754
UNIT 4
Maintenance of the Body
20
Lymph is eventually delivered to one of two large
ducts
in the
thoracic region. Te
right lymphatic duct
drains lymph from the
right upper limb and the right side of the head and thorax (Fig-
ure 20.2a). Te much larger
thoracic duct
receives lymph from
the rest of the body. It arises anterior to the first two lumbar verte-
brae as an enlarged sac, the
cisterna chyli
(sis-ter
9
nah ki
9
li), that
collects lymph from the two large lumbar trunks that drain the
lower limbs and from the intestinal trunk that drains the digestive
organs. As the thoracic duct runs superiorly, it receives lymphatic
drainage from the le± side of the thorax, le± upper limb, and the
le± side of the head. Each terminal duct empties its lymph into
the venous circulation at the junction of the internal jugular vein
and subclavian vein on its own side of the body (Figure 20.2b).
Homeostatic Imbalance
20.1
Like the larger blood vessels, the larger lymphatics receive their
nutrient blood supply from a branching vasa vasorum. When lym-
phatic vessels are severely inflamed, the related vessels of the vasa
vasorum become congested with blood. As a result, the pathway
of the associated superficial lymphatics becomes visible through
the skin as red lines that are tender to the touch. Tis unpleasant
condition is called
lymphangitis
(lim
0
fan-ji
9
tis;
angi
5
vessel).
Lymph Transport
Te lymphatic system lacks an organ that acts as a pump. Under
normal conditions, lymphatic vessels are low-pressure conduits,
and the same mechanisms that promote venous return in blood
vessels act here as well—the milking action of active skeletal
muscles, pressure changes in the thorax during breathing, and
valves to prevent backflow. Lymphatic vessels are usually bun-
dled together in connective tissue sheaths along with blood ves-
sels, and pulsations of nearby arteries also promote lymph flow.
In addition to these mechanisms, smooth muscle in the walls
of all but the smallest lymphatic vessels contracts rhythmically,
helping to pump the lymph along.
Even so, lymph transport is sporadic and slow. Movement
of adjacent tissues is extremely important in propelling lymph
through the lymphatics. When physical activity or passive move-
ments increase, lymph flows much more rapidly (balancing the
greater rate of fluid loss from the blood in such situations). For
this reason, it is a good idea to immobilize a badly infected body
part to hinder flow of inflammatory material from that region.
Homeostatic Imbalance
20.2
Anything that prevents the normal return of lymph to the
blood—such as when tumors block the lymphatics or lymphatics
are removed during cancer surgery—results in short-term but
severe localized edema (
lymphedema
). Usually, however, the ves-
sels remaining in the area grow and drainage is reestablished.
²o summarize, the lymphatic vessels:
Return excess tissue fluid to the bloodstream
Return leaked proteins to the blood
Carry absorbed fat from the intestine to the blood (through
lacteals)
Check Your Understanding
1.
What is lymph? Where does it come from?
2.
Name two lymphatic ducts and indicate the body regions
usually drained by each.
3.
What is the driving force for lymph movement?
For answers, see Appendix H.
Lymphoid Cells and Tissues
Describe the basic structure and cellular population
of lymphoid tissue. Differentiate between diffuse and
follicular lymphoid tissues.
In order to understand the role of the lymphoid organs in the
body, let’s investigate their components—lymphoid cells and
lymphoid tissues—before considering the organs themselves.
Lymphoid Cells
Te lymphoid cells consist of immune system cells found in
lymphoid tissues together with the supporting cells that form
the “scaffolding” of those tissues.
Lymphocytes
, the main warriors of the immune system, arise
in red bone marrow (along with other formed elements). Tey
then mature into one of the two main varieties of lymphocytes—
T cells
(T lymphocytes)
or
B cells
(B lymphocytes)
—that
protect the body against antigens. (
Antigens
are anything that
provokes an immune response, such as bacteria and their tox-
ins, viruses, mismatched RBCs, or cancer cells.) Activated ² cells
manage the immune response, and some of them directly attack
and destroy infected cells. B cells protect the body by producing
plasma cells
, daughter cells that secrete antibodies into the blood
(or other body fluids). Antibodies mark antigens for destruction
by phagocytes or other means. Chapter 21 explores the roles of
the lymphocytes in immunity.
Macrophages
play a crucial role in body protection and the
immune response by phagocytizing foreign substances and by
helping to activate ² cells. So, too, do the spiny-looking
den-
dritic cells
that capture antigens and bring them back to the
lymph nodes.
Last but not least are the
reticular cells
, fibroblast-like cells
that produce the reticular fiber
stroma
(stro
9
mah), which is the
network that supports the other cell types in lymphoid organs
and tissues
(Figure 20.3)
.
Lymphoid Tissue
Lymphoid tissue
is an important component of the immune
system, mainly because it (1) houses and provides a prolifera-
tion site for lymphocytes and (2) furnishes an ideal surveillance
vantage point for lymphocytes and macrophages.
Lymphoid tissue, largely composed of a type of loose con-
nective tissue called
reticular connective tissue
, dominates all
the lymphoid organs except the thymus. Macrophages live on
the fibers of the reticular connective tissue network. Huge num-
bers of lymphocytes squeeze through the walls of postcapillary
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