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CHP 15

Terms in this set (136)

The lymphatic system (lim-FAT-ik) consists of four elements
a fluid called lymph, lymphatic vessels that transport the lymph,
a number of structures and organs containing lymphocytes
within a filtering tissue (lymphatic tissue), and red bone marrow
The lymphatic system assists in
circulating body
fluids and helps defend the body against disease-causing agents.
Most components of blood plasma filter through
blood capillary
walls to form interstitial fluid.
After interstitial fluid passes
into lymphatic vessels, it is referred to as
lymph (LIMFclear
fluid)
So, interstitial fluid and lymph are very similar; the major
difference between them is
location.
Interstitial fluid is found
between
cells
but lymph is located within
vessels and
lymphatic tissue.
Lymphatic tissue is a specialized form
reticular connective
tissue (see Table 3.4) that contains large numbers of lymphocytes.
lymphocytes are
agranular white
blood cells
Two types of lymphocytes participate in adaptive
immune responses
B cells and T cells (described shortly).
The lymphatic system has three primary functions:
Drains excess interstitial fluid.
Transports dietary lipids.
Carries out immune responses.
Lymphatic vessels drain
excess interstitial fluid from
tissue spaces and return it to the
blood.
Would it be possible for circulation of the blood without the lymphatic vessels draining excessive interstitial fluid for tissue spaces and returning it to the blood?
No
Lymphatic vessels transport the _______ and ______ ________ absorbed by
the gastrointestinal tract, to the blood
Lymphatic vessels transport the
lipids and lipid-soluble vitamins (A, D, E, and K), absorbed by
the gastrointestinal tract, to the blood.
Lymphatic tissue initiates
highly specific responses directed against
particular microbes
or abnormal cells
With the assistance of macrophages, T and
B cells recognize
foreign cells, microbes, toxins, and cancer
cells.
These chemical substances that are recognized as foreign
by the immune system are called
antigens and provoke
an immune response.
B cells make up about ____ percent of the lymphocytes
in the body
B cells make up about 15-30 percent of the lymphocytes
in the body
Most of the B cells differentiate into
plasma cells that protect us against disease by producing
plasma cells that protect us against disease by producing
antibodies, proteins that combine with and destroy specific
foreign substances (antigens).
memory B cells
Some B cells become
long-lived memory B cells, which can mount an even
stronger immune response if the same antigen attacks the
body at a later date.
T cells, which make up _______ percent of the lymphocytes in
the body,
T cells, which make up 70-85 percent of the lymphocytes in
the body,
The four
major types of T cells are
T helper cells, T cyototoxic cells, T
regulatory cells, and T memory cells
T helper cells cooperate
with
B cells to amplify antibody production by plasma cells
Following activation by T helper cells, T cytotoxic cells
destroy target cells on contact by causing them to rupture or by
releasing cytotoxic (cell-killing) substances.
T regulatory cells
(formerly called T suppressor cells) can
turn off the immune response
by suppressing T cells
Why is turning off the immune response important sometimes?
this is important in combating an
autoimmune disease (a disease caused by a reaction against the
body's own cells).
T regulatory cells also protect beneficial
intestinal
bacteria, which aid digestion and produce some B vitamins
and vitamin K.
T memory cells
"remember" an antigen
and mount a more vigorous response if the same antigen attacks
the body in the future.
person who is overly reactive to an antigen that is tolerated by
most other people is said to be
allergic (hypersensitive
The antigens
that induce an allergic reaction are called
allergen
Type I (anaphylactic) reactions
are the most
common type of allergic reaction and occur within a few minutes
after a person previously exposed to an allergen is reexposed to it
anaphylactic shock
which may
occur in a susceptible individual who has just received a triggering
drug or been stung by a wasp, wheezing and shortness of breath as
airways constrict are usually accompanied by shock due to vasodilation
and fluid loss from blood. This life-threatening emergency is usually
treated by injecting epinephrine to dilate the airways and
strengthen the heartbeat.
