The Lymphatic System and Lymphoid Organs and Tissues
the lymphatic capillary, it forces the endothelial minivalve
ﬂaps shut, preventing lymph from leaking back out as the pres-
sure moves it along the vessel.
Proteins in the interstitial space are unable to enter blood
capillaries, but they enter lymphatic capillaries easily. In addi-
tion, when tissues become inﬂamed, lymphatic capillaries de-
velop openings that permit uptake of even larger particles such
as cell debris, pathogens (disease-causing microorganisms such
as bacteria and viruses), and cancer cells. Te pathogens can
then use the lymphatics to travel throughout the body. Tis
threat to the body is partly resolved by the fact that the lymph
“detours” through the lymph nodes, where it is cleansed of de-
bris and “examined” by cells of the immune system.
A special set of lymphatic capillaries called
transports absorbed fat from the small intestine to the bloodstream.
Lacteals are so called because of the milky white lymph that drains
through them (
milk). Tis fatty lymph, called
drains from the ﬁngerlike villi of the intestinal mucosa.
Larger Lymphatic Vessels
From the lymphatic capillaries, lymph ﬂows through successively
larger and thicker-walled channels—ﬁrst collecting vessels, then
trunks, and ﬁnally the largest of all, the ducts (Figure 20.1).
collecting lymphatic vessels
have the same three tunics as
veins, but the collecting vessels have thinner walls and more in-
ternal valves, and they anastomose more. In general, lymphatics
in the skin travel along with superﬁcial
, while the deep
lymphatic vessels of the trunk and digestive viscera travel with
. Te exact anatomical distribution of lymphatic
vessels varies greatly between individuals, even more than it
does for veins.
Te largest collecting vessels unite to form
which drain fairly large areas of the body. Te major trunks,
named mostly for the regions from which they drain lymph, are
, and the single
Inferior vena cava
Right jugular trunk
(b) Major lymphatic trunks and ducts in relation to veins and surrounding
Anterior view of thoracic and abdominal wall.
(a) General distribution of collecting lymphatic vessels
and regional lymph nodes.
duct into vein
Drained by the right lymphatic duct
Drained by the thoracic duct
The lymphatic system.