Innovative techniques are being used to help researchers investigate and understand lymphatic function. Understanding the mechanics of lymph transport can offer valuable insight to patients living with lymphedema, and will lead to better clinical management and new therapeutic targets. Here we provide a brief overview of our current understanding of lymph transport, and what it reveals about best practices in lymphedema management.
Arteries transport blood throughout the body, bathing cells and tissues with the nutrients and oxygen they need. Veins collect only a portion of the fluid delivered by the arteries, the rest of it is picked up and returned to the cardiovascular system via the lymphatic system.
The lymphatic system is organized much like a tree, starting with very small vessels located in the body’s tissues called initial lymphatics. This is where lymph fluid first enters the system. These initial lymphatics then coalesce into larger lymphatic vessles known as ‘collectors’, which connect to lymph nodes and then further coalesce into larger vessels or trunks. These larger vessels drain lymph into either the right or left subclavian vein (located at the neck), and back into the blood stream.
The lymphatic system is often thought of as a primarily passive vascular system, much like veins, characterized by low fluid pressure and periodic one-way valves that prevent backflow. Understandably, patients often wonder why the excess fluid in their limbs doesn’t just find another way out via veins or remaining healthy lymphatic vessels, or why contracting the limb (which compresses the tissues and increases pressure on the fluid) doesn’t help pump out the excess fluid.
Why is swelling so difficult to reduce in patients with lymphedema?
If the volume of fluid in tissues is determined by how much is delivered minus what is removed (which it is), and increased volume of fluid in the tissues is a driving force pushing fluid into initial lymphatics (which it is), then why in the case of lymphedema can’t the veins, or remaining healthy lymphatic vessels relieve lymphatic swelling? And shouldn’t contracting the limb help pump the excess fluid out? The simple answer is that the lymphatic system is far from being a simple passive drainage system.
It is true that fluid entering a tissue increases fluid pressure within the tissue (called interstitial pressure) which in turn decreases the amount of new fluid deposited by the vascular system, and helps push fluid out of the tissue and into the lymphatic vessels. A good example of this is when a person is put on a saline I.V. drip, which causes a spike in lymphatic flow due to the excess volume of fluid in the body.
However, the process of draining tissues of fluid is actually far from passive – interstitial hydrostatic tension (fluid build up) and muscle contraction (limb movement) can help, but the primary driver of lymph flow is the pumping of a repeating functional unit of the lymphatic system called the lymphangion. As you move more centrally up the lymphatic vasculature you begin to see lots of smooth muscle, and the appearance of a lymphangions organized one after another, like the cars of a freight train. The lymphangion has valves like a vein to allow one-way fluid flow, but thanks to its smooth muscle it also produces a rhythmic pumping motion to propel fluid from one lymphangion to the next.
So instead of a “drain” like the lymphatic system is often referred to as, the system may be better described as a vacuum. Indeed, once lymph fluid is in the ‘collector’ lymphatics, fluid mechanics suggests it needs to be pushed uphill against a significant pressure gradient. Increased pressure in the tissue can help, but the primary driver of this lymph flow is the pumping of lymphangions. Consequently, poorly functioning lymphatics creates a pool of fluid that can’t be passively removed.
Considering the mechanics of lymph transport, what treatments should and shouldn’t work?
The observation that the lymphatic system is more like a vacuum than a drain can help explain why compression garments can be very effective at restricting new edema, but very ineffective at reducing existing edema. An increase in fluid pressure in the compressed area can help reduce the volume of fluid that is deposited into the tissue from the circulatory system, but it can’t help squeeze out fluid that has already found its way there. There simply aren’t sufficient passive drains for it to exit through.
This observation also explains why manual lymphatic drainage is so effective – by redirecting fluid into areas where active lymphatics still exist. Conversely, it also explains why Pneumatic Compression Devices (which provide rhythmic massage typically in the direction of dysfunctional or insufficient lymph nodes), have limited effectiveness for treating secondary lymphedema.
New techniques to non-invasively study lymphatic function in animals and humans are also emerging and will continue to expand our understanding, and evolve best practices in lymphedema management and treatment.