621
CHAPTER SIXTEEN
Lymphatic System and Immunity
of the abdominal vessels and forced into the thoracic ves-
sels. Once again, the valves of the lymphatic vessels prevent
lymph backfl
ow.
Obstruction of Lymph Movement
The continuous movement of fl uid from interstitial spaces
into blood capillaries and lymphatic capillaries stabilizes the
volume of fluid in these spaces. Conditions that interfere
with lymph movement cause tissue fl
uid to accumulate in
interstitial spaces, producing edema. For example, a surgeon
removing a cancerous breast tumor also usually removes
nearby axillary lymph nodes to prevent associated lymphatic
vessels from transporting cancer cells to other sites (metas-
tasis). Removing the lymphatic tissue can obstruct drainage
from the upper limb, causing edema (see ±
g. 16.6
b
).
Edema can swell the lower limbs of a person with congestive heart
failure to enormous proportions. Elevating the lower limbs relieves
edema somewhat.
PRACTICE
7
What factors promote lymph F
ow?
8
What is the consequence of lymphatic obstruction?
16.5
LYMPH NODES
Lymph nodes (lymph glands) are located along the lymphatic
pathways. They contain many
lymphocytes
and
macrophages
(histiocytes) that ±
ght invading pathogens.
Structure of a Lymph Node
Lymph nodes vary in size and shape but are usually less than
2.5 centimeters long and are somewhat bean-shaped
(f g. 16.9)
.
Figure 16.10
illustrates a section of a typical lymph node.
Blood vessels and nerves join a lymph node through the
indented region of the node, called the
hilum.
The lymphatic
vessels leading to a node (afferent vessels) enter separately at
various points on its convex surface, but the lymphatic ves-
sels leaving the node (efferent vessels) exit from the hilum.
A
capsule
of connective tissue with many ±
bers encloses
each lymph node. The capsule extends into the node and
partially subdivides it into compartments. Masses of lym-
phocytes (B cells) and macrophages in the cortex, called
lymph nodules,
(lymph follicles) are the functional units of
the lymph node.
Lymph nodules are found singly or in groups associated
with the mucous membranes of the respiratory and diges-
tive tracts. The
tonsils,
described in chapter 17 (p. 657), are
partially encapsulated lymph nodules. Aggregates of nodules
called
Peyer’s patches
pervade the mucosal lining of the dis-
tal small intestine. Within the Peyer’s patches are scattered
16.4
LYMPH MOVEMENT
The hydrostatic pressure of tissue fl uid drives lymph into
lymphatic capillaries. However, muscular activity largely
infl
uences the movement of lymph through the lymphatic
vessels.
Lymph Flow
Lymph, like venous blood, is under relatively low hydrostatic
pressure. It may not fl
ow readily through the lymphatic ves-
sels without help from contracting skeletal muscles in the
limbs, pressure changes from the action of skeletal muscles
used in breathing, and contraction of smooth muscles in the
walls of the larger lymphatic trunks. Lymph fl ow peaks dur-
ing physical exercise, due to the actions of skeletal muscles
and pressure changes associated with breathing.
Contracting skeletal muscles compress lymphatic ves-
sels. This squeezing action moves the lymph inside a vessel,
but because the lymphatic vessels have valves that prevent
backfl
ow, the lymph can move only toward a collecting duct.
Additionally, the smooth muscles in the walls of the larger
lymphatic trunks may contract and compress the lymph
inside, forcing the fl
uid onward.
Breathing aids lymph circulation by creating a relatively
low pressure in the thorax during inhalation. At the same
time, the contracting diaphragm increases the pressure in
the abdominal cavity. Consequently, lymph is squeezed out
Filaments
anchored to
connective
tissue
Movement of
tissue fluid
Epithelial
cell
Flow of lymph
FIGURE 16.8
±Tissue±F
uid enters lymphatic capillaries through
F
aplike valves between epithelial cells.
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