Lymphatic vessels begin as
lymphatic capillaries
These capillaries
are closed at
one end and located in the spaces between cells
lymphatic capillaries unite to form
larger
lymphatic vessels which resemble small veins
in structure but have thinner walls and more valves
At intervals
along the lymphatic vessels, lymph flows through
lymph nodes, encapsulated masses of B cells and T cells.
In the skin, lymphatic
vessels lie in the
subcutaneous tissue and generally follow the
same route as veins
lymphatic vessels of the viscera generally follow
arteries, forming plexuses (networks) around them
Tissues
that lack lymphatic capillaries include
avascular tissues (cartilage, the epidermis, and the cornea of the eye), the central
nervous system, portions of the spleen, and red bone marrow.
Lymphatic capillaries are found throughout the body except in
avascular tissues, the central nervous system, portions of the
spleen, and red bone marrow.
Lymphatic capillaries have greater permeability than
blood capillaries
and thus can absorb large molecules such as proteins and
lipids.
Lymphatic capillaries are also slightly larger in
diameter
than blood capillaries and have a unique structure that permits
interstitial fluid to flow into them but not out
unique structure that permits
interstitial fluid to flow into them
but not out
The ends of
endothelial cells that make up the wall of a lymphatic capillary
overlap
When pressure is greater in the interstitial
fluid than in lymph, the
cells separate slightly, like the opening
of a one-way swinging door, and interstitial fluid enters the
lymphatic capillary.
When pressure is greater inside the lymphatic
capillary,
the cells adhere more closely, and lymph cannot
escape back into interstitial fluid
Attached to the lymphatic
capillaries are
anchoring filaments, which contain elastic fibers.
The anchoring filaments extend out from the lymphatic capillary,
attaching
(anchoring) lymphatic endothelial cells to surrounding
tissues.
When excess interstitial fluid accumulates and causes
tissue swelling,
the anchoring filaments are pulled, making the
openings between cells even larger so that more fluid can flow
into the lymphatic capillaries.
In the small intestine, specialized lymphatic capillaries called _________ carry dietary lipids into lymphatic
vessels and ultimately into the blood
In the small intestine, specialized lymphatic capillaries called
lacteals (LAK-te- -als; lact-milky) carry dietary lipids into lymphatic
vessels and ultimately into the blood
The presence of these lipids causes the lymph draining from the
small intestine to appear creamy white; such lymph is referred to
as
chyle
Elsewhere in the body, lymph is a
clear,
pale-yellow fluid.
Lymph passes from lymphatic capillaries into
lymphatic vessels and
then through lymph nodes.
As lymphatic vessels exit lymph nodes
in a particular region of the body, they unite to form
lymph trunks
The principal lymph trunks are the
lumbar, intestinal, bronchomediastinal,
subclavian, and jugular trunks
The lumbar
trunks drain lymph from the
free lower limbs, the wall and viscera
of the pelvis, the kidneys, the adrenal glands, and the abdominal
wall.
The intestinal trunk drains lymph from the
The intestinal trunk drains lymph from the stomach, intestines,
pancreas, spleen, and part of the liver
The bronchomediastinal
trunks (brong-ko--me--de--as-TI
-
-nal) drain lymph from the
The bronchomediastinal
trunks (brong-ko--me--de--as-TI
-
-nal) drain lymph from the
thoracic wall, lung, and heart.
The subclavian trunks drain
the free
upper limbs.
The jugular trunks drain
the head and neck
Lymph passes from lymph trunks into two main channels
the
thoracic duct and the right lymphatic duct and then drains into venous blood
All lymph returns to the bloodstream through the
thoracic (left) lymphatic duct and right lymphatic duct.
The thoracic (left lymphatic) duct is about
38-45 cm (15-18 in.) long and begins as a dilation called the
cisterna chyli
The thoracic duct is the
main duct for the return of
lymph to blood
The cisterna chyli
receives lymph from the
right and left lumbar trunks and from
the intestinal trunk.
