Lymphatic muscle cells are unique in the body, functioning like both smooth muscle and cardiac cells simultaneously.
Muscular contraction and relaxation phases are managed by a complex coordination of extrinsic and intrinsic factors, including local tissue composition, lymph contents and autonomic neural reflexes.
A 2020 Review in Biology by Solari et al, presents an excellent description of factors which modulate lymph flow, and the cellular and molecular mechanisms involved (1).
The following is a summary on my understanding of the main points, and how they relate to the forces created during application of the Dr Vodder Method of MLD.
Lymph Flow is Constant
Lymph is formed from circulating tissue fluid, constantly fed from the blood capillary and continuously removed by the lymph vessel system, for filtering, cleaning, and return to the venous circulation.
Lymph vessels use muscular contractions to propel the lymph from the periphery toward the heart.
Lymph is passed through multiple lymph node stations along the way, where it is processed and rendered harmless by immune system interactions within the lymph nodes.
Lymph Vessels
There are three layers forming the lymph vessel wall. The internal lining of endothelial cells also forms the bicuspid valves between individual lymphangions.
The muscular layer appears in sheets around the lymphangion and is thicker through the mid-section than around the valves.
There are longitudinally and transverse layers similar to other smooth muscle organs, and in the larger lymphatics there may be also be a spiral layer.
A fibrous outer layer (adventitia) has a higher than usual proportion of elastin fibres.
80% of all the lymph vessels are located just under the skin where they work continuously to remove harmful molecules or microorganisms that have crossed the skin barrier.
Intrinsic and extrinsic mechanisms of lymph-motoricity
Lymphatic smooth muscle responds to a variety of stimuli which can be classified as; intrinsic i.e. stimulus from within the vessel, or extrinsic where muscle contraction is initiated from outside the vessel.
Intrinsic mechanisms are the primary driver of lymph transport in the superficial system
There are several intrinsic mechanisms which induce lymph pumping.
Firstly, lymphatic muscle is auto myogenic, a property they share with cardiac pacemaker cells and which generates around 6 contractions of the lymphangion each minute. These routine contractions in the most distal lymph collectors create a pressure gradient between the loose connective tissue and the initial lymph vessel lumen which favours the uptake of interstitial fluid during lymph formation.
The effectiveness of intrinsic lymphatic pumping can be seen in the visible contours of our surface features, such as the tendons on the back of your hand.
The large cohort of lymph collector vessels under the skin creates and maintains a sub-atmospheric pressure in the soft tissue, so that normal atmospheric pressure compresses the soft tissue and less compressible features such as tendons stand out (Read more).
Intrinsic pumping can be up and down regulated by the endothelial cell signalling in response to shear forces from the internal flow of lymph, and by the composition and temperature of the lymph. This ensures that lymph flow is responsive to, and able to maintain local tissue volume and pressure homeostasis.
Extrinsic stimuli are the dominant driver of lymphatic pumping in the deep system.
Movements outside the lymphangion can also increase lymph-motoricity by transmitting stretch and shear forces to the vessel wall via fibrous components of the connective tissue.
The pulsation of arteries in the perivascular sheath, movement of skeletal muscles, and to a lesser degree peristalsis of digestive organs, all contribute to increased lymph pumping though mechanic movement around the lymph vessels.
Respiratory movements are a strong extrinsic stimulus to lymph flow and activation of the diaphragm is a vital component of drawing lymph from the abdominal vessels into the thoracic compartment. increasing lymph flow 10-fold in rat models..
How they work together
Lymph is formed at the interface between the initial lymph vessels and the loose connective tissue, with low intraluminal pressures created by proximal movements creating a suction force to draw interstitial, fluid into the lymphatic vasculature.
Intrinsic lymph pumping, responsive to tissue load and composition is the dominant force in the periphery, propelling lymph toward more proximal collector vessels.
In the deep system, extrinsic forces are recruited to continue propelling the lymph toward the heart. Autonomic reflexes may also moderate lymphatic muscle cells, particularly in the thoracic duct where they co-ordinate the flow of lymph with movement in the diaphragm.
Mechanical vs molecular stimulus of angiogenesis and vascular function.
Even though endothelial cells line all blood and lymph vessels, they exhibit specific variations in their response to stimuli, especially in terms of promoting new vessel formation (angiogenesis) and in the signalling pathways for regulating the contraction or relaxation of smooth muscle cells.
In blood vessels both mechanisms are mediated by molecular stimuli, such as the concentration of oxygen, carbon dioxide and nitrous oxide molecules.
