As blood circulates, some of its fluid components push out of the
capillary bed into the surrounding tissue. This material forms lymph,
a special protein-containing tissue fluid that bathes the cells.
Lymphatic vessels reabsorb part of this lymph to return it to the
circulation, thereby maintaining tissue fluid balance. The lymphatics
also engage in absorption of fats and other substances from the
digestive tract. Lymph node structures along the route of the lymphatics
filter out foreign materials and disease-causing agents from the
general circulation. Other lymphatic system structures include the
tonsils, spleen, and thymus.
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hydrostatic pressure: fluid diffusion and reabsorption
Capillary hydrostatic pressure (filtration pressure) forces fluid
out of the blood capillaries. Hydrostatic pressure results from
the heart forcing blood through the narrow arterial part of capillaries.
The fluid contains oxygen and nutrients that move into the surrounding
tissue where they are less concentrated. Similarly, the tissue contains
carbon dioxide and waste products that move into the capillaries
where they are less concentrated. This process of substances moving
from areas of higher concentration to areas of lower concentration
Fluid reabsorption begins in the lymph capillaries that are throughout
the body near blood capillaries. Lymph capillaries are small microscopic
tubes that collect extracellular fluid. The walls of lymph capillaries
comprise loosely joined cells. The overlapping edges of the cells
form mini-valves that allow extracellular fluid to pass into the
capillary and prevent fluid from flowing back into the tissue. Unlike
blood capillaries, lymph capillaries are blind-end tubes that lead
away from the tissue.
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Lymph travels through the lymph capillaries to small lymph vessels.
Like veins, the walls of lymph vessels have smooth muscle that contracts
and propels lymph away from the tissues. Lymph vessels contain valves
that prevent lymph from flowing backward.
The lymph vessels converge into two main collecting ducts: the
shorter right lymphatic duct and the longer thoracic duct. The right
lymphatic duct drains lymph from the right side of the head, neck,
thorax, and right upper extremity into the right subclavian vein.
Lymph from the rest of the body flows into the thoracic duct that
empties into the left subclavian vein. The thoracic duct begins
in the abdomen as an expanded sac called the cisterna chyli. When
lymph empties into the veins, it forms plasma (the liquid part of
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organs: nodes, nodules, spleen, thymus gland, tonsils
The lymphoid organs are the lymph nodes, spleen, thymus, and groups
of lymph nodules in both the oral cavity (tonsils) and small intestine,
and appendix (Peyer's patches). A connective tissue capsule surrounds
the lymph nodes. The nodes have an outer cortex and inner medulla.
Within the medulla is the germinal center that produces lymphocytes.
These infection-fighting white blood cells produce antibodies that
identify and destroy antigens.
Designed like filters, lymph nodes remove antigens (foreign bodies)
from lymph. Each lymph node has several sinuses (inner chambers)
that contain lymphocytes. Lymph nodes also contain macrophages that
help clear the lymph of bacteria, cellular debris, and other foreign
material. Macrophages attack, ingest (engulf), then kill antigens
in a process called phagocytosis. Small extensions of the macrophage
pull the antigen inside.
Lymph nodules are groups of lymphocytes arranged in round clusters.
Many lymph organs contain lymph nodules within their substances.
Unlike lymph nodes, they cannot filter lymph.
The spleen is the largest lymphoid organ. It has two types of tissue:
the red pulp, which contains many red blood cells (erythrocytes)
and macrophages; and the white pulp, which stores lymphocytes. The
macrophages in the red pulp remove foreign substances and damaged
or dead erythrocytes and platelets from the blood. And, the red
pulp stores platelets, which are important for blood clotting. The
lymphocytes within the white pulp are used for the body immune system.
In the thymus gland lymphocytes become specialized. The thymus
plays an important role in lymphocyte specialization and immunity.
The tonsils are paired lymph nodules in the oral cavity. These
patches of lymph tissue produce lymphocytes. The location of each
pair (palatine, pharyngeal, and lingual) determines its name. The
tonsils protect the throat and respiratory system. Sometimes, the
tonsils cannot remove all the invading microorganisms and become
infected. If the infection is severe and chronic, the tonsils may
require tonsillectomy (surgical removal).