Unit 4 Circulation and Body Defense

PART 3: THE LYMPHATIC SYSTEM

INTRODUCTION

The ability to ward off the pathogens that produce disease is called resistance.

Lack of resistance is called susceptibility.

Resistance to disease can be grouped into two broad areas.

o Nonspecific resistance or innate immunity is present at birth and includes defense

mechanisms that provide general nonspecific protection against invasion by a wide range of

pathogens. It represents an “early warning system” designed to prevent microbes from

gaining access into the body and to help eliminate those that do gain access.

o Specific resistance or adaptive immunity involves activation of specific lymphocytes that

combat a particular pathogen or other foreign substance. It is designed to detect and

eliminate any microbes that are not destroyed by innate immune mechanisms.

The body system that carries out immune responses is the lymphatic system.

LYMPHATIC SYSTEM STRUCTURE AND FUNCTION

The lymphatic system consists of a 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).

o Interstitial fluid and lymph are basically the same. Their major difference is location.

The lymphatic system functions to:

1. Drain interstitial fluid

2. Return leaked plasma proteins to the blood

3. Transport dietary fats and lipid-soluble vitamins

4. Protect against invasion by nonspecific defenses and specific immune responses.

ZOOL-1072/HEAL-2073 J. Taylor Red River College 1

Objectives1. List the components and major functions of the lymphatic system2. Describe the organization of lymphatic vessels3. Explain the formation and flow of lymph4. Compare the structure and functions of primary and secondary lymphatic organs and tissues

Unit 4 Circulation and Body Defense

Lymphatic Vessels and Lymph Circulation Lymphatic vessels begin as blind-ended lymph capillaries in tissue spaces between cells.

o Interstitial fluid drains into lymphatic capillaries, thus forming lymph.

o Lymph capillaries merge to form larger vessels, called lymphatic vessels, which convey

lymph into and out of structures called lymph nodes.

Lymphatic Capillaries Lymphatic capillaries are found throughout the body except in avascular tissue, the CNS,

portions of the spleen, and red bone marrow.

Lymphatic capillaries have a slightly larger diameter than blood capillaries and have

overlapping endothelial cells that work as one-way valves for fluid to enter the lymphatic

capillary (Figure 22.2).

Anchoring filaments attach endothelial cells to surround tissue.

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.

Lymph Trunk and Ducts The principal lymph trunks, formed from the exiting vessels of lymph nodes, are the

lumbar, intestinal, bronchomediastinal, subclavian, and jugular trunks (Figure 22.3).

The thoracic duct begins as a dilation called the cisterna chyli (Figure 22.3) and is the

main collecting duct of the lymphatic system.

o 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.

o It drains lymph into venous blood via the left subclavian vein.

The right lymphatic duct is much smaller than the thoracic duct (Figure 22.3).

o The right lymphatic duct drains lymph from the upper right side of the body.

o It drains lymph into venous blood via the right subclavian vein.

Formation and Flow of Lymph Interstitial fluid drains into lymph capillaries.

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).

o Lymph flows as a result of the milking action of skeletal muscle contractions and

respiratory movements.

ZOOL-1072/HEAL-2073 J. Taylor Red River College 2

Unit 4 Circulation and Body Defense

o It is also aided by lymphatic vessel valves that prevent backflow of lymph.

Lymphatic Organs and Tissues The primary lymphatic organs are those in which cells become immunocompetent (capable of

mounting an immune response):

o Red bone marrow (B cells)

o Thymus gland (T cells).

The secondary lymphatic organs are the lymph nodes and spleen. Included as secondary

lymphatic tissues are lymphatic nodules.

o Most immune responses occur in secondary lymphatic organs and tissues.

Thymus Gland The thymus gland lies between the sternum and the heart and functions in immunity as the site

of T cell maturation (Figure 22.5).

The thymus gland is large in the infant and after puberty is largely replaced by adipose and

areolar connective tissue.

Lymph Nodes Lymph nodes are encapsulated oval structures located along lymphatic vessels (Figures 22.1

and 22.6).

