The lymphatic system is made up of the lymphatic organs and the lymph vessel system. Besides its immunological function in fighting infections, the latter is important in the transport of fluids. Hereby, it is closely related to the blood circulatory system.

The lymphatic capillaries that emanate out to the periphery of the body represent the starting point of the lymph vessel system. These capillaries unite to become first preliminary collectors and then collectors. The lymph is further directed from the lymphatic branches to the veins. From the preliminary collectors on, the lymph vessels have valves that allow the lymph to flow in one direction. Like filter stations, lymph nodes are also incorporated in the lymph vessels.
The lymph transport takes place passively by the movement of the extremities and the joints and the resulting constriction of the lymph vessels. Moreover, the lymph is transported actively by the ordered contraction of single lymph vessel cells which are restricted by valves and contract about 10-times per minute. If the lymph vessel cells are excited, they can even contract up to 60-times per minute. The lymph follows the least path of resistance towards the proximally widening lymph vessels.
Parts of the blood leak out in the capillary tissue bed as intercellular fluid. The tissue fluid is drained via the veins and partially as lymph via the lymph vessels. Lymph and blood are then reunited via the Vena cava superior.

Lymphedema is marked by a visually discernible and palpable amount of fluid that collects in the interstitium. This results from the mechanical insufficiency of the lymphatic system that, in turn, is caused by a chronic inflammatory illness, by valve failure, by blockages from tumors or by interruptions due to operations. Because of the mechanical insufficiency, the interstitial fluid cannot be sufficiently drained off. This results in a back-up and collection of liquid in the spaces between the cells.

In order to drain off the excess fluid, the following physical drainage therapies come into question: manual lymph drainage, compression bandages/stockings, compression machines, special motion therapy.

If pressure is applied to the human body, the tissue tension increases, because fluids do not allow themselves to be compressed. Soft tissue consists mainly of water and the redirection of fluid is only limitedly possible. As fluids can drain off into the vessels, pressure applications affect the vascular system.
Externally, the compression pressure can result from e.g. compression stockings, and, internally, by the expansion of the muscle resisting the compression agent.

If the skeleton musculature is contracted without compression, the interstitial pressure increases and is transmitted to connective tissue-like connections to the vascular wall of the veins and the lymph vessels. The skeleton musculature works like a muscle pump. Superficial veins are pressed against the skin and, thus, squeezed. By means of the valves, the flow is directed proximally. In the muscular diastole (relaxation) phase, the vessels can then refill. The compression augments the efficiency of the muscle pump. Also with regard to the manual lymph drainage, the tissue pressure as well as the vascular pressure are both locally increased. Also the joint pumps support the directed transport of fluid.

For over 100 years it has been known that vessels atrophy with decreasing use. Thus, inflammations can additionally restrict the vascular supply of oxygen and nutrients to body areas. Since intermittent negative pressure treatment causes more shear stress, this stress increases blood flow that, in turn, causes atrophying capillaries to re-form and become even stronger. Consequently, more fluid can be taken up and drained off. Anastomoses can also benefit from this growth. It is known, for example, that scar formation can interfere with the formation of lympho-lymphatic anastomoses and, thus, hinder wound healing. Since positive effects have been observed under Vacumed treatment, one could see this as indirect proof. This could even be an advantage over conventional means to promote capillary formation.

By means of the intermittent negative pressure, the Vacumed device works like an external muscle pump. During the negative pressure phase, the superficial lymph vessels, because of the cavity, are expanded somewhat greater than the surrounding more solid tissue. Through the resulting pressure gradients, the vessels are filled with more lymph. The normal pressure then suffices to press the fluid via the lymph vessel system proximally.
This could explain why the intermittent negative pressure therapy can be successful to treat lymphedemas.