In the neck, the thoracic duct also receives
lymph from the
left jugular, left subclavian, and left bronchomediastinal
trunks
Therefore, the thoracic duct receives lymph
from the
left side of the head, neck, and chest, the left free upper
limb, and the entire body inferior to the ribs
The thoracic duct in turn drains lymph into
venous blood at the
junction of the left internal jugular and left subclavian veins.
Most components of blood plasma, such as nutrients, gases, and
hormones, filter freely through the capillary walls to form
interstitial
fluid.
More fluid filters out of
blood capillaries, however,
than returns to them by reabsorption
The excess filtered fluid—
about 3 liters per day—drains into
lymphatic vessels and becomes
lymph
Because most blood plasma proteins are too large to leave
blood vessels, interstitial fluid contains only a small amount of
protein.
Proteins that do leave blood plasma cannot return to the
blood directly by diffusion because
the concentration gradient
(high level of proteins inside blood capillaries, low level outside)
opposes such movement.
The proteins can, however, move readily
through the more permeable
lymphatic capillaries into lymph
Thus, an important function of lymphatic vessels is to return
lost
blood plasma proteins and plasma to the bloodstream.
Without
this return of lymph (plasma lost from the blood) to the blood
the blood volume would drop precipitously and the cardiovascular system would cease to function. Therefore, the lymphatic vessels
are a key part of the cardiovascular pathways in the body.
If filtration greatly exceeds reabsorption, the result is
edema
(swelling), an abnormal increase in interstitial fluid volume
Edema can result from either
excess filtration
or inadequate reabsorption
Like veins, lymphatic vessels contain
valves, which ensure the
one-way movement of lymph
lymph drains
into venous blood through the
right lymphatic duct and the thoracic
duct at the junction of the internal jugular and subclavian
veins
Thus, the sequence of fluid flow is
1. blood capillaries (blood)
2. interstitial spaces (interstitial fluid)
3. lymphatic capillaries (lymph)
4. lymphatic vessels (lymph)
5. lymphatic ducts (lymph)
6. junction of the internal jugular and subclavian veins (blood).
Two "pumps" that aid the return of venous blood to the heart
maintain the flow of lymph
Skeletal muscle pump
Respiratory pump
The "milking action" of skeletal
muscle contractions compresses
lymphatic vessels (as well as
veins) and forces lymph toward the junction of the internal
jugular and subclavian veins
Lymph flow is also maintained by pressure
changes that occur during
inhalation (breathing in)
Lymph flows
from the abdominal region, where the pressure is higher, toward
the thoracic region, where it is lower
When the pressures reverse
during exhalation (breathing out), the valves
in lymphatic vessels
prevent backflow of lymph.
In addition, when a lymphatic vessel
distends
the smooth muscle in its wall contracts, which helps
move lymph from one segment of the vessel to the next
Lymphatic organs and tissues, which are widely distributed
throughout the body, are classified into two groups based on their
functions.
Primary lymphatic organs
secondary lymphatic organs
and tissues
Primary lymphatic organs are the sites where
cells divide and become immunocompetent (im-u--no--KOM-petent),
that is, capable of mounting an immune response
The primary
lymphatic organs are the
red bone marrow (in flat bones
and the epiphyses of some long bones of adults) and the thymus
Pluripotent stem cells in red bone marrow give
rise to mature,
immunocompetent B cells and to pre-T cells (immature
T cells), which migrate to and become immunocompetent
T cells in the thymus.
The secondary lymphatic organs
and tissues are the sites where
most immune responses occur
The secondary lymphatic organs
and tissues include
They include lymph nodes, the spleen, and lymphatic nodules
(follicles) (see Figure 15.1).
The thymus, lymph nodes, and
spleen are considered organs because
each is surrounded by a
connective tissue capsule
lymphatic nodules, in contrast, are not
organs because
they lack such a capsule
The thymus is a
bilobed organ located in the mediastinum between
the sternum and the aorta
An enveloping layer of connective tissue holds
the
two lobes closely together,
but a connective tissue capsule
encloses each lobe
separately.