They contain T cells, macrophages, follicular dendritic cells, and B cells.

Lymph enters nodes through afferent lymphatic vessels, and exits through efferent lymphatic

vessels.

o Foreign substances filtered by the lymph nodes are trapped by nodal reticular fibers.

o Macrophages then destroy some foreign substances and cells by phagocytosis

o Lymphocytes bring about the destruction of other cells by immune responses.

Lymph nodes are the site of proliferation of plasma cells and T cells.

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.

Spleen 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.

The spleen consists of white and red pulp (Figure 22.7).

ZOOL-1072/HEAL-2073 J. Taylor Red River College 3

Unit 4 Circulation and Body Defense

o The white pulp is lymphatic tissue.

Its T lymphocytes directly attack and destroy antigens in blood, primarily via

cytolysis.

Its B lymphocytes develop into antibody producing plasma cells, and the

antibodies inactivate antigens in blood.

Macrophages destroy antigens in blood by phagocytosis.

o The red pulp consists of venous sinuses filled with blood and splenic cords consisting of

RBCs, macrophages, lymphocytes, plasma cells, and granulocytes.

Macrophages remove worn-out or defective RBCs, WBCs, and platelets by

phagocytosis

The spleen stores blood platelets in the red pulp.

The red pulp is involved in the production of blood cells during the second

trimester of pregnancy.

The spleen is often damaged in abdominal trauma. A splenectomy may be required to prevent

excessive bleeding.

Lymphatic Nodules Lymphatic nodules are oval-shaped concentrations of lymphatic tissue.

o They are scattered throughout the lamina propria of mucous membranes lining the GI

tract, respiratory airways, urinary tract, and reproductive tract. This is the mucosa-

associated lymphatic tissue (MALT).

o Specific aggregations of MALT include:

1. Peyer’s patches - lymphatic nodules in the ileum of the small intestine.

2. Tonsils - multiple aggregations of large lymphatic nodules embedded in a

mucous membrane at the junction of the oral cavity and the pharynx.

They include the pharyngeal (adenoid), palatine, and lingual tonsils

(Figure 23.1).

They are situated strategically to protect against invasion of foreign

substances and participate in immune responses by producing

lymphocytes and antibodies.

ZOOL-1072/HEAL-2073 J. Taylor Red River College 4

Unit 4 Circulation and Body Defense

Components of the Lymphatic System and Lymph Drainage

ZOOL-1072/HEAL-2073 J. Taylor Red River College 5

Unit 4 Circulation and Body Defense

Lymphatic Drainage into Subclavian Veins

ZOOL-1072/HEAL-2073 J. Taylor Red River College 6

Unit 4 Circulation and Body Defense

Some Practice Multiple-Choice Questions

1) Which is NOT a function of the lymphatic and immune system? a) Draining excess interstitial fluidb) Maintaining pH blance in the bodyc) Transporting dietary lipidsd) Carrying out immune responses

2) What is the major difference between lymph and interstitial fluid?a) Composition of electrolytesb) White blood cells are present in lymphc) Locationd) Types of proteins present

3) What is lack of resistance known as?a) Pathogenicb) Innatec) Specificd) Susceptibility

4) What causes lymph from the small intestines to appear white?a) Proteinsb) WBCc) RBCd) Lipids

5) Which organ is not considered a member of the immune system?a) Spleenb) Lymph nodec) Thymusd) Pancreas

6) Which vessel delivers lymph to the left subclavian vein?a) Left axillary veinb) Jugular trunkc) Thoracic ductd) Right lymphatic duct

7) What vessel does lymph from the right foot empty into? a) Left axillary veinb) Jugular trunkc) Thoracic ductd) Right lymphatic duct

8) What is the function of the spleen?a) Removes worn out blood cellsb) Circulates lymph

ZOOL-1072/HEAL-2073 J. Taylor Red River College 7

Unit 4 Circulation and Body Defense

c) Cleanses interstitial fluidd) Cleanses lymph

ZOOL-1072/HEAL-2073 J. Taylor Red River College 8