Chapter 20
The Lymphatic System and Lymphoid Organs and Tissues
753
20
inside
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
lacteals
(lak
9
te-alz)
transports absorbed fat from the small intestine to the bloodstream.
Lacteals are so called because of the milky white lymph that drains
through them (
lact
5
milk). Tis fatty lymph, called
chyle
(“juice”),
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).
Te
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
veins
, while the deep
lymphatic vessels of the trunk and digestive viscera travel with
the deep
arteries
. 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
lymphatic trunks
,
which drain fairly large areas of the body. Te major trunks,
named mostly for the regions from which they drain lymph, are
the paired
lumbar
,
bronchomediastinal
,
subclavian
, and
jug-
ular trunks
, and the single
intestinal trunk
(Figure 20.2b)
.
Left jugular
trunk
Internal
jugular veins
Left subclavian
trunk
Left subclavian
vein
Esophagus
Trachea
Ribs
Left lumbar
trunk
Left broncho-
mediastinal
trunk
Entrance of
thoracic duct
into vein
Thoracic duct
Hemiazygos
vein
Intestinal trunk
Inferior vena cava
Right jugular trunk
Right lymphatic
duct
Right subclavian
trunk
Right subclavian
vein
Right broncho-
mediastinal trunk
Brachiocephalic
veins
Superior
vena cava
Azygos vein
Cisterna chyli
Right lumbar
trunk
(b) Major lymphatic trunks and ducts in relation to veins and surrounding
structures.
Anterior view of thoracic and abdominal wall.
(a) General distribution of collecting lymphatic vessels
and regional lymph nodes.
Cervical
nodes
Entrance of
right lymphatic
duct into vein
Internal
jugular vein
Entrance of
thoracic duct
into vein
Thoracic
duct
Cisterna
chyli
Collecting
lymphatic
vessels
Axillary
nodes
Aorta
Inguinal
nodes
Regional
lymph
nodes:
Drained by the right lymphatic duct
Drained by the thoracic duct
Figure 20.2
The lymphatic system.
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