Lymphatic System

Lymphatic vessels absorb excess interstitial fluid and transport this fluid—now called lymph—to ducts that drain into veins. Lymph nodes, and lymphoid tissue in the thymus, spleen, and tonsils produce lymphocytes, which are white blood cells involved in immunity.

The lymphatic system has three basic functions: (1) it transports interstitial (tissue) fluid, initially formed as a blood filtrate, back to the blood; (2) it transports absorbed fat from the small intestine to the blood; and (3) its cells—called lymphocytes—help to provide immunological defenses against disease-causing agents.

The smallest vessels of the lymphatic system are the lymphatic capillaries (fig. 13.34). Lymphatic capillaries are microscopic closed-ended tubes that form vast networks in the intercellular spaces within most organs. Because the walls of lymphatic capillaries are composed of endothelial cells with porous junctions, interstitial fluid, proteins, microorganisms, and absorbed fat (in the intestine) can easily enter. Once fluid enters the lymphatic capillaries, it is referred to as lymph.

From merging lymphatic capillaries, the lymph is carried into larger lymphatic vessels called lymph ducts. The walls of

Interstitial Space Blood Lymph
Figure 13.34 The relationship between blood capillaries and lymphatic capillaries. Notice that lymphatic capillaries are blind-ended. They are, however, highly permeable, so that excess fluid and protein within the interstitial space can drain into the lymphatic system.

lymph ducts are similar to those of veins. They have the same three layers and also contain valves to prevent backflow. Fluid movement within these vessels occurs as a result of peristaltic waves of contraction (chapter 12). The smooth muscle within the lymph ducts contains a pacemaker that initiates action potentials associated with the entry of Ca2+, which stimulates contraction. The activity of the pacemaker, and hence the peristaltic waves of contraction, are increased in response to stretch of the vessel. The lymph ducts eventually empty into one of two principal vessels: the thoracic duct or the right lymphatic duct. These ducts drain the lymph into the left and right subclavian veins, respectively. Thus interstitial fluid, which is formed by filtration of plasma out of blood capillaries (a process described in chapter 14), is ultimately returned to the cardiovascular system (fig. 13.35).

Before the lymph is returned to the cardiovascular system, it is filtered through lymph nodes (fig. 13.36). Lymph nodes contain phagocytic cells, which help to remove pathogens, and germinal centers, which are sites of lymphocyte production. The tonsils, thymus, and spleen—together called lymphoid organs— likewise contain germinal centers and are sites of lymphocyte production. Lymphocytes are the cells of the immune system that respond in a specific fashion to antigens, and their functions are described as part of the immune system in chapter 15.

Lymphatic System Human

Figure I 3.35 The relationship between the circulatory and lymphatic systems. This schematic illustrates that the lymphatic system transports fluid from the interstitial space back to the blood through a system of lymphatic vessels. Lymph is eventually returned to the vascular system at the subclavian veins.

Test Yourself Before Your Continue

1. Compare the composition of lymph and blood and describe the relationship between blood capillaries and lymphatic capillaries.

2. Explain how the lymphatic system and the cardiovascular system are related. How do these systems differ?

3. Describe the functions of lymph nodes and lymphoid organs.

Heart and Circulation

Adenoid Tonsil

Cervical lymph nodes Right lymphatic duct

Right subclavian vein

Axillary lymph nodes

Bone marrow

Lymphatics of mammary gland

Cisterna chyli

Thymus

Spleen

Inguinal lymph nodes

Adenoid Tonsil

Cervical

Thymus

Spleen

Right subclavian vein

Axillary lymph nodes

Bone marrow

Lymphatics of mammary gland

Cisterna chyli

Inguinal lymph nodes

Lymphatic System Has Mammary Glands

Lymph node

Figure 13.36 The location of lymph nodes along the lymphatic pathways. Lymph nodes are small bean-shaped bodies, enclosed within dense connective tissue capsules.

Lymph node

Figure 13.36 The location of lymph nodes along the lymphatic pathways. Lymph nodes are small bean-shaped bodies, enclosed within dense connective tissue capsules.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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  • Largo
    Where are human lymph nodes located?
    5 years ago
  • ZARA
    Which bigger lymph capillary and blood capillary?
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    What type of gland is mammary gland?
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  • mira
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  • Ricky Scott
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  • agamennone
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  • FIORETTA
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  • Teegan Gibson
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