Lymphatic Physiology

Overview
  • Lymphatic vessels serve as conduits for return of protein- and particle-laden interstitial fluid back to the circulation. These vessels direct this fluid through a series of lymph nodes and in doing so play an important anatomical role in immune function.
Vessel Architecture
  • The architecture of lymphatic vessels allows for unidirectional entry of fluid along with high molecular weight proteins and particulates that would normally not be able to cross blood vessels. This is a result of the loose connections between certain lymphatic endothelial cells that result in the creation of large inter-cellular flaps. Fluid and particles can enter the lymphatic vessel through the flaps but cannot exit, thus creating a one-way valve.
Lymph Flow
  • Lymph fluid flows from the ends of lymphatic capillaries into progressively larger lymphatic vessels that ultimately combine into the thoracic duct. The phenomenon which appears to actuate lymphatic flow from the interstitium back to the vasculature appears to be intermittent squeezing of lymphatic capillaries by surrounding muscles. Because lymphatic vessels posses intra-vascular unidirectional valves, similar to those found in veins, squeezing of the lymphatic vessel tends to cause directional flow toward the vasculature.
Role in Interstitial Fluid Maintenance
  • Overview
    • Lymphatic Vessels ultimately drain the interstitial fluid and return it to the circulation. This function is critical for homeostasis of the interstitial fluid for a variety of reasons.
  • Maintenance of Low Interstitial Oncotic Pressure
    • Interstitial fluid generally has a minimal oncotic pressure due to its low protein concentration. However, small amounts of plasma proteins do leak out of the vasculature and into the interstitial fluid. If these proteins were not rapidly removed by lymphatic vessels, their concentration would rise and progressively increase the interstitial fluid oncotic pressure. This would severely derange microcirculatory Starling Forces and result in excessive filtration of water into the interstitial fluid.
  • Maintenance of Interstitial Volume
    • In certain pathological scenarios excess fluid can leak from the microcirculation into the interstitial fluid. In such cases lymphatic vessels act as over-flow mechanisms and drain excessively filtered fluid. Excess fluid in the interstitium naturally increases the interstitial pressure and this happily increases the lymphatic flow rate.
  • Maintenance of Interstitial Pressure
    • Many studies have demonstrated that the interstitial fluid normally possesses a negative hydrostatic pressure. It is thought that the lymphatic muscular pumping mechanism described above is the the mechanistic source of negative hydrostatic pressures within interstitial fluid.