II. Vessels of the Lymphatic System From smallest to largest the lymph vessels are:
Lymph capillaries Lymph vessels (called lymphatics) Lymph trunks Lymph ducts
A. Lymph Capillaries Lymph capillaries are the smallest lymph vessels. They interweave with blood capillaries throughout the tissue space. Unlike blood capillaries they are closed at one end (closed-ended). This means they start at the tissue level. They can be found as a single vessel or in extensive plexuses. The interstitial fluid collected from body tissues is absorbed into these vessels.
Lymph capillaries are more permeable than blood vessels. They can absorb proteins, pathogens, and debris that are too large to enter a blood capillary. This is due to the fact that they are a single layer of endothelial cells where each cell slightly overlaps the next, forming mini-valves. The mini-valves allow interstitial fluid to get into the lymph capillary but not out. Lymph capillaries are found in all vascular tissues except the spleen, bone marrow, and CNS. In the CNS the role of lymph is performed by the cerebrospinal fluid.
Lacteals are specialized lymph capillaries present in the innermost wall of the small intestine. They absorb fats from food. The lymph here is milky white rather than clear (lacte = milk). Lacteals drain into larger lymph vessels which also eventually drain into the blood vessels.
B. Lymphatics and Lymph Trunks Lymphatics are like veins but they are slightly larger and have thinner walls and more valves. Superficial lymphatics travel parallel to superficial veins. Deep lymphatics form networks around the arteries and deep veins. Lymphatics are not continuous vessels. They enter and leave a series of lymph nodes as they carry lymph from the tissue space back to the veins. The lymph nodes are found along the length of the lymphatics and filter the lymph as it passes through. When a lymphatic carries lymph into a lymph node it is called an afferent lymphatic. When a lymphatic carries the filtered lymph away from a node it is called an efferent lymphatic.
Lymph trunks are larger vessels than the lymphatics and carry lymph to the even larger lymph ducts.
C. Lymph Ducts Lymph ducts are two main lymphatic vessels that receive lymph from the trunks and carry it to subclavian veins for reentry into the blood stream.
The two lymph ducts are:
The right lymph duct begins in the right shoulder area and collects lymph
from only ¼ of the body. It collects lymph draining from the right side of the
head and neck, the right side of the thorax, and the right upper extremity. The
right lymph duct connects to the right subclavian vein and empties its lymph
into the vein.
The left lymph duct or thoracic duct is much larger and is located anterior to
the vertebral column. This duct collects lymph from the other ¾ of the body. It
begins at L2 as the cisterna chyli, a large sac that receives lymph from both
of the lower extremities and intestinal trunk. Running superiorly, it also drains
the left side of the neck and head, the left side of the thorax, and the left
upper extremity. The thoracic duct connects to the left subclavian vein and
empties its lymph into the vein.
III. Lymphatic Tissue and Lymph Nodes A. Lymphatic Tissue (Lymphoid Tissue) Lymphatic tissue is comprised of special reticular fibers and large populations of white blood cells, especially lymphocytes. Lymphoid tissues contain centers of generation for white blood cells.
Diffuse lymphatic tissue is found in mucous membranes of the gastrointestinal (GI), respiratory, urinary, and reproductive tracts. Altogether this tissue is called Mucosal Associated Lymphoid Tissue (MALT). Note that these areas are places where pathogens have easy access to the body.
Lymphatic nodules are oval-shaped concentrations of lymphatic tissue. These nodules do not have a capsule. They are found in the GI, respiratory, and urinary tracts. Large accumulations of lymphatic nodules are found in the GI tract (Peye r's patches), vermiform appendix, and throat (tonsils).