In lymph vessels, the stimulus are both mechanical, with stress and shear forces in the surrounding tissue directing the growth of new lymph vessels (lymph angiogenesis), and shear forces created by the internal flow leading to changes in lymph pumping.
How does MLD increase lymph-motoricity
The precise and specific movements of the Dr Vodder method of MLD, have been shown to increase the rate of lymph pumping by activating both intrinsic and extrinsic stimuli.
Movement in the skin without any downward compression, opens the initial lymph vessels, reducing the intraluminal pressure and encouraging lymph formation. The increased lymph volume stretches the endothelial lining resulting in an increase in intrinsic lymph pumping.
The two directional nature of each movement transfers stretch and shear forces into the tissues which also activating extrinsic mechanisms of lymph pumping. As we working from distal to proximal along the lymph pathway, the lymph vessel is both loaded internally and stimulated by the movements in the surrounding connective tissue.
The profound effect of MLD on automatic balance will also affect lymph flow systemically, as high sympathetic levels constrict peripheral lymph vessels. Rebalancing autonomic tone dilates lymph vessels increasing the overall capacity of the system.
MLD therefore has both local and systemic effects on lymph flow.
Summary
Both intrinsic and extrinsic forces work together to ensure a steady lymph flow from the periphery to the heart. MLD can directly impact lymphatic pumping by stimulating both intrinsic and extrinsic control mechanisms.
In contrast to blood vessels which respond primarily to molecular stimuli, lymph vessels are sensitive to stretch and shear forces both internally and externally.
Taking into account the significance of gentle shear forces in tissue healing, the mechanisms by which MLD increases lymph flow and promotes tissue repair become obvious.
Mechanical forces are the key, and this is exploited exquisitely by the specific direction and force of movements in the skin and tissue creating by the original Dr Vodder method of Manual Lymph Drainage.
Solari, E.; Marcozzi, C.; Negrini, D.; Moriondo, A. Lymphatic Vessels and Their Surroundings: How Local Physical Factors Affect Lymph Flow. Biology 2020, 9, 463. https://doi.org/10.3390/biology9120463
Simple Summary
Lymphatic vessels are responsible for the drainage of liquids, solutes, and cells from interstitial spaces and serosal cavities. Their task is fundamental in order to avoid fluid accumulation leading to tissue swelling and edema. The lymphatic system does not possess a central pump, instead lymph is propelled against an overall hydraulic pressure gradient from interstitial spaces to central veins thanks to two pumping mechanisms, which rely on extrinsic forces or the intrinsic rhythmic contractility of lymphatic muscle cells embedded in vessel walls. This latter mechanism can very rapidly adapt to subtle changes in the microenvironment due to hydraulic pressure, lymph flow-induced wall shear stress, liquid osmolarity, and local tissue temperature. Thus, endothelial and lymphatic muscle cells possess mechanosensors that sense these stimuli and promote a change in contraction frequency and amplitude to modulate lymph flow accordingly. In this review, we will focus on the known physical parameters that can modulate lymph flow and on their putative cellular and molecular mechanisms of transduction.
Abstract
Lymphatic vessels drain and propel lymph by exploiting external forces that surrounding tissues exert upon vessel walls (extrinsic mechanism) and by using active, rhythmic contractions of lymphatic muscle cells embedded in the vessel wall of collecting lymphatics (intrinsic mechanism). The latter mechanism is the major source of the hydraulic pressure gradient where scant extrinsic forces are generated in the microenvironment surrounding lymphatic vessels. It is mainly involved in generating pressure gradients between the interstitial spaces and the vessel lumen and between adjacent lymphatic vessels segments. Intrinsic pumping can very rapidly adapt to ambient physical stimuli such as hydraulic pressure, lymph flow-derived shear stress, fluid osmolarity, and temperature. This adaptation induces a variable lymph flow, which can precisely follow the local tissue state in terms of fluid and solutes removal. Several cellular systems are known to be sensitive to osmolarity, temperature, stretch, and shear stress, and some of them have been found either in lymphatic endothelial cells or lymphatic muscle. In this review, we will focus on how known physical stimuli affect intrinsic contractility and thus lymph flow and describe the most likely cellular mechanisms that mediate this phenomenon.
Great read to ease into this topic. In the article it mentions the rate of intrinsic pumping can be influenced by temperature of the lymph, so I was wondering what is the ideal temperature for lymph and at what temperature does flow become affected?
A great read