HUMAN BIOLOGY

ImmunIty and dIsease 159

What is the lymphatic system?

• The lymphatic system consists of lymph nodes, lymph vessels,
  and other organs and tissues that collect and filter tissue fluid.


• Lymph vessels return tissue fluid lost from capillaries to the
  blood, transport fats, and carry debris and foreign material to
  lymph nodes.


• Lymph nodes, the spleen, and the thymus all function in
  body defense.

takE-homE mEssaGE

lymph The tissue fluid that

moves in lymph vessels.

lymph nodes Organs of

the lymphatic system that

filter lymph; they are located

at intervals along lymph

vessels.

lymph vascular system

Lymph capillaries and other

vessels of the lymphatic

system.

lymphatic system Organs

and tissues that return tis-

sue fluid to the cardiovascu-

lar system and have roles in

defense.

spleen Lymphoid organ

that filters blood and

serves as a reservoir for

lymphocytes.

thymus Lymphoid organ

where T cells multiply and

mature.

Lymphoid organs and lymphatic tissues

are specialized for body defense

Several elements of the lymphatic system operate in body

defenses. These parts include the lymph nodes, the spleen,

and the thymus. They also include the tonsils and patches

of tissue in the small intestine, in the appendix, and in air-

ways leading to the lungs.

As Figure 9.3 shows, the lymph

nodes are located at intervals

along lymph vessels. Before lymph

enters the bloodstream, it trick-

les through at least one of these

nodes. A lymph node has several

chambers where white blood cells

accumulate after they have been

produced in bone marrow. During

an infection, lymph nodes become

battlegrounds where armies of lym-

phocytes form and where foreign

agents are destroyed. Macrophages

in the nodes help clear the lymph

of bacteria and other unwanted

substances (Figure 9.4B).

The spleen is the lymphatic sys-

tem’s largest organ. It filters blood

and also serves as a holding sta-

tion for lymphocytes. The spleen

has inner chambers filled with soft

red and white tissue called “pulp.”

The red pulp is a storage reservoir of red blood cells

and macrophages. (In a developing embryo, the spleen

produces red blood cells.) In the white pulp, masses of

lymphocytes are arrayed close to blood vessels. If an

invader reaches the spleen during an infection, the lym-

phocytes are mobilized to destroy it, just as they are in

lymph nodes.

The thymus is where T cells multiply and become spe-

cialized to combat specific foreign antigens. You will soon

be learning more about how these cells function.

With this background on the parts of the lymphatic sys-

tem, we are ready to look in more detail at bodily defenses,

beginning with those of body surfaces.

the lymph vascular system functions
in drainage, delivery, and disposal

The lymph vascular system consists of lymph capil-
laries and other vessels that collect water and dissolved
substances from tissue fluid and transport them to ducts
of the cardiovascular system. The lymph vascular system
has three functions, which we could call the “three Ds”—
drainage, delivery, and disposal.
To begin with, the system’s vessels are drainage
channels. They collect water and solutes that have leaked
out of the blood in capillary beds (due to fluid pressure
there) and return those substances to the bloodstream.
The system also picks up fats the body has absorbed from
the small intestine and delivers them to the bloodstream.
Finally, lymphatic vessels transport foreign material and
cellular debris from body tissues to the lymph vascular
system’s disposal centers, the lymph nodes.
The lymph vascular system starts at capillary beds
where fluid enters the lymph capillaries (Figure 9.4A).
These capillaries don’t have an obvious entrance. Instead,
water and solutes move into their tips at flaplike “valves.”
These are areas where endothelial cells overlap.
Lymph capillaries merge into larger lymph vessels. Like
veins, these vessels have smooth muscle in their walls and
valves that prevent back flow. They converge into collecting
ducts that drain into veins in the lower neck. This is how
the lymph fluid is returned to circulating blood. Move-
ments of skeletal muscles and of the rib cage (during
breathing) help move fluid through the lymph vessels, just
as they do for veins.

Figure 9.4 Animated! Lymph capillaries collect fluid and
direct it through lymph nodes. A This diagram shows lymph
capillaries at the start of the lymph vascular system. B The
macrophage shown here has intercepted a bacterium (pink)
that can cause tuberculosis.

flaplike “valve”

formed from

overlapping cells

at the tip of a

lymph capillary

blood

capillary bed

lymph

capillary

tissue fluid

A

B

SPL/Science Source

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