Bailing the Leaky Ship
The lymphatic system is another network that transports material throughout the body. While most of the plasma that leaks from capillaries into tissue is returned via those same capillaries, some fluid does not. If this interstitial fluid remained in the tissues and was allowed to further accumulate, swelling (edema) would occur. The lymphatic system is a means by which this leaked fluid is returned to the circulatory system.
Additionally, as this fluid washes through the tissues, it invariably collects cellular debris and pathogens. To identify and mount an immune response against these materials, the lymphatic system determines if the materials are pathogens.
Composition of Lymph
Once in the capillary, high blood pressure causes plasma to be pushed into the interstitial tissues, the tissues outside the vessels. This lowers the pressure of the remaining plasma in the capillary so that at the end of the capillary (before it becomes a venule) the pressure is much lower.
Plasma is rich in proteins, such as albumin, which are too large to leak into the tissues. So, while the blood pressure is lower at the venous end of the capillary, the protein content remains high and continues to exert an osmotic drawing force on the fluid in the tissues. In this way, the proteins act as solutes to attract water and this attractive force is stronger than the blood pressure trying to push out more fluid. Therefore, the power imbalance favors fluid returning to the capillary and leaving some plasma (minus proteins) outside the capillary in the tissues. This plasma-minus-proteins is now called lymph.
Anatomy of a Word
filtration
Filtration is when fluid leaves the capillary, and adsorption is when tissue fluid returns to the capillary.
With little protein content and no pump (such as the heart) exerting pressure on it, the lymph has very little reason to move into the lymphatic circulation. It depends on pressure exerted in the interstitial tissues by surrounding organs—especially muscle. The action of walking, breathing, or any movement in general causes the position of organs to change, temporarily increasing the pressure of the lymph. This external pressure forces the stranded lymph into the lymphatic system.
Lymphatic Circulation
The lymphatic system begins with narrow, thin-walled, and blind-ended vessels that collect the lymph from tissues and funnel it back toward the heart. Along the way, these vessels release the lymph into small organs called lymph nodes, which act like a filtering system. Lymph nodes are filled with phagocytic cells (macrophages) and lymphocytes (immune cells). Within the lymph nodes, the macrophages remove and examine the debris, and if any pathogens are found, present the molecules (antigens), signifying the material as a pathogen to the local lymphocytes in order to stimulate an immune response. After leaving the lymph node, the lymph continues in even larger lymphatic vessels until the fluid, now cleaned of debris and pathogens, is returned to the circulation via the subclavian veins.
Lymphatic Capillaries and Vessels
Capillaries are the only vessels that allow for the direct exchange of materials. So to move from the tissues into lymphatic circulation, the lymph must flow through capillaries. The ones it uses are called lymphatic capillaries. The pressure exerted upon the fluid forces the lymph between the loose and overlapping junctions of these capillaries. In essence, the junctions function as unidirectional valves to allow the lymph to come in, but not go out. Additionally, the endothelial cells of the lymphatic capillaries are attached to the peripheral connective tissue cells via fibers. These fibers help keep the lumen, or inside space, of the lymphatic capillary open as pressure pushes the fluid into the vessel.
Located within capillary beds of the circulatory system, lymphatic capillaries are in the perfect locations to collect lymph as it fails to return to the circulatory system via the systemic capillaries. Additionally, in the gastrointestinal tract, large lymphatic capillaries are present in every protrusion (villus) of the intestinal surface. Here, material absorbed from the intestines can quickly and easily make its way into the lymphatic system, and then be cleaned and screened in the lymph nodes before being passed along to the circulatory system.
Lymphatic capillaries empty their contents into larger lymphatic vessels that are composed of the same three layers (tunics) as the circulatory vessels. Lymphatic vessels most closely resemble veins with their large luminal diameter, compared to the thin wall of the vessel. Also like veins, the lymphatic vessels contain unidirectional valves that assist in the movement of the low-pressure lymph on its journey back to the heart.
How can lymph vessels be distinguished from veins?
An easy way to distinguish a lymphatic vessel from a vein is by the presence or absence of RBCs. While lymphatic vessels will have lymphocytes and white blood cells, only veins will contain red blood cells.
Large vessels combine to form lymphatic trunks. These trunks empty into one of two lymphatic ducts that transfer the drained lymph to one of the two subclavian veins, completing its return to the circulatory system.