Chapter 22: The Lymphatic System
Organization of the Lymphatic System, p. 764
The lymphatic system
has 4 main parts:
: a fluid similar to plasma, but without the plasma proteins.
): the network that carries lymph from peripheral tissues to the
and lymphoid organs
: found throughout the body.
and other immune system cells.
Functions of the Lymphatic System, p. 764
The main function of the lymphatic system
is to produce, maintain and distribute lymphocytes
Lymphocytes are produced and stored in lymphoid tissues (such as the tonsils), and lymphoid organs
(such as the spleen and thymus), and are also produced in red bone marrow. To function, the
lymphocytes must detect problems and travel to the site of injury or infection.
Lymphocytes circulate in the blood, enter the interstitial fluid through capillaries, and return to the blood
stream through lymphatic vessels. The circulation of fluids from blood plasma to lymph and back to the
venous system also transports other immune system cells (such as macrophages and microphages) as
well as hormones, nutrients and waste products.
Lymphatic Vessels, p. 765
The vessels that carry lymph are lymphatic vessels
. The lymphatic system begins with the smallest
vessels: the lymphatic capillaries
differ from blood capillaries in 4 ways:
They start as pockets rather than tubes.
They have larger diameters.
They have thinner walls.
In section, they are flat or irregular.
The endothelial cells of lymphatic capillaries are not tightly bound together, but they overlap. The
overlap acts as a one-way valve that allows fluids, solutes and larger materials such as viruses, bacteria
and debris to enter the vessel, but prevents them from returning to the intercellular space.
Special lymphatic capillaries (lacteals
) in the small intestine transport lipids from the digestive tract.
From the lymphatic capillaries, lymph flows into a system of larger lymphatic vessels similar to veins,
containing many one-way valves that give lymph vessels a lumpy, beaded appearance. Lymphatic
vessels often travel toward the thoracic cavity in association with veins, but are generally smaller.
From the lymphatic capillaries, the lymphatic system is divided into the superficial lymphatics
. The superficial lymphatics
are located in the skin, mucus membranes, and serous
membranes lining body cavities. The deep lymphatics
are larger vessels that accompany deep arteries
The superficial lymphatics
and deep lymphatics
join to form the large lymphatic trunks
that empty into 2
major collecting vessels: the thoracic duct
and the right lymphatic duct
The base of the thoracic duct