection)
4. Spleen
a. The spleen is the largest mass of lymphatic tissue in the body
and is found in the left hypochondriac region between the
fundus of the stomach and the diaphragm.
b. The spleen consists of white and red pulp (Figure 22.7))
1) The white pulp is lymphatic tissue.
a) Its T lymphocytes directly attack and destroy
antigens in blood, primarily via cytolysis.
b) Its B lymphocytes develop into antibody
producing plasma cells, and the antibodies
inactivate antigens in blood.
c) Macrophages destroy antigens in blood by
phagocytosis.
2) The red pulp consists of venous sinuses filled with
blood and splenic cords consisting of RBCs,
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macrophages, lymphocytes, plasma cells, and
granulocytes.
a) Macrophages remove worn-out or defective
RBCs, WBCs, and platelets by phagocytosis
b) The spleen stores blood platelets in the red pulp.
c) The red pulp is involved in the production of
blood cells during the second trimester of
pregnancy.
3) The spleen is often damaged in abdominal trauma. A
splenectomy may be required to prevent excessive
bleeding (Clinical Connection).
5. Lymphatic Nodules
a. Lymphatic nodules are oval-shaped concentrations of
lymphatic tissue.
1) They are scattered throughout the lamina propria of
mucous membranes lining the GI tract, respiratory
airways, urinary tract, and reproductive tract.
2) This is the mucosa-associated lymphatic tissue (MALT).
b. Peyer’s patches are lymphatic nodules in the ileum of the small
intestine.
c. Tonsils are multiple aggregations of large lymphatic nodules
embedded in a mucous membrane at the junction of the oral
cavity and the pharynx.
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1) They include the pharyngeal (adenoid), palatine, and
lingual tonsils (Figure 23.1).
2) They are situated strategically to protect against
invasion of foreign substances and participate in
immune responses by producing lymphocytes and
antibodies.
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