Components of the Lymphatic System
Are Found Throughout the Body
the components and functions of the
the flow of lymph.
the structures and functions of the
primary and secondary lymph organs.
ach day, about 20 liters of fluid filter out of
the blood capillary walls to form
in te rstitia l
f l u i d ,
the fluid that surrounds the cells of body
tissues. Although chemically similar to blood
plasma, interstitial fluid has less protein than blood plas-
ma because most blood protein molecules are too large to
filter through the capillary wall. About 17 L of this inter-
stitial fluid are reabsorbed at the venous ends of the capil-
To maintain a constant blood volume, the remaining 3
L must also reenter the cardiovascular system. That is just
one of the many jobs of your lymphatic system.
The Lymphatic System Has
Three Functions: Drainage,
Transport, and Immunity
The lymphatic system is basically a complex drainage sys-
tem, much like a water sewage system in a city. Like sewage
moving into and through the sewer pipes, interstitial fluid
is carried through the system and eventually empties into
the veins, where it again becomes part of the blood plasma.
The lymphatic system also helps transport lipids and
lipid-soluble vitamins (A, D, E, and K) from the digestive
tract to the blood. These materials do not dissolve well in
the watery plasma, so sending them to the blood via the
lymphatic system helps slow their entry into the blood sup-
ply and allows them to be dispersed more evenly through-
out the body.
Finally, the lymphatic system contains various cells
that participate in immune responses. These cells are
lymphocytes, a type of white blood cell, as discussed in
Chapter 10. Specifically, the lymph system uses T lympho-
cytes (T cells) and B lymphocytes (B cells) in its immune
response to fight foreign cells (such as bacteria, fungi, and
viruses), other foreign substances, and abnormal cells.
The Lymphatic System Consists
of Lymph, Lymphatic Vessels,
and Several Structures and Organs
Like the blood vessels of your cardiovascular system, the
lymphatic system is a type of vascular system for moving
fluid. It consists of lymph, lymphatic vessels, a number of
structures containing lymphatic tissue, and red bone mar-
row. Figure 12.1 is an overview of the system; we will dis-
cuss some of the structures shown in Figure 12.1 in more
detail in the sections that follow.
Lymphatic vessels begin as lymphatic capillaries
(see Figure 12.2). These tiny vessels begin in the tissues
and carry the fluid that forms there. They are closed at
one end and located in the spaces between cells. Lymphat-
ic capillaries are slightly larger than blood capillaries and
are uniquely structured to permit interstitial fluid to flow
into—but not out—of them. The endothelial cells that
make up the wall of a lymphatic capillary are not attached
end to end; rather, the ends overlap, like roof shingles.
Just as blood capillaries unite to form venules and
veins, lymphatic capillaries unite to form larger and larger
lymphatic vessels. Lymphatic vessels resemble veins in
structure but have thinner walls and more valves. Once in-
terstitial fluid enters lymphatic vessels, it is called lymph.
From the lymphatic vessels, lymph eventually passes
into one of two main channels. The thoracic duct is the
main lymph-collecting duct. It receives lymph from the left
side of the head, neck, and chest; the left upper limb; and
the entire body below the ribs. The right lymphatic duct
drains lymph from the upper right side of the body. Even-
tually the lymph again becomes part of the blood plasma.
Lymphatic organs and tissues, which are widely dis-
the body, are classified into two
groups based on their functions: primary lymphatic organs
and secondary lymphatic organs and tissues.
The primary lymphatic organs include the red bone
marrow and the thymus. These primary lymphatic
organs are the sites where stem cells divide and develop
into mature B cells and T cells.
The secondary lymphatic organs and tissues include
the lymph nodes, the spleen, and lymphatic nodules.
Most immune responses occur in these sites. Lymphatic
tissue is a specialized form of reticular connective
tissue that contains large numbers of lymphocytes.
344 CHAPTER 12
The Lymphatic System and Immunity