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RESPIRATORY ANATOMY: BIOS 255 Anatomy And Physiology III With Lab
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BIOS255 – Anatomy & Physiology IIIProf Haneen Salhieh, M.S.
Week 4: Lymphatic System
The lymphatic system is a multifaceted system consisting of lymphatic vessels, organs, and tissues that
penetrate almost every body tissue. The different organs include the thymus, tonsils, red bone marrow,
spleen, and lymph nodes. These organs are densely populated with lymphocytes that will fight against
pathogens and activate an immune response. Recall that lymphocytes are produced in red bone marrow.
Lipids are hydrophobic substances that are not easily transported in blood and often require being
transported by carrier proteins.
Overview of the Lymphatic System
Before we dive into the specific components, it is essential to be familiar with the primary roles of this
body system. The lymphatic system has three functions:
fluid recovery
immunity
lipid absorption.
Fluid Recovery
Recall that fluid is continually filtered from blood capillaries into tissues. The blood capillaries will reabsorb
about 85% of this interstitial fluid and put it back into venous circulation. The remaining 15% remains in
the tissues, accounting for 2-4 liters of water and 25% plasma proteins. Returning the remaining fluid and
proteins to blood is essential to ensure fluid homeostasis. Vessels of the lymphatic system are
responsible for reabsorbing this excess fluid called lymph. Failure to return lymph could result in edema
or circulatory failure.
Immunity
Along with the excess tissue fluid that is recovered, the lymphatic system will also pick up any chemicals
or foreign cells from tissues. The lymph (recovered fluid) will pass through lymphatic vessels and lymph
nodes containing immune cells like lymphocytes that seek out and destroy pathogens, abnormal cells, or
potentially harmful foreign substances. This ensures that the lymphatic fluid that enters the bloodstream is
free of pathogens that could spread throughout the body,
Lipid Absorption
The lymphatic system absorbs dietary fats and fat-soluble vitamins from the digestive system. The small
intestine contains special lymphatic vessels called lacteals that absorb these nutrients and transport
them to the bloodstream via lymph. Dietary lipids are too large to be transported through blood capillaries
and enter the lacteals.
Lymph
Lymph is a colorless, clear fluid similar to blood plasma but with lower protein concentration. This fluid is
the recovered excess fluid from tissues essential to ensure fluid balance. The composition of this fluid is
different from place to place in the body. Lymph also contains bacteria, viruses, cellular debris, hormones,
and other substances that are absorbed from tissues. This lymph is filtered as it passes through lymph
nodes enroute to the bloodstream.
Lymphatic Vessels
The lymphatic vessels lack a pump, such as the heart, and rely on the stretching of the vessels due to an
increase of lymph to open valves that allow unidirectional flow for the lymph. Much like venous return,
lymphatic flow can be produced by skeletal muscle pumps, and due to their proximity to many arteries,
the pressure within those arteries can drive lymph circulation.
, BIOS255 – Anatomy & Physiology III
Prof Haneen Salhieh, M.S.
The lymphatic vessels are similar to blood vessels except that they are closed at one end and provide a
one-directional flow for fluid, and their cells are not joined by tight junctions. These vessels penetrate
nearly every body tissue except for bone, bone marrow, cartilage, and the cornea.
Lymphatic vessels contain thin endothelial cells overlapping like the shingles of a roof that act as open
and close valves. When pressure in the tissue is high, it pushes the valve-like structure and fluid flows
inward. When pressure is higher in the lymphatic vessel than in the tissue, the valves are closed.
The smallest lymphatic vessels that first collect lymph are known as lymphatic capillaries. As the
lymphatic capillaries converge along their path, they become larger vessels called collecting vessels.
These will empty lymph into the lymph nodes to be filtered. Once lymph leaves the lymph node from the
other side, it will continue traveling down the collecting vessels and often encounter additional lymph
nodes.
Lymphatic capillaries are thin-walled vessels that help to reabsorb fluid found in the interstitial space.
The collecting vessels converge to form lymphatic trunks that drain a major portion of the body. These
are named based on the part of the body they drain. The lymphatic trunks include paired lumbar,
broncho-mediastinal, subclavian, and jugular trunks, and a solitary intestinal trunk. The lumbar and
intestinal trunks form a sac called the cisterna chyli.
The lymphatic trunks then converge to form the largest vessels known as collecting ducts. These
include the right lymphatic dunk and the thoracic duct.
The right lymphatic duct receives lymph from the right arm, right thorax, and right head. This
large vessel will empty the lymph into the bloodstream via the right subclavian vein.
The thoracic duct, which is larger and longer, receives lymph from everywhere else – so the left
thorax, left arm, left head, and all of the body below the diaphragm. This duct empties into the left
subclavian vein.
Let’s recap the route from the tissue fluid back into the bloodstream:
Interstitial fluid → lymphatic capillaries → collecting vessels → lymphatic trunks → collecting ducts →
subclavian veins
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