I'm learning laws of salting in the Shulchan Aruch. One concept I'm battling to understand is "דם שפירש ממקום למקום" - blood that moved from one place to another. Here's a good example of my confusion: see YD 69:18. The Shach in ע"ד says that blood in the flesh that did not move is permitted, but that's only "if it didn't move at all from its place". A simple reading of this seems to imply that the Shach (and most other Rishonim and Acharonim) did not understand the concept of a circulatory system, since they seem to have a concept of a fixed place where each drop of blood belongs, and where it is permitted to eat it - but as soon as that blood moves out of its place, it becomes prohibited.
Perhaps the Mechaber disagrees with this interpretation of דם שפירש ממקום למקום, applying it only to blood the moved between two separate pieces of meat, or at least that exited this piece of meat and was later re-absorbed? In the argument over whether salting is supposed to remove all the blood or most of the blood, the Rambam comes down on the latter side, which is why he requires chalita, to seal the blood in so that it doesn't leak out when you later cook it. If that's the case, then he surely cannot agree with the way the Shach understands it, because whatever blood didn't come out with the salting has surely "moved" during the process - yet the Rambam permits it!
How do you understand this concept? And if, indeed, it is based on a misconception of the circulatory system, how would the halacha be affected?