Maintenance of the Body
from the primitive inferior vena cava. Te lymphatics of the pelvic
region and lower extremities form from sacs on the iliac veins.
Except for the thymus, which is an endodermal derivative,
the lymphoid organs develop from mesodermal mesenchymal
cells that migrate to particular body sites and develop into re-
ticular tissue. Te thymus, the ﬁrst lymphoid organ to appear,
forms as an outgrowth of the lining of the primitive pharynx.
It then detaches and migrates caudally to the thorax where it
becomes inﬁltrated with lymphocyte precursors derived from
hematopoietic tissues elsewhere in the embryo’s body.
Except for the spleen and tonsils, the lymphoid organs are
poorly developed before birth. Shortly aFer birth, they become
heavily populated by lymphocytes, and their development par-
allels the maturation of the immune system.
Check Your Understanding
What is MALT? List several components of MALT.
List several functions of the spleen.
Which lymphoid organ develops ﬁrst?
For answers, see Appendix H.
Although the functions of the lymphatic vessels and lym-
phoid organs overlap, each helps maintain body homeostasis in
unique ways, as summarized in
. Te lym-
phatic vessels help maintain blood volume. Te macrophages of
lymphoid organs remove and destroy foreign matter in lymph
and blood. Additionally, lymphoid organs and tissues provide
sites from which the immune system can be mobilized. In
Chapter 21, we continue this story as we examine the inﬂam-
matory and immune responses that allow us to resist a constant
barrage of pathogens.
Te ﬁrst of these, the
jugular lymph sacs
, arise at the junctions
of the internal jugular and subclavian veins and form a branching
system of lymphatic vessels throughout the thorax, upper extremi-
ties, and head. Te two main connections of the jugular lymph sacs
to the venous system are retained and become the right lymphatic
duct and, on the leF, the superior part of the thoracic duct. Cau-
dally the elaborate system of abdominal lymphatics buds largely
Smooth muscle in
the intestinal wall
Follicles of a
Peyer’s patch (aggregated lymphoid nodules).
Cross section of wall of the ileum of the small intestine (20
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Lymphatic vessels, lymph nodes, and lymph make up the
lymphatic system. Lymphatic vessels return ﬂuids that have
leaked from the blood vascular system back to the blood.
Lymphoid organs and tissues protect the body by removing
foreign material from the lymph and blood streams, and provide
a site for immune surveillance.
Distribution and Structure of Lymphatic Vessels
Lymphatic vessels form a one-way network—lymphatic
capillaries, collecting vessels, trunks, and ducts—in which ﬂuid
ﬂows only toward the heart. Te right lymphatic duct drains
lymph from the right arm and right side of the upper body; the
thoracic duct receives lymph from the rest of the body. Tese
ducts empty into the blood vascular system at the junction of the
internal jugular and subclavian veins in the neck.
Lymphatic capillaries are exceptionally permeable, admitting
proteins and particulate matter from the interstitial space.
Pathogens and cancer cells may spread through the body via the
Te ﬂow of lymphatic ﬂuid is slow; it is maintained by skeletal
muscle contraction, pressure changes in the thorax, and
contractions of the lymphatic vessels. Valves prevent backﬂow.
Immune System; Topic: Anatomy Review, pp. 7–9.