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Anatomy Study Guide: Chapter 12 Part 1: The Lymphatic System pg.390-395
The blood delivers nutrients and oxygen to the cells by the fluid leaving the capillaries and bringing
everything directly to the cell, in the spaces between cells (interstitial spaces).
The function of the lymphatic system is to return fluids from interstitial spaces in the cells, to the
circulatory system.
The lymphatic system is a one-way system, with lymph flowing only toward the heart.
If it were not for the lymphatic system, some of the fluid volume of the blood would stay behind in the
area of the cells and produce edema, which is a swelling of the tissue due to fluid accumulation. The
circulatory system would also not have enough volume to function.
Lymph is composed mostly of water and small amounts of dissolved proteins.
Lymph capillaries are very porous. The endothelial cells that make up the walls of the lymph capillaries
overlap one another, forming minivalves that allow lymph fluid in, but close when pressure tries to force
fluid out.
Lymphatic collecting vessels, as the larger lymphatic vessels are called, get progressively larger as they
get closer to the heart.
Lymph is finally returned to the venous part of the circulatory system through the right lymphatic duct,
which drains the right arm and the right side of the head, and the thoracic duct which drains the rest of
the body.
The subclavian veins, on each side of the body, receive the lymph from each of these ducts.
The lymphatic system moves lymph similarly to the venous system through adjacent skeletal muscle
contractions, valves, and smooth muscle contractions in the larger vessels.
Lymph nodes vary in size and shape, but are usually less than an inch in size and distributed along the
lymphatic vascular system. They collect foreign material from the lymph and deal with most of it.
Lymph nodes are enclosed in a fibrous capsule within surrounding connective tissue, and branches of
the capsule called trabeculae divide nodes into compartments.
Inside lymph nodes macrophages engulf and destroy bacteria, viruses etc. Lymphocytes are a type of
white blood cell that also deal with foreign substances.
The outer part of the lymph node is the cortex, which has lymphocytes in groupings called follicles. The
centers of many of the follicles enlarge when B cells are generating daughter cells called plasma cells,
which release antibodies.
The inner medulla contains phagocytic macrophages, which “eat” biological agents such as bacteria,
viruses and dead or dying cells.
Other lymphocytes “in transit” called T-cells circulate between lymph, lymph nodes and blood to
perform their function.
Lymph enters the convex side of lymph nodes through afferent vessels and leaves through efferent
vessels on the concave side.
A sinus is a cavity through which lymph flows inside the lymph node. The hilus is the indented region
where lymph leaves the lymph node.
Fewer efferent lymphatic vessels than afferent ones mean that lymph stays in the lymph node longer,
where there is more time for lymphocytes and macrophages to do their work.
When an infection occurs, lymph nodes frequently become swollen due to the large amount of foreign
matter trapped inside for the lymphocytes and phages to deal with.
Lymph nodes are not the only structures which perform this type of function.
Other lymphoid organs include the spleen, thymus gland, tonsils and Peyer’s patches.
The spleen is located next to the stomach and filters blood.
Tonsils are located in the throat and trap and remove bacteria coming in.
Peyer’s patches are much the same as tonsils, but are located in the small intestine.
Peyer’s patches and tonsils are made of mucosa-associated lymphatic tissue (MALT), which guard
against the foreign matter entering these areas.
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The thymus gland is most active during youth and is located in the upper thoracic region. It produces
thymosin and other hormones, which affect the function of lymphocytes.
Figures to study and memorize:
Figure 12.1 Relationship of the lymphatic vessels to the blood vessels of the cardiovascular circuit.
Figure 12.2 Distribution and special structural features of lymphatic.
Figure 12.3 Distribution of lymphatic vessels and lymph nodes.
Figure 12.4 Structure of a lymph node.
Figure 12.5 Lymphoid organs.