The tonsils are multiple clusters of large lymphatic nodules forming a ring of lymphatic tissue around the entrance to the throat. They are:
Pharyngeal tonsils (adenoids): located on the posterior wall of the
nasopharynx. Palatine tonsils: located on either side of the throat and are the ones that are
usually removed. Lingual tonsil: a cluster at the base of the tongue, and also sometimes
Pathology Note: As we have seen, the tonsils are lymphoid tissues that can entrap bacteria and other pathogens in their reticular fibers. This helps to ensure immunity against a wide range of organisms and is important especially in childhood to build up immunity. However, bacteria may occasionally overwhelm the lymphoid tissue in the tonsils and cause infection. Chronically infected tonsils may need to be removed surgically.
B. Lymph Nodes
Lymph nodes are the most numerous lymph organs. They are oval shaped filters located along lymph vessels. Lymph passes through many nodes on its journey back to the blood. Each node acts to clean and filter the lymph as it passes through. Typically, by the time lymph reenters the blood stream it has been cleansed of pathogens and cellular debris
Lymph node size varies from 1mm - 25mm in length. There are superficial and deep sets of nodes. They are particularly concentrated as superficial and deep nodes in the areas of the neck, axilla, and inguinal regions. Lymph nodes contain populations of white blood cells and are locations where some white blood cells reproduce.
Typically, several afferent lymphatics enter the node. One efferent lymphatic leaves the node.
The node consists of an outer capsule of dense connective tissue. Inside the capsule, the outer region is called the cortex and the inner region is called the medulla. These areas contain large numbers of T-cells and B-cells. The T-cells continuously circulate into and out of the node. B-cells become plasma cells in areas of the cortex called germinal centers. Subsequently, large numbers of mature antibody-producing B cells are present in lymph nodes.
Reticular fibers (loose connective tissue fibers) throughout the node have large numbers of macrophages sitting on them to phagocytose wastes, debris, and disease-
causing organisms (pathogens). The macrophages of the node attack pathogens and initiate the immune responses of the B-cells and T-cells. When the body is invaded by pathogens it is called an infection. Lymph nodes play a vital role in the elimination of pathogens from the body.
Massage Note Caution: When lymphocytes in the nodes proliferate in response to pathogens the node swells. Therefore, swollen nodes are contraindicated for massage because they usually indicate that an infection is present. Massage may flush the pathogen out of the node before it is destroyed and therefore spread the infection through the body.
IV. Lymph Organs A. Primary Lymph Organs: are the sites of production and maturation of B-cells and T- cells, producing immune competent B-cells and T-cells. The two primary lymph organs are:
Bone marrow - hemocytoblasts in red bone marrow make lymphocytes. B-
lymphocytes mature in bone marrow. Thymus - as a lymphoid organ, T-lymphocytes mature in the thymus
B. Secondary Lymph Organs: contain white blood cells that have traveled to them from the primary lymph organs: bone marrow and thymus. Secondary lymph organs are
Lymph nodes: the most abundant secondary lymph organs (see above). Tonsils: Although they don't have a capsule the tonsils are often
considered organs. Immune responses can occur here, as they do in the
nodes, causing the tonsils to swell. Spleen: the spleen is the largest mass of lymphoid tissue in the body. This
fist-sized capsule containing blood vessels and lymphoid tissue is just
below the diaphragm in the left upper quadrant of the abdomen, curling
around the anterior aspect of the stomach.
The spleen filters blood, not lymph. The spleen contains two types of tissues. There are many venous
sinuses (called red pulp) where blood is stored and cleaned by
reticuloendothelial cells. There are also areas of concentrated lymphoid tissue called white pulp.
Lymphocytes in the spleen monitor blood for pathogenic organisms.
Macrophages remove old platelets and erythrocytes, and also remove debris from the blood.
The spleen has no afferent lymphatics, only efferent lymphatics.
Blood entering the spleen can leave as blood or as lymph, but the spleen does not filter lymph fluid.
The spleen has a layer of smooth muscle which can contract
forcefully during the fight or flight reaction. If more blood is needed (as in severe blood loss), the sympathetic nervous system will cause contraction of the smooth muscle to squeeze the spleen and push extra blood into circulation for increased blood volume. The spleen is fragile and easily ruptured by trauma to the abdomen.