1. Tissue fl
uid formation
a. Tissue fl
uid originates from plasma and includes
water and dissolved substances that have passed
through the capillary wall.
b. Tissue fl
uid generally lacks large proteins, but
some smaller proteins are F
ltered out of blood
capillaries into interstitial spaces.
c. As the protein concentration of tissue fl
increases, colloid osmotic pressure increases.
2. Lymph formation
a. Increasing hydrostatic pressure in interstitial
spaces forces some tissue fl
uid into lymphatic
capillaries. This fl
uid becomes lymph.
b. Lymph formation prevents accumulation of excess
tissue fl
uid (edema).
3. Lymph function
a. Lymph returns the smaller protein molecules and
uid to the bloodstream.
b. It transports foreign particles to the lymph nodes.
1. Lymph fl ow
a. Lymph is under low pressure and may not fl ow
readily without external aid.
b. Contraction of skeletal muscles and low pressure
in the thorax created by breathing movements
move lymph.
The lymphatic system is closely associated with the
cardiovascular system. It transports excess fl
uid to the
bloodstream, absorbs fats, and helps defend the body
against disease-causing agents.
1. Lymphatic capillaries
a. Lymphatic capillaries are microscopic, closed-
ended tubes that extend into interstitial spaces.
b. They receive tissue fl
uid through their thin walls.
c. Lacteals are lymphatic capillaries in the villi of the
small intestine.
2. Lymphatic vessels
a. Lymphatic vessels are formed by the merging of
lymphatic capillaries.
b. They have walls similar to veins, but thinner, and
valves that prevent backfl
ow of lymph.
c. Larger lymphatic vessels lead to lymph nodes and
then merge into lymphatic trunks.
3. Lymphatic trunks and collecting ducts
a. Lymphatic trunks drain lymph from large body
b. Trunks lead to two collecting ducts—the thoracic
duct and the right lymphatic duct.
c. Collecting ducts join the subclavian veins.
In a sense, aging of the immune system begins before birth,
when nonself T cells are selected for destruction, via pro-
grammed cell death (apoptosis), in the thymus. The immune
system begins to decline early in life. The thymus reaches its
maximal size in adolescence and then slowly shrinks. By age
seventy, the thymus is one-tenth the size it was at the age of
ten, and the immune system is only 25% as powerful.
The declining strength of the immune response is why
elderly people have a higher risk of developing cancer and
succumb more easily to infections that they easily fought off
at an earlier age, such as infl uenza, tuberculosis, and pneu-
monia. Encephalitis due to infection by the West Nile virus
may cause very minor symptoms in young people, but it can
kill the elderly. HIV infection progresses to AIDS faster in peo-
ple older than forty. AIDS is more difF cult to diagnose in older
people, sometimes because physicians do not initially suspect
the condition, instead attributing the fatigue, confusion, loss
of appetite, and swollen glands to other causes. However,
11% of new cases of AIDS occur in those over age F fty.
Interestingly, numbers of T cells diminish only slightly
with increasing age, and numbers of B cells not at all.
However, activity levels change for both types of lympho-
cytes. T cell function controls production of B cells, so effects
on B cells are secondary. The antibody response to antigens
is slower, and as a result, vaccines that would ordinarily be
effective in one dose may require an extra dose. The propor-
tions of the different antibody classes shift, with IgA and IgG
increasing, and IgM and IgE decreasing. A person may pro-
duce more autoantibodies than at a younger age, increasing
the risk of developing an autoimmune disorder.
Elderly people may not be candidates for certain medi-
cal treatments that suppress immunity, such as cancer che-
motherapy and steroids to treat inflammatory disorders,
because of the declining function of the immune system.
Overall, the immune system enables us to survive in a world
that is home to many microorganisms. Clinical Application
16.1 looks at the devastation of immunity that is AIDS.
When is maximum size of the thymus reached?
Explain the decline in strength of the immune response in elderly
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