Rich points to two articles (part 1; part 2) by Dr. Agar on Home Dialysis Central. I really enjoyed the articles in no small part because I agree so completely. It's always a joy to read something well written and authoritative that confirms your own belief.
The critical concept is in Part 1, he in fact writes "Okay...focus here...it is crucial that you understand this next bit!
"When HD removes a waste or fluid from your blood, it only reduces your blood level of that substance. When your blood level drops, a gradient (difference) forms between the blood and interstitium. This occurs because now the interstitium has more of the substance than your blood does.
Your body wants fluid, salts, and wastes to be in balance between your three fluid compartments. Keeping this balance, called homeostasis, is the main job of the kidneys. To restore balance, dissolved substances (solutes) always move from a compartment with higher level to one with a lower level.
So, as the blood level of a solute falls, this forms a gradient between your blood and interstitium. The solute will then move out of the interstitium and back into your blood. And this creates a second gradient between the interstitium and your cells. In this way, removing "X" from your blood will remove "X" from your interstitium, which, in turn, will remove "X" from your cells. Each of these steps takes time."
This is the crucial concept. The way I imagine the situation is each molecule is a color - a pixel of color. Red, green and blue, red are the small easy to remove molecules potassium and phosphorus, green are the harder to remove molecules phosphorus and beta2, and blue are water and salts.
At the start of dialysis all three compartments are white (the D65 point) because the colors are balanced between the compartments. If the red - small molecules come out fastest then there will be a color shift away from red to teal. I think you would see waves of color rippling through the body. As the blood reentered the blood stream from the venous needle it would be most teal and as it mixed in the blood compartment the blood stream would take on a blue/green tinge.
This tinge would spread to the interstitum and finally to the cells. This kaleidescope would continue and based on Dr. Agar's explanation you'd see a shift towards purple in the blood compartment because the green molecules would be removed faster than they could be replaced from the interstitium.
A high ultrafiltration rate would turn the blood compartment yellow as water is removed faster than it can be replaced. Another element that Dr. Agar did not discuss is that the blood does not circulate around the body evenly. My understanding is that at rest the blood tends to stay in one quarter of the body or another. I think if we could see the body as a whole we would see more rapid color shifts in the quarter of the body with the an extremity access (not sure of the impact of a catheter).
Dr. Agar's article was in two parts - I think my response will be in three.