Download Major Functions of the Lymphatic System

Major Functions of the Lymphatic System
The lymphatic system is essential for our survival. This system has
three main functions:
1. To collect and recycle the excess interstitial fluid and its dissolved
substances,
2. To absorb fats and other substances from the digestive tract (this
topic will be discussed in the Digestive System Unit), and
3. To initiate and coordinate an immune response to remove cellular
debris, bacteria, toxins, fungi, parasites, and viruses that
accumulate in our bodies.
Because this system has the two very different functions of
maintaining the proper fluid balance in the body and protecting the
body from harmful infections, we will begin its study by 1)
investigating the lympathic vessels and lymph which function in fluid
balance and then 2) investigate how these structures along with
lymphatic cells, tissues and organs function in protecting the body
from infections. Infection can be viewed as the invasion and
multiplication of microorganisms that are not normally present
within the body. An infection may remain at the location where it
entered the body, or it may spread through the body via blood or
lymphatic vessels. Immunity is the state of having sufficient
defenses (resistance) against infections that might disrupt
homeostasis. Immunity involves both non-specific, inherent
components (innate immunity) and specific, acquired from
previous exposure components (adaptive immunity).
It is important to realize that although immunity will be considered
here in the context of human anatomy and physiology, it is not
restricted to humans or animals. The ability to defend itself from
“non-self” invaders appears, as early as in bacteria defending
themselves from viral attacks, and it is an inherent homeostatic
mechanism present in all types of cells, plants, and animals. As
organisms evolved, so did the immune system. Thus, while the innate
system is present in all animals, only vertebrates present the adaptive
response.
Lymph Tissue and Lymphatic Vessels
Like the circulatory system that carries blood throughout the body,
the lymphatic system is made of a series of vessels, capillaries, and
organs. These structures collect excess fluid and cellular debris from
the tissues and return them back to the blood.
In the circulatory system, blood flows from arteries, through
capillaries and into veins to be returned to the heart. On its way
through the capillaries, some of the fluid passes out across the
capillary wall and into the interstitial fluid in a process called
capillary filtration. This filtration tends to occur across the arterial
end of the capillary, with most of the filtered fluid being reabsorbed at
the venous end of the capillary. This leaves a small amount of fluid
that remains in the interstitial spaces between cells. This filtered fluid
is mostly plasma plus any plasma proteins that might have leaked
from the blood vessel as well. This excess interstitial fluid is collected
by the lymphatic system. The fluid flows through the lymphatic
vessels until it is returned to the circulatory system to again become a
component of blood. Once interstitial fluid passes into lymphatic
vessels, it is called lymph. Lymph is a clear, pale-yellow fluid
connective tissue.
The lymphatic system consists of many different tissues and organs
that are found throughout the body. Some organs provide the
environment for the development and maturation of leukocytes.
Other tissues and organs trap pathogen and are the sites where
leukocytes can interact with the pathogen. The circulatory and
lymphatic systems interact to connect these organs and tissues.
Fluid moves from blood capillaries into the interstitial spaces. Most of
the fluid returns to the blood, but some of the fluid moves from the
interstitial spaces into lymphatic capillaries to become lymph. To
collect the lymph from the interstitial space, lymph capillaries
originate in the blood capillary beds, and lymph vessels run parallel to
the veins. At intervals along the lymphatic vessels, lymph flows
through lymph nodes. Fluid collected in the lymph system is returned
to the heart via veins in the chest. Unlike the circulatory system, the
lymphatic system does not flow through a closed, circular system.
There are no lymph arteries. Lymph fluid is not pumped around the
body. Instead, the lymph system collects the lymph into vein-like
structures called lymph vessels and returns it to the bloodstream.
Lymph Vessels and Capillaries
The lymphatic system contains both capillaries and vessels.
Lymphatic vessels begin as capillaries. Both of these structures are
thin walled, which allows lymph to be transported across the
membrane and collected in the vessels. Lymphatic capillaries have
greater permeability than blood capillaries and can absorb large
molecules such as proteins and lipids. The endothelial cells that make
up the wall of a lymphatic capillary lack a basement membrane
loosely attach to each other and slightly overlap. Interstitial fluid
enters the lymphatic vessel when the pressure is greater in the
interstitial fluid than in lymph and nothing in the interstitial fluid is
excluded from entering the lymphatic capillaries. When pressure is
greater inside the lymphatic capillary, the endothelial cells prevent
lymph from passing back into the interstitial spaces by acting like a
one-way swinging door. Lymphatic capillaries are found wherever
blood capillaries are located except in the central nervous system and
bone marrow.
Anatomy of a Lymphatic Vessel
Lymphatic capillaries unite to form larger lymphatic vessels.
Structurally, lymphatic vessels are similar to veins because they also
have one-way valves that function like gates to ensure the lymph only
flows in one direction. Like veins, skeletal muscle contraction exerts
pressure on the lymph vessels and forces the lymph forward through
them. Lymph vessels are like one-way roads, with the lymph being
collected at the capillary beds and travels through the body into the
thoracic cavity. Lymph is deposited in one of two large ducts in the
chest region: the right lymphatic duct and the thoracic duct. The
lymph then travels from these ducts into venous circulation via the
subclavian and jugular veins.Unlike the cardiovascular circulation,
the lymphatic circulation lacks a pump like the heart. Lymphatic
vessels are low-pressure vessels similar to veins and the same muscle
pump and respiratory pump that promote venous return also
facilitate lymph flow. Therefore, even though there is some smooth
muscle in lymphatic vessels, movement of the body is important to
lymph circulation.
Common Dysfunctions of the Lymphatic Circulation
Anything that would disrupt the flow of lymph could contribute to
significant swelling of tissues (edema). Lymphedema is a condition
of localized fluid retention and a tissue swelling caused by a
compromised lymphatic system. Lymphedema can be primarily
caused genetically or secondarily due to injury or obstruction of
lymphatic vessels. It is most frequently seen after lymph node
dissection, surgery and/or radiation, in which lymphatic system
damage is caused during the treatment of cancer, usually breast
cancer. Lymphedema may also be associated with parasitic infections
in which parasites obstruct lymph vessels. Symptoms may include
fatigue, a swollen limb or localized fluid accumulation in other body
areas, including the head and neck, discoloration of the skin overlying
the swollen tissue and eventually deformity (elephantiasis).