The State Change Hypothesis for Venous Blood Flow asserts that the blood in the veins is propelled upward at least in part by a sudden change in volume of the blood itself and is prevented from returning by the valves.
The idea of a state change in biological fluids is described at length in Gerald Pollack’s book and the idea as applied to blood flow has now been mentioned by Stefan Lanka.
Here we see a section of vein including two valves. The section is narrowed in the middle as if compressed by the muscles of the body, thereby pushing the blood upwards. If in addition to this, the blood were to suddenly expand for some reason then this would clearly assist in the procedure and make for a more effective pumping action, clearing a greater percentage of water volume than would otherwise be possible.

The assertion is that the blood does not expand gradually as with a slow heating process but that it exists in two distinct states and can make an almost instantaneous transition between the two, triggered by a relatively small energy change whether it be mechanical, thermal or possibly electrical. A state change is accompanied by both a change in energy state (either sudden release or gradual absorption) and also a change in molecular structure.
The structure of biological or ‘EZ’ water is shown here as a tightly packed hexagonal structure. The water is much more dense than ordinary water and is constructed when water comes into contact with the lining of the arteries.
This state is a ‘high energy’ state in that it requires the input of a certain amount of energy to create the extra bonds necessary to maintain its structure.

So the idea is that the blood is shaped upstream into some dense high energy state similar to EZ water by the heart, arteries and possibly capillaries. Then as it enters the veins, it will change state either suddenly or gradually, expanding in the process and pushing itself past the valves and upstream towards the heart.
EZ water is constructed by several means; by input of heat, by adsorption to hydrophilic surfaces and also via the construction of vortices (Pollack). The heart and the arteries are certainly good sources of heat, and spiral currents have been observed in both the heart and arteries.

Stefan Lanka identifies the dense water as ‘PI water’ and mentions a similar scheme for driving the blood through the veins. In addition, he claims that energy is stored as angular momentum in the spinning red blood cells and that there is later on an exchange of this energy with the PI water in the blood vessels.
Summary
The heart generates energy in the form of heat and momentum which is imparted to the blood via a vortex stream and stored as intermolecular bonds until required by the, veins at which point the energy is released and converted back to mechanical energy to assist the blood flow back to the heart.
The valves are automatically closed behind the flow by vortices in the blood flow: see Blood Flow in the Heart
Question 1: What is the precise nature of the dense fluid created by the blood flow?
Rene Quinton replaced all the blood in a dog with seawater and the animal survived with no long term damage, indicating salt water is sufficient for circulation and that other specialised properties of blood are not strictly necessary. Moreover, the stopping of a dogs heart did not immediately lead to cessation of blood flow so even the kinetic energy imparted by the heart is not really needed. Blood Flow in the Heart
Question 2: What is the mechanism by which this fluid is expanded?
The cooling of the blood as it moves from the core to the extremities has been proposed; after all, a cold shower is invigorating but a hot bath makes us feel sluggish.
Question 3: How is the expansion of the fluid in each segment organised to create a coherent flow?
Note that if all the venous blood expands at once then there is no need for the valves from this point of view; the scheme only works if one segment at a time is expanded.
Related pages: Blood Flow in the Heart Stefan Lanka and Pi water Vortical Flow The Heart is not a Pump Rene Quinton
References:
Cells Gels and the Engines of Life – Gerald Pollack
The Fourth Phase of Water – Gerald Pollack
https://fully-human.org/wp-content/uploads/2019/10/Pollack_The-fourth-phase-of-water.pdf
Stefan Lanka: What biology IS – body and soul biology and the substance life is made of
https://odysee.com/@counterpropaganda2020:a/Stefan-Lanka–What-biology-IS—Body-and-soul-biology-and-the-substance-life-is-made-out-of:9
PI water: More than just energised H2O – Stefan Lanka
https://wissenschafftplus.de/uploads/article/pi-water-understanding-water-recognising-life.pdf