Extensions of the capsule, called
trabeculae (tra-BEK-u- -le-little beams), penetrate inward and
divide each lobe into lobules
Each thymic lobule consists of a
a deeply staining outer cortex
and a lighter-staining central medulla
The cortex
is composed of large numbers of
T cells and scattered dendritic
cells, epithelial cells, and macrophages
Immature T cells
migrate from red bone marrow to the
cortex of the thymus,
where they proliferate and begin to mature.
Dendritic cells
(den-DRIT-ik; dendr-a tree), so named because they
have long,
branched projections that resemble the dendrites of a neuron,
assist the maturation process
Each of the specialized epithelial
cells in the cortex has several long
processes that surround
and serve as a framework for as many as 50 T cells
These
epithelial cells help
pre-T cells mature into T cells
Additionally,
the epithelial cells produce
thymic hormones that are thought to
aid in the maturation of T cells
Only about___percent of developing
T cells survive in the cortex.
Only about 2 percent of developing
T cells survive in the cortex.
The remaining cells die via
apoptosis (programmed cell death).
Thymic macrophages help
clear out the debris of dead and dying cells
The surviving T cells
enter the
medulla
The medulla consists of
widely scattered, more mature T cells,
epithelial cells, dendritic cells, and macrophages
T cells that leave the thymus via
the blood are carried to
lymph nodes, the spleen, and other lymphatic
tissues where they colonize parts of these organs and tissues.
Because of its high content of lymphoid tissue and a rich blood
supply, the thymus has
a reddish appearance in a living body
With age, however, fatty infiltrations replace the
lymphoid tissue
and the thymus takes on more of the yellowish color of the invading
fat, giving the false impression of reduced size
However, the
actual size of the thymus, defined by its connective tissue capsule
does not change
It is after puberty that
adipose and areolar connective tissues
begin to replace the thymic tissue.
By the time a person
reaches maturity, the functional portion of the gland is
reduced
considerably, and in old age the functional portion may weigh
only 3 g
Before the thymus atrophies, it populates
the
secondary lymphatic organs and tissues with T cells
However,
some T cells continue to proliferate in the thymus throughout an
individual's lifetime
but this number decreases with age.
The bilobed thymus is largest at
puberty and then the functioning portion atrophies with age
Located along lymphatic vessels are about 600 bean-shaped
lymph nodes
They are scattered throughout the body, both
superficially and deep, and usually occur in groups
Large groups of lymph nodes are present near the
mammary glands
and in the axillae and groin.
like the thymus,
are covered by a
capsule of dense connective tissue that extends
into the node
The capsular extensions, called
trabeculae (tra-BEK-u- -le-), divide the node into compartments give support, and provide a route for blood vessels into the interior
of a node.
Route of lymph flow
through a lymph node:
1. Afferent lymphatic vessel
2. Subcapsular sinus
3. Trabecular sinus
4. Medullary sinus
5. Efferent lymphatic vessel
The parenchyma (functional tissue) of a lymph node is divided
into a
superficial cortex and a deep medulla
Within the outer
cortex are
egg-shaped aggregates of B cells called lymphatic
nodules (follicles).
Most lymphatic nodules
in the outer cortex are
secondary lymphatic nodules
which form in response to an antigen (a foreign substance) and
are sites of plasma cell and memory B cell formation.
After B cells
in a primary lymphatic nodule recognize an antigen, the
primary
lymphatic nodule develops into a secondary lymphatic nodule
The center of a secondary lymphatic nodule contains a region of
light-staining cells called a
germinal center
In the germinal center
are
B cells, follicular dendritic cells (a special type of dendritic cell),
and macrophages.
When follicular dendritic cells "present" an
antigen,
B cells proliferate and develop into antibody-producing
plasma cells or memory B cells
Memory B cells persist after an
after an
immune response and remember having encountered a specific antigen.
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