The Tidal Nature of Blood Flow
There are a few core, basic principles of physiology that are currently outside of the understanding and models of both Industrial Medicine (modern medicine as it's been coopted by industry for profit at the expense of health) and even alternatives to this system, like Functional or Integrative Medicine.
One of these is the tidal nature of blood circulation in the body. Like all living systems on planet earth, human physiology follows a circadian rhythm coupled to the relative movement of the sun.
The central importance of daily patterns to health is increasingly understood when it comes to things like sleep or meal timing. But it still hasn't permeated the models to the extent it needs to for clinicians to fully understand pathology or how to interpret labs.
This very much includes understanding the rhythms of blood flow in the body. We have this idea that we can do blood tests to see what disease you "have" or even how much of a certain substance you "have." Really we're just measuring particular substances in the blood at a given time. No more, no less.
The amount of something, like a hormone or a protein, that's in the blood may not tell us how much is in the body at all. In fact, it may be that as the amount in the periphery of the body where blood is taken to be measured goes down, the amount in the center of the body, like in the brain and other organs, actually goes up.
This is a nice quote from a 1977 study on daily and seasonal rhythms of serotonin (5-HT), which illustrates this point nicely, particularly the last sentence.
"A pronounced diurnal pattern of 5-HT, with a minimum in the afternoon and higher values in the morning and evening, was similar to the diurnal variations in plasma free tryptophan. Platelet 5-HT was significantly higher at 8 a.m. than 4 p.m. in spring but no longer in autumn, and was similar in both men and women. It appeared that peripheral diurnal variations in 5-HT were opposite in phase to the central 5-HT rhythm."
Wirz-Justice, Anna, M. Lichtsteiner, and H. Feer. “Diurnal and Seasonal Variations in Human Platelet Serotonin in Man.” Journal of Neural Transmission 41, no. 1 (March 1, 1977): 7–15. https://doi.org/10.1007/BF